**Recent Advances in Atmospheric-Pressure Plasma Technology**

Editor

**Bogdan-George Rusu**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editor* Bogdan-George Rusu "Petru Poni" Institute of Macromolecular Chemistry Romania

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This is a reprint of articles from the Special Issue published online in the open access journal *Applied Sciences* (ISSN 2076-3417) (available at: https://www.mdpi.com/journal/applsci/special issues/atmospheric pressure plasma).

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## **Contents**

## **About the Editor**

#### **Bogdan-George Rusu**

Bogdan-George Rusu, PhD. Phys., is researcher at the "Petru Poni" Institute of Macromolecular Chemistry, Iasi, Romania. He graduated from the Faculty of Physics, University "Al. I. Cuza", Ias, i, Medical Physics section. He also has an M.Sc. degree in Plasma Physics, Spectroscopy and Polymer Physics and a PhD in Physics at the University Al. I. Cuza, Faculty of Physics, Ias, i and at the Institut Europeen des Membranes, Universit ´ e de Montpellier II, Montpellier, France. ´

Plasma technology diagnosis, plasma treatment, and plasma polymerizations have been the focus of Rusu Bogdan-George's research for more than 15 years. As his main scientific interest, he mentions: the processing of materials with plasma (dielectric barrier discharge DBD at atmospheric pressure plasma treatment, plasma polymerization, and physical and photochemical changes induced by the UV–polymer laser interaction); the deposition of thin layers (using various techniques such as magnetron discharge sputtering, hollow cathode deposition, and pulsed laser deposition); and the preparation of nanoparticle suspension solutions (C, Si, and ZnO) using laser ablation in different liquids and testing them in biomedical applications.

The scientific results in these fields were realized through 33 scientific articles, of which 24 were published in specialized journals rated by ISI; and 47 communications at national/international conferences, including 10 oral presentations.

### *Editorial* **Recent Advances in Atmospheric-Pressure Plasma Technology**

**Bogdan-George Rusu**

"Petru Poni" Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, 700487 Iasi, Romania<EMAIL_ADDRESS>
#### **1. Introduction**

In recent years, plasma technology has presented an alternative in the processing and development of new materials. Of the plasma processing methods developed thus far, atmospheric pressure plasma is the most economically accessible, due to the low costs of this technology. Among the first applications of plasmas in atmospheric pressure was the treatment of various polymeric materials. Plasma treatment efficiently improves the surface wettability and adhesion properties of different types of materials. This is possible because plasma parameters such as particle density, collision frequency, mean kinetic energy of the particles, and the presence of chemical active species will cause a large variety of elementary processes both within the plasma volume and on the plasma–material interface. Consequently, the quality and magnitude of the polymer surface treatment, including etching, functionalization, and crosslinking, might also be controlled [1]. Generally, the plasma used in material processing can be generated in different gases, pressure ranges, or discharge geometries.

Another technique developed later was that of a plasma polymerization technique. Plasma polymerization is a versatile method for the deposition of films with functional properties, which are suitable for a range of applications. These plasma polymers have different properties than those fabricated via conventional polymerization: the plasmapolymerized films are usually branched, highly cross-linked, insoluble, and adhere well to most substrates [2].

In the last decade, new applications of plasmas at atmospheric pressure have been developed, such as: plasma medicine, plasma agriculture, plasma used in food industry, in 3D printing technology, textile industry, and so on. This Special Issue attempts cover all these applications, publishing both target applications of the atmospheric pressure plasma and also the fundamental aspects that appear in the case of these types of discharge.

#### **2. Review of Special Issue Contents**

Contributions to this Special Issue focus on different aspects of Atmospheric-Pressure Plasma Technology, giving valuable examples of applied research in the field. This Special Issue of *Applied Sciences*, "Recent Advances in Atmospheric-Pressure Plasma Technology", includes one review [3] and eight original papers [4–11], providing new insights into the application of atmospheric pressure plasma technology.

Domonkos et al [3] discussed the application of cold atmospheric pressure plasma (CAPP) technology in different configurations. Their manuscript outlines the application of CAPP in medicine for the inactivation of various pathogens (e.g., bacteria, fungi, viruses, sterilization of medical equipment, and implants) in the food industry (e.g., food and packing material decontamination), agriculture (e.g., disinfection of seeds, fertilizer, water, soil). In medicine, plasma also holds great promise with regard to direct therapeutic treatments in dentistry (tooth bleaching), dermatology (atopic eczema, wound healing), and oncology (melanoma, glioblastoma).

Asghar et al. [4] discussed the changes induced in the atmospheric pressure plasma jets (APPJs) characteristics of the gas mixture; more precisely, argon/oxygen (Ar/O2) mixture.

**Citation:** Rusu, B.-G. Recent Advances in Atmospheric-Pressure Plasma Technology. *Appl. Sci.* **2022**, *12*, 10847. https://doi.org/ 10.3390/app122110847

Received: 18 October 2022 Accepted: 24 October 2022 Published: 26 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The voltage–current waveform signals of APPJ discharge, gas flow rate, photo-imaging of the plasma jet length and width, discharge plasma power, axial temperature distribution, optical emission spectra, and irradiance were investigated.

Chodkowski et al. [5] focused on fabrication and physicochemical properties investigations of silica-multiwalled carbon nanotubes/poly(dimethylsiloxane) composite coatings deposited on the glass supports activated by cold plasma in air or argon. They found that the type of plasma influences the surface properties of the deposited coating.

AlShunaifi et al. [6] realized a two-dimensional numerical model to simulate operation conditions in the non-transferred plasma torch, used to synthesis nanosilica powder. The numerical results showed good correlation and good trends with the experimental measurement. This study allowed us to obtain more efficient control of the process conditions and better optimization of this process in terms of the production rate and primary particle size.

Suenaga et al. [7] realized an evaluation of the effect of plasma gas species and temperature concerning the reactive species produced and the bactericidal effect of plasma. Nitrogen, carbon dioxide, oxygen, and argon were used as the gas species, and the gas temperature of each plasma varied from 30 to 90 ◦C. The results demonstrate that the plasma gas type and temperature have a significant influence on the reactive species produced, and the bactericidal effect of plasma and the disinfection process could be improved by properly selecting the plasma gas species and temperature.

Suenaga et al. [8] design and build a multi-gas temperature-controllable plasma jet that can adjust the gas temperature of plasmas with various gas species and evaluated its temperature control performance. By varying the plasma jet body temperature from −30 ◦C to 90 ◦C, the gas temperature was successfully controlled linearly in the range of 29–85 ◦C for all plasma gas species.

Papadimas et al. [9] realized a single dielectric barrier discharge (SDBD)-based actuator. The consumed electric power was measured, and the optical emission spectrum was recorded in the ultraviolet–near infrared (UV–NIR) range. The average temperature of the neutral species over the actuator was found to be around 410 K at the maximum power level.

Khan et al. [10] reported a dielectric barrier discharge (DBD) plasma rotational reactor for the direct treatment of wheat flour. The primary research goal was to determine the effects of short-period cold plasma treatment of DBD type on flour and dough properties. The obtained results showed a 6–7% increase in flour hydration due to cold plasma treatment, which also contributes to hydrogen bonding due to changes in the bonded and free water phase.

Rueda et al. [11] report a study which is focused on gas treatments (NOx abatement) by dielectric barrier discharge (DBD) at atmospheric pressure. Two diagnostic methods are considered to evaluate the discharging ratio on the reactor surface: an image processing method and a DBD equivalent circuit analysis, both presented in this paper. The experimental results show good agreement between the two methods. These two strategies work very well and provide remarkably coherent results under different intensity conditions.

This Special Issue has attracted 38 citations and received more than 8700 views, demonstrating the growing interest in this topic.

The Editor would like to express his appreciation to the contributors for their dedication and excitement during the process of compiling their individual essays for this Special Issue. The Multidisciplinary Digital Publishing Institute (MDPI) editorial team members also deserve praise for their professionalism and commitment to publishing this Special Issue. We hope that the readers will enjoy this collection of papers and will be motivated to envision fresh concepts for future developments in plasma research technology.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**

## *Article* **Combined Image Processing and Equivalent Circuit Approach for the Diagnostic of Atmospheric Pressure DBD**

**Vanesa Rueda 1,2, Rafael Diez 2,\*, Nicolas Bente 1 and Hubert Piquet 1,\***

**Abstract:** The framework of this paper is the study of gas treatments (NOx abatement) by dielectric barrier discharge (DBD) at atmospheric pressure. To investigate the impact of various solutions for electrical energy injection on the treatment process, two diagnostic methods are considered to evaluate the discharging ratio on the reactor surface: an image processing method and a DBD equivalent circuit analysis, both presented in this paper. For the image analysis, the discharge area is first translated into gray levels, then segmented using the Otsu's method in order to perform the discharging ratio diagnostic. The equivalent circuit approach, derived from the classical Manley's diagram analysis, includes the behavior of the part of the reactor in which no discharge is happening. The identification of its parameters is used to estimate the discharging ratio, which evaluates the percentage of the reactor surface covered by the discharge. Experimental results with specifically developed power supplies are presented: they show a good agreement between the two methods. To allow a quantitative comparison of the discharge uniformity according to the operating conditions, the statistical analysis of gray level distribution is performed: non-uniform discharges with intense energy channels are shown to be clearly distinguished from more diffuse ones.

**Keywords:** dielectric barrier discharge; partial surface discharging; discharging ratio; image processing; equivalent circuit; Manley diagram; NOx abatement; DBD diagnostic; atmospheric pressure; discharge uniformity

#### **1. Introduction**

Non-Thermal Plasma (NTP) are broadly used for applications such as ozone generation, treatment of surfaces, medical treatments, disinfection, and UV production [1–6]. In recent years, NTP has also become a rising technology for environmental protection. Many studies have reported effective removal of pollutants such as nitrogen oxides (NOx), sulfur dioxide (SO2), carbon dioxide (CO2), particulate matter, and volatile organic compounds (VOC) [1,7], with treatments usually processed at atmospheric pressure.

Dielectric Barrier Discharges (DBDs) are NTP produced in reactors, including at least one insulating layer between two metallic electrodes. In comparison with other NTP reactors, such as corona discharges and electron beams, DBDs are characterized by the absence of sparks, thanks to the dielectric barriers which limit the local current rise and by the possibility of obtaining uniform discharges, where the NTP covers the entire surface of the reactors, guaranteeing a quality treatment.

DBDs operate in filamentary or homogeneous mode [8,9]. In the homogeneous mode, the plasma is uniform and diffuse [1,8] and it presents a glowing aspect. For some applications, it has been proven to improve the process: for instance, thin films deposit requires homogeneous discharges in order to obtain a uniform treatment and prevent local damage on the treated surface [2]. The filamentary mode is characterized by streamers, intense energy channels distributed over the electrodes surface, randomly, or sometimes following

**Citation:** Rueda, V.; Diez, R.; Bente, N.; Piquet, H. Combined Image Processing and Equivalent Circuit Approach for the Diagnostic of Atmospheric Pressure DBD. *Appl. Sci.* **2022**, *12*, 8009. https://doi.org/ 10.3390/app12168009

Academic Editor: Bogdan-George Rusu

Received: 8 July 2022 Accepted: 8 August 2022 Published: 10 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

geometrical patterns [10]. Ozone generation, UV emission for instance, benefits from this mode. At atmospheric pressure, DBD generally operates in filamentary mode, and offer capabilities to initiate very valuable chemical transformations. According to their highly desirable characteristics, DBDs and packed-bed DBDs are widely used for pollutant removal applications. This is the case for the prospective application of our studies (NOx abatement) [7].

Experimental conditions such as the reactor geometry, materials, gas mixture, pressure, and electrical supply waveforms not only influence the discharge mode, but also the characteristics of the power injection into the plasma and the full or partial coverage of the reactor's surface by the NTP. Obtaining operating conditions where the surface of the reactor is fully covered by the plasma is indeed highly desirable for gas treatments: it means that each volume of the gas injected into the reactor will receive the effect of NTP treatment, contrary to the situation where only part of the reactor's surface is covered by the NTP, and thus part of the flowing gas may not receive the expected treatment. Such opposite situations are shown in Figure 1 (side view of a cylindrical reactor, detailed in Section 2, showing the streamers distribution seen through the transparent walls made with quartz).

**Figure 1.** Typical discharge appearance with partial discharging and different β percentages (white frame showing the limits of the reactor).

The partial discharging is quantified by the β (no units) percentage, which takes the 1 value when the surface is fully covered and 0 value when the discharge is OFF [8,11,12]. Among the diagnostic criteria used for the diagnostic of NTP delivered by DBD, this β discharging ratio is very important to improve the understanding of the efficiency of the gas treatments (NOx abatement in our case).

Different diagnostic methods of the discharge mode, based on its appearance, have been proposed. Ref. [9] introduced a simple method that detects the homogeneous glow discharge using the Manley figure and the voltage and current waveforms. This method is based on a definition of the homogeneous APGD (Atmospheric Pressure Glow Discharge), where only one current pulse per voltage half-cycle is generated. Other diagnostic methods include Intensified Charge-Coupled Device (ICCD), high-speed imaging, optical emission spectrum, numerical simulations, and image processing [13].

Given that the image processing method is effective, simple, and does not require expensive equipment, it is a good solution to analyze the discharges under different experimental conditions. However, if the aim is to detect the conventional glow discharge (APGD), it can only be used with short exposure time (~10 ns, which requires an expensive camera). When higher exposure times are used, the filaments cannot be always distinguished. Nevertheless, the image processing can be still used to analyze the uniformity of the discharge: in this paper an image processing algorithm is set up to study the uniformity of the discharges rather than the discharge mode (homogeneous or filamentary). This analysis of the discharge uniformity is based on the image gray level histogram [13] and produces, as a result, an estimation of the β partial discharging ratio. Additional statistical analysis of the images also allows a quantified comparison of the uniformity of the discharges.

Other diagnostics of DBD systems can be also achieved on the basis of measured electrical waveforms. Using these measurements, it is possible to identify the parameters of the well-known Manley diagram [14] and its associated DBD's equivalent circuit [15]. Originally based on the assumption that the discharge is uniform (with parameters of the equivalent circuit corresponding to a full use of the surface of the electrodes, i.e., β = 1), this approach can be simply extended [8,16] to consider partial discharging situation.

In order to implement complementary approaches for diagnostic of the discharges obtained in a DBD reactor for NOx abatement studies, the two aforementioned approaches (image processing and equivalent circuit) are considered in this paper. The article is organized as follows: Section 2 presents first the reactor, the power supply and the experimental test-bench, for which measurements are used in this paper. Then, the image processing algorithm for diagnostic with estimation of the β percentage of the surface covered by the discharge is detailed. Finally, the equivalent circuit, considering partial discharging phenomenon and the identification of its parameters (chiefly the β ratio) using the electrical measured waveforms, is presented.

In Section 3, the application of the two methods at different operating conditions obtained with the test-bench is proposed and the correspondence between the obtained β percentage values is discussed.

#### **2. Materials and Methods**

#### *2.1. Experimental Setup*

The schematic diagram of the experimental setup is presented in Figure 2. The system consists of a DBD reactor, a gas blending system, a power supply, measurement instruments, and the control interface:

**Figure 2.** Experimental setup (**a**) and reactor dimensions (**b**).

Two different electrical supplies are used for the experimentations: a sinusoidal voltage source and a square current source. The sinusoidal voltage waveforms (Figure 3b) are generated by a function generator connected to an audio amplifier. Since the output voltage of the amplifier is too low to ignite a discharge, the amplifier output is connected to a high-voltage transformer. Frequency and peak voltage are the degrees of freedom made available by this supply: a maximum frequency of 20 kHz and a maximum voltage of 12 kV can be generated.

**Figure 3.** Typical electrical waveforms (*vDBD*, blue and *iDBD*, green): (**a**) Square Current Source: here, *fsw* = 80 kHz, *J* = 1 A, *d* = 10%. (**b**) Sinusoidal Voltage Source: here, *vDBD\_peak* = 11.2 kV, *f* = 10 kHz.

A specific power supply has been also developed to create the plasma and to control the energy injected into the NTP. This one delivers square current waveforms (Figure 3a), which is generated by the power converter presented in [18,19]. This converter consists of a DC current source connected in cascade with a full-bridge current inverter and a step-up transformer. The converter delivers current pulses, whose shape is controlled by three degrees of freedom: frequency *fsw*, current amplitude *J*, and pulses duration Δ*tp*. (Δ*tp* = *d*/(*fsw\*2*), *d* being the duty cycle, in the range 5–95%). The maximum output current *J* is 1.2 A, and the maximum voltage is 11 kV. The frequency *fsw* can be varied from 80 kHz to 200 kHz. In order to control the rise of the reactor temperature, this supply is operated in "Burst mode" [1]: pulses trains are separated by power injection pauses; the durations of both bursts and pauses are fully controlled.

 This camera is installed 40 cm from the axis of reactor and provides images such as the ones displayed in Figure 1. The area of interest is 28 mm high (outer diameter of the reactor) and 60 mm long (length of the metallic mesh). The camera acquires images with 2304 × 1536 pixels; once cropped for the area of interest, these values reduce to 437 × 248 (the difference in proportions results from a very slight misalignment of the camera with respect to the reactor axis). Despite such a device having an exposure time much longer than the lifetime of the streamers, as will be shown below, very interesting information concerning the distribution of the plasma can be drawn from the analysis of these images.

#### *2.2. Image Processing Algorithm*

The aim of the image processing is to calculate the β discharging ratio, which defines the surface covered by the plasma. The algorithm is achieved in two main steps:

The initial image captured by the camera (Figure 4) first receives basic manipulations, which makes the file lighter and speeds up further treatments.

First of all, the image is cropped (Figure 5a), so as to keep only the reactor (useful region) with the same dimensions as its side view: L width and 2.R height. Then, in order to reduce the size of the file containing the picture and speed up its manipulation, the true-color RGB image is converted into a grayscale image. Each element of the resulting matrix contains only the gray level, ranging from 0 to 255, calculated as a weighted sum of the corresponding red, green and blue pixels. The gray level (*GL*) is calculated with the Python coded function (1), which implements recommendation Rec.709 [20]; these weights are considered to better represent human perception of red, green, and blue than equal weights.

$$GL = 0.2125 \times R + 0.7154 \times G + 0.0721 \times B \tag{1}$$

The obtained image is presented on Figure 5b; in all the pictures presented in this paper, for better readability, the grayscale images are shown with a color scale from violet (background), the lowest intensity, to yellow, the highest.

**Figure 4.** Initial image, acquired by the camera.

**Figure 5.** Initial cropped image (**a**), and grayscale transformed (**b**).

#### 2.2.1. Geometrical Transformation

The geometrical transformation is introduced in order to compensate the deformation of the discharge representation, which is caused by the cylindrical shape of the reactor, captured on the flat sensor of the camera. For that purpose, the Abel transformation [21–23] initially considered has been finally withdrawn, because the plasma distribution around the gas gap does not present the cylindrical invariance which is mandatory. A new method for calculation is proposed below.

On the taken image, the reactor cylindrical shape is flattened by the camera, as shown on Figure 6. The reactor has length *L* and is placed along the Z axis; the camera's optical axis is on the X axis. The plane (X, Y) is an orthogonal cross-section of the reactor (Figure 6b). The points *m*(*yp*) cover the external surface of the reactor, at outer radius r, for θ ranging from −90◦ to 90◦. The point M(y), with y ranging from −R to +R, is the orthogonal projection, captured by the camera, of the points *m*(*yp*). Only the outer electrode is shown. Several geometrical assumptions are made:

**Figure 6.** Principle of the geometrical transformation (projection): (**a**) 3D view of the reactor and the camera. (**b**) Orthogonal cross-section of the reactor. (**c**) Acquired picture. (**d**) Transformed picture.

Starting from the captured image, the goal of the geometrical transformation is to build the projected output image (Figure 6d): it is the rollout of the DBD reactor external surface. The height ratio between the captured image (Figure 6c) and the projected one (Figure 6d) is therefore equal to π/2. The width is the same for both images and equal to the length L of the reactor.

In order to build Figure 6d, each M(y) point of the acquired image is used to define the *m* - *yp* point at vertical position *yp*; if a streamer appears on the original picture as a spot with Δ*z*Δ*y* size, on the transformed image it is spanned into Δ*z*Δ*yp* with:

$$y\_p = R \times \theta \, = \, R \times \sin^{-1}\left(\frac{y}{R}\right) \tag{2}$$

$$
\Delta y\_p = \frac{\Delta y \times R}{\sqrt{R^2 - y^2}} = \frac{\Delta y}{\cos(\theta)}\tag{3}
$$

Equation (2) is obtained by a basic trigonometric manipulation: the length *yp*, which is also a circle segment, is equal to the radius *R* multiplied by the *θ* angle. The ratio Δ*yp*/Δ*y* from Equation (3) is a first-order approximation obtained with first y-derivative of Equation (2). Using these equations, the image is projected to cylindrical coordinates and rolled out. This process is repeated along the Z axis in all planes (X, Y) of the reactor to obtain the full picture. Figure 7b shows the obtained image in cylindrical coordinates.

**Figure 7.** Projected unrolled image of the outer surface of the reactor: (**a**) Acquired grayscale image— Cylindrical coordinates output image (**b**). Red rectangle highlights the inner tube of the reactor.

#### 2.2.2. Image Segmentation

*Step 1—area of interest:* as can be seen in the unrolled picture Figure 7b, the lower and upper parts of the discharge look brighter than the central part of it. The coaxial geometry of the reactor and the transparent quartz walls cause this effect. In the central part of the discharge, the inner electrode creates a solid background for the pictures, whereas in the upper and lower parts, the gas gap between the two electrodes and quartz barriers produces an optical effect that intensifies the discharge. To avoid biased results due to this optical effect and according to the assumption (c) that the plasma volume has no depth, the image is cropped again, erasing the parts which do not present a solid background. They correspond to the gas gap and only the central part of the picture, which has the height of the diameter of the inner tube (red frame figure below), is kept (Figure 8b)—one should notice that this figure is different from the original one, Figure 7a.

**Figure 8.** Cylindrical coordinates output image (**a**), and its cropped central region with solid background (**b**).

*Step 2—image segmentation*: the discharge area is segmented using a thresholding technique, as shown in Figure 9. It means that a gray-level value called "threshold value" is used as the delimiter between the background and the discharge area. All pixels with values less than the threshold value are considered as background, and pixels with values greater than the threshold are considered as the discharge area. In order to find the threshold value, Otsu's method is used [24]: this method chooses a threshold value that minimizes the variance of the thresholded background and discharge pixels. The ability of this method to segment the image even with low intensity discharges (operating points at low power) of uniform appearance is remarkable (as will be shown in Section 3).

**Figure 9.** Segmented Discharge area (violet is background, yellow is discharge area).

*Step 3—image analysis*: the calculation of the β percentage is achieved with the ratio between the number of pixels of the discharge, obtained with the segmentation, and the total number of pixels. Other statistical results quantifying the discharge's uniformity are also presented in Section 3.3.

#### *2.3. Partial Surface Discharging Analysis via the Electric Model*

The DBD is commonly modelled in the literature by the electric circuit shown in Figure 10. *Cd* represents the capacitive behavior of the dielectric barrier(s), in this case the series equivalent capacitance of the two quartz cylinders. The gas is modeled as a dielectric (*Cg*) as long as it is not conductive (|*vgas*|< *V*th, thus *igas* = 0), and after breakdown it is represented with a constant voltage (*vgas* = ±*V*th), the sign of which depends on the direction of the gas current [15,18,19].

**Figure 10.** Simplified electric circuit model of the DBD, with current paths in ON–green and OFF– black plasma states (**a**) and its Manley's diagram (**b**).

The parameters of this model (*Cd*, *Cg* and *V*th) are found with experimental waveforms for voltage and current on the DBD, then tracing the electrical charge vs. voltage diagram (known as Manley's diagram) as shown in Figure 10.

The slopes of the parallelogram are used to define the dielectric and series equivalent capacitances (respectively, *Cd* for the ON state and *Ceq* for the OFF state—*Ceq* being the value of the series connection of *Cd* and *Cg*). From the latter, the gas capacitance (*Cg*) is calculated. The zero-cross with the voltage axis determines the threshold voltage (*V*th). The area enclosed by the parallelogram is equal to the energy injected during one period to the DBD, so it can be used to determine the average power.

It is well known that *V*th, *Cg*, and *Cd* are physical parameters determined by the gas composition, pressure, dielectric material, and geometry; these three values define the behavior of the equivalent circuit, which remains predictive as long as the surface covered by the plasma on the electrodes, remains stable. Indeed, variations in the identified parameters as a function of the electric conditions given by the power supply are common, even if the reactor geometry does not change [11,12]. As shown in Figure 11, this can be observed in the case of the DBD reactor used in this work: here, the electrical energy injected into the plasma, controlled by the power supply, is changed. Accordingly, the discharge covers the whole surface of the reactor ((a), β = 1) or only part of it ((b), β < 1). Note that, concerning the identified parameters of the equivalent circuit (Figure 10), *Ceq* is similar for both cases, however, identified *Cd* and *V*th change significantly: being physical parameters of the reactor, such changes are not acceptable.

As observed in the pictures beside the Manley's diagrams, this phenomenon in the model identification takes place when the discharge occurs in a partial area of the available electrode surface (Figure 11b). Depending on the operating conditions of the power supply, the discharge area extends until it fully covers the electrodes, and the values measured using Manley's diagram coincide with the theoretical ones. In that scenario, the classic DBD electric model, shown in Figure 10, completely represents the load. Nonetheless, for the partial surface discharging case, a modified model is proposed in the literature [8,11,25]. In consequence, a different equivalent electric model has to be used, where *Cd*, and *Cg* are both split into the non-discharging and discharging area (α and β percentages respectively, which fulfil: α + β = 1), as displayed in Figure 12.

**Figure 11.** Change of the DBD identified model parameters (*Ceq*, *Cd* and *V*th) for the same reactor, with different supply conditions, leading to different discharging ratio: β =1(**a**) and β <1(**b**).

**Figure 12.** DBD electrical model for partial discharging area (**a**), proposed in [11], along with its modified Manley's diagram (**b**).

In this case, the α and β parameters are calculated having previously identified *Cd*, *Ceq*, *Cg*, and *V*th, using the Manley's diagram for a discharge covering all the surface. For the partial discharging area condition, *Ceq* does not change, as the series capacitor of the two equivalent circuits is the same when there is no discharge (Figures 10 and 12): this is proven experimentally in the example of Figure 11, with an error below 2.4% between the two measurements.

The percentage of the discharge area (*β*) can be found with the slope when the discharge has been established (*C* ), as follows:

$$\beta \, \, = \, \frac{\mathbf{C}' - \mathbf{C}\_{eq}}{\mathbf{C}\_d - \mathbf{C}\_{eq}} \tag{4}$$

The breakdown voltage, *V*th, can be verified according to Equation (5), using the charge variation in the OFF state, Δ*QdbdOFF* (black segments on Figure 12)

$$V\text{th } = \frac{\Delta Qdbd\_{OFF}}{2 \times \mathcal{C}\_{\mathcal{S}}} , \text{OFF state} \tag{5}$$

#### **3. Results**

The application of the two diagnostic methods for β discharging ratio, described in the previous section, is now presented and discussed.

#### *3.1. Agreement of the Circuit Analysis Method and Image Processing for β Ratio Diagnostic*

Two operating points obtained at 5 kHz with the sine voltage generator, corresponding to two different power levels (10 W and 25 W), are selected. Figure 13 presents the obtained results in each column (top-down): the electrical waveforms (voltage—blue, and current green), the Manley diagram build with these data, the acquired reactor's pictures, the segmented and cropped (area of interest) grayscale image together with the β discharging ratio obtained from the equivalent circuit approach and with the image processing. The last data is associated with the margin error (the difference between the maximum and minimum values) estimated through repeated calculations using three different images acquired in steady state conditions (see below) during three different experiments.

**Figure 13.** Application of both diagnostic methods: (**a**) low power level (10 W). (**b**) high power (25 W). Measures are acquired in steady state.

It should be mentioned that the coverage of the discharge area varies from the time the system is started, with a typical transient shown in Figure 14. Here, the record starts when the power supply is turned ON, with a gas flow already stabilized (see Figure 2). For the presented operating conditions (f = 10 kHz, P = 30 W), one can observe during the

transient the rise of the discharging surface (and accordingly the β ratio), which definitively stabilizes only after approximatively 120 s. For steady-state process analyses, data should only be acquired after this system time constant.

**Figure 14.** Partial discharging ratio β variation during the startup transient.

The two methods described in the previous section are now applied on a set of measures acquired with operating points covering a large domain (sine voltage power supply). This diversity of considered conditions is aimed at comparing the results of the two methods, whose results are presented on Figure 15: for each operating condition, three measurements are acquired in steady state during different experiments. The displayed value is the average of these three results, and the error bar indicates the maximum and minimum values. One can notice the very good agreement with a difference which remains generally below 10%. In most cases, the β discharging ratio obtained with the electrical circuit approach remains in the error range of the image processing method. The latter is obtained, using several images repeatedly acquired for each operating point; this systematic uncertainty remains below 12%.

**Figure 15.** Partial discharging ratio β obtained with image processing and equivalent circuit analyzes. Results are grouped by values of supply frequency; adjustments of injected power are achieved with the peak value of imposed *vDBD* voltage. Displayed values are obtained in steady state.

#### *3.2. Application to Rectangular Current Power Supply Operated in Burst Mode*

As shown in Figure 16, results obtained with the rectangular current power supply are difficult to analyze through the equivalent circuit approach: indeed, each burst of current pulses injected into the DBD creates an electrical transient (see voltage and current waveforms on Figure 16a), which has no time to vanish before the end of the burst. As a result, the Q-V Manley diagram exhibits a poorly trapezoidal shape, as can be appreciated in Figure 16b: this makes it very difficult to select the slopes to be used for the calculation of the circuit's parameters.

**Figure 16.** Rectangular current supplied system: electrical measurements of a whole burst (**a**), with DBD voltage (blue) and current (green), and associated Q-V Manley diagram (**b**).

Taking advantage of the very good convergence of both methods for β ratio diagnostic highlighted on Figure 15, it is proposed to admit:

Using Equations (4) and (5), which translate the behavior of the equivalent circuit of Figure 12, it becomes possible to build and to draw the parallelepipedal figure, which should be considered.

Note that the image acquisition shows an averaged behavior of the system. As for the β value provided by the analysis of this image, therefore, this approach provides an equivalent average behavior over the burst. The image processing of the acquired picture shown in Figure 17a gives: β = 56%. Together with already known circuit parameters (*Cd* = 90 pF, *Cg* = 135 pF, *V*th = 2585 V, independent from operating conditions), Figure 17b shows the Manley figure (red).

**Figure 17.** Rectangular current supplied system: processed image corresponding to Figure 16 (**a**), giving a value: β = 56% experimental Manley diagram (**b**), blue and the theoretical one (red), obtained with the known parameter set for the equivalent circuit: *Cd* = 90 pF, *Cg* = 135 pF, *V*th = 2585 V.

#### *3.3. Effect of the Supply Waveform on the Discharge Uniformity: Additional Diagnostic Provided by the Image Processing*

In addition to the segmentation process, other statistical diagnostics of the images can provide very useful information about the discharge. We now consider the operating conditions obtained with the electrical waveforms shown in Figure 3, with two different generators; the waveforms are completely different but, in fact, they have the same power level (approx. 40 W).

The appearance of the discharges [26] shown in Figure 18 are very different: the discharge obtained with the sinusoidal voltage source (column (b)) appears much more diffuse than the one pumped with the rectangular current source (column (a)): in the original picture (column (a)), the spots which can be related to the streamers are clearly visible.

**Figure 18.** Image diagnostic of operating conditions obtained with square current source (column (**a**)) and sine voltage power supply (column (**b**))—electrical waveforms presented in Figure 3—from top to bottom: original image, grayscale, result of geometrical transformation, segmented image (central area), gray levels frequency.

The gray-levels histograms of the last row in Figure 18 present the result of a statistical analysis, which presents good matching with the discharge appearance and offers a quantification solution to translate the visual effect into a number. This analysis was proposed by [13] and is achieved as previously on the only central part of the image (red frame); it is carried out after the translation of the image into gray levels and its geometrical transformation (before the segmentation): the horizontal axis of the histogram presents the gray level (from 0 to 255), and the vertical axis presents the frequency of pixels for each level. Based on the histogram results, statistical measures as the mean gray level and the standard deviation are used to characterize the discharge. Below, the parameters of this analysis are commented:

Table 1 presents the resulting parameters of the image processing for both cases shown in Figure 18.

**Table 1.** Grayscale images statistical properties (electrical waveforms presented on Figure 3 and images in Figure 18).

As can be seen, the SD of the case obtained with the rectangular current source is significantly higher than cases with the sine voltage. Despite the fact that cases worked with similar electrical power, the intensity of the first case is higher than for the second. Accordingly, the discharges obtained by the sinusoidal voltage source are more uniform, diffuse, and weak than the discharges obtained by the square current source. This observation has been verified with a set of different operating points in a wide range of the β parameter.

The Manley figures of both supply conditions showed that DBD works in a filamentary mode, even if diffuse and uniform discharges appeared with the sinusoidal voltage source.

One can notice that the image (a) is much more luminous than the (b) one—this is confirmed by the difference of the values of the Gray level Intensity/mm2. Nevertheless, the Otsu's algorithm has been clearly able to differentiate the background and the area covered by the discharge. In this way, the discharge segmentation process is validated under different image conditions.

#### **4. Conclusions**

Two methods to evaluate the β discharging ratio of a dielectric barrier discharge are proposed in this paper. The equivalent circuit-based method uses recorded waveforms of voltage and current of the DBD reactor to build the Manley's diagram and to identify the equivalent circuit parameters and the β ratio. The image processing method uses pictures of the discharge captured through the transparent walls of the reactor. A two-step algorithm is presented and validated. First, a geometrical transformation compensates the distortion due to the shape of the reactor. Second, the segmentation of the images based on the Otsu's method is used to identify the discharge area and then to analyze it, based on the gray-level histograms.

These two methods were proved to work very well and to provide remarkably coherent results, under different intensity conditions of the discharge, when pumped with different electrical waveforms obtained with two very different power supplies. Additionally, a comparison between the discharges produced by a square current source and a sinusoidal voltage source with similar power levels has shown that more uniform discharges are obtained with the sinusoidal voltage source, and more intense discharges are obtained with the square current source. These observations have been correlated with statistical properties of the gray level acquired images, which provide a quantified appreciation of the discharge uniformity, very useful for achieving the comparison of the performances.

**Author Contributions:** Conceptualization, R.D. and H.P.; Investigation, V.R., R.D., N.B. and H.P.; Software, V.R. and N.B.; Supervision, R.D. and H.P.; Validation, V.R.; Visualization, V.R. and N.B.; Writing—original draft, V.R., R.D. and H.P.; Writing—review & editing, R.D., N.B. and H.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank the French-Colombian cooperation program ECOS Nord— Colciencias-ICETEX, as well as Javeriana University project 10035.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Enhancement of Wheat Flour and Dough Properties by Non-Thermal Plasma Treatment of Wheat Flour**

**Muhammad Jehanzaib Khan 1,\*, Vojislav Jovicic 1,\*, Ana Zbogar-Rasic 1 and Antonio Delgado 1,2**

**Abstract:** Demand to improve food quality attributes without the use of chemicals has risen exponentially in the past few years. Non-thermal plasma (NTP) (also called 'cold plasma') is becoming increasingly popular for this purpose due to its unique low-temperature and non-chemical nature. In the present research, the concept of in situ dielectric barrier discharge (DBD) plasma treatment inside a rotational reactor for the direct treatment of wheat flour was experimentally analyzed. The primary research goal was to determine the effects of short-period NTP treatment of DBD type on flour and dough properties. For this purpose, the influence of different operating parameters was tested, i.e., treatment time, the amount of flour placed in the reactor and the environmental (air) temperature. Changes in the structural attributes of the most commonly used flours (type 550 and 1050) and their respective doughs were studied using a set of analytical techniques. Rheological analysis demonstrated the ability of NTP to significantly intensify the visco-elastic properties of dough produced from wheat flour type 550 that was treated for less than 180 s. This indicated that plasma treatment enhanced intermolecular disulphide bonds in gluten proteins, which resulted in stronger protein–starch network formations. However, longer treatment times did not result in a significant increase in the visco-elastic properties of wheat dough. The obtained results showed a 6–7% increase in flour hydration due to NTP treatment, which also makes a contribution to hydrogen bonding due to changes in the bonded and free water phase. Experimental findings further confirmed the dependence of NTP treatment efficiency on environmental air temperature.

**Keywords:** wheat flour; dough; rheology; non-thermal plasma; dielectric barrier discharge; wheat functionality; hydration properties; microstructure

#### **1. Introduction**

Cereals and grains are abundantly used worldwide, constituting a major part of human food consumption. Cereals and their products are energy sources for humans and animals, and contain carbohydrates, proteins, fiber and vitamins (such as E and B), along with magnesium and zinc [1–3]. Grains are consumed either whole or in powder form, which forms exist in different granulations and have different chemical compositions [4]. Generally, a grain consists of three parts: (1) bran, i.e., the outer layer, mostly rich with dietary fiber; (2) germ, i.e., the inner layer, consisting of lipids and proteins; and (3) endosperm, i.e., the bulk of a kernel, full of starch, proteins and minerals [5]. A powder form, i.e., flour, is a blend of these parts in different proportions, leading to different flour classifications.

Plant-based foods are often subjected to thermal or chemical processing prior to consumption [6]. However, nowadays, the customer requirement to preserve nutrient ratios by reducing the treatment temperatures for preserving food quality and reducing the use of chemical treatment agents have urged food engineers to search for other, suitable, non-chemical food processing methods. Modern consumers demand rich organoleptic,

**Citation:** Khan, M.J.; Jovicic, V.; Zbogar-Rasic, A.; Delgado, A. Enhancement of Wheat Flour and Dough Properties by Non-Thermal Plasma Treatment of Wheat Flour. *Appl. Sci.* **2022**, *12*, 7997. https://doi.org/10.3390/ app12167997

Academic Editor: Bogdan-George Rusu

Received: 28 June 2022 Accepted: 8 August 2022 Published: 10 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

additive-free food, environmentally friendly processing, freshness and high sensory and nutritional attributes [7,8]. On the other hand, insufficient processing of food products can lead to the proliferation of bacterial, parasitic and pathogenic infections in humans [9]. Food-borne illnesses have significantly increased in recent years due to consumption of food that has been inadequately processed and therefore correct storage is additionally important to prevent mould formation, pest infestation and grain germination [6]. Microorganisms and pathogens occur in grains stored in humid environments, in which bran absorbs a significant amount of water, thus lowering the concentration of gluten and starch in the kernel and ultimately affecting end product quality [5,10].

Conventional thermal processing measures for food, for boosting shelf life, hygiene and structural properties, can be very detrimental to food nutrient ratios and sensory and organoleptic qualities [11]. Currently employed sterilization and processing techniques, such as chemical preservation [12,13], addition of bio-preservatives [14], mild heat treatments [15], microwave processing [16], ultrasound processing [17], reduction of water activity [18], high hydrostatic pressure technology [19,20] and vacuum and hurdle techniques [21], provide limited benefits in comparison with their adverse alterations of food properties, such as shape, color, taste, smell, structure, nutrient degradation, formation of toxic byproducts, etc. [9,22].

Wheat is one of the most important and the world's leading staple crop, which is full of nutrients [3]. The main factors affecting the quality of wheat-based products are gluten proteins, which are responsible for the visco-elastic properties of end products [23]. Their properties affect the rheological behavior and the gas-holding capability of a dough, as well as the texture and final volume of baked products. Wheat kernels are composed of 70–75% starch, 10–14% proteins, with the remainder constituted by minerals [24]. Wheat proteins are divided into two groups, i.e., non-gluten and gluten proteins. Gluten proteins comprise around 80–85% of the total protein contents of wheat flour and are further divided into gliadins and glutenins. Gliadins are monomeric proteins, which are mainly responsible for viscous and extensibility behavior during dough network formation. Glutenins are polymeric or aggregative proteins which influence the elastic and cohesiveness properties of dough networks.

The number and distribution of sulphur–sulphur (S-S) bonds, which are formed due to the presence of cysteine residues in gliadins and glutenins, significantly influence dough properties. However, S-S bonds are formed differently in two protein types: in gliadins, they lead to the formation of intramolecular S-S bonds, while in glutenins intermolecular S-S bonds are formed [25]. The S-S bonds for dough network formation start to activate during the kneading process, as soon as the wheat flour is hydrated due to the addition of water. During the network formation, these S-S bonds are in the form of fibrils. Macrofibrils of protein aggregates are formed by the formation of intermolecular S-S bonds. These fibrils then transform to a continuous film, which finally forms a homogeneous and fine gluten matrix [26,27]. On the other hand, several low-molecular weight thiols, containing a free SH group, are present in wheat flour, and have strong dough-weakening effects. The tripeptide glutathione (GSH) has the biggest impact; it is present in reduced (GSH) and oxidized, glutathione disulfide (GSSG) forms. A free SH group reacts with glutenin and forms glutathionylated protein (PSSG), which negatively affects the formation of intermolecular S-S bonds.

Wheat flour is normally processed not only to improve hygiene, shelf life and texturerelated properties, but also to minimize the negative effects of GSH and GSSH [28]. A common industrial practice is to use chemical additives, e.g., ascorbic acid (i.e., vitamin C) as a reducing agent, which is extensively used to improve wheat flour textural properties (e.g., elastic and viscous properties). Its oxidized form dehydroascorbic acid (DHA) is responsible for the removal of free SH groups in flour, hence preventing the glutathione to create PSSG. The reaction mechanism is shown in Equations (1) and (2). Ascorbic acid easily converts to DHA in the presence of an enzyme and is limited by the availability of atmospheric oxygen during kneading. DHA reacts with GSH to form ascorbic acid and

GSSG, thus limiting the formation of PSSG. As a result, the number of S-S intermolecular bonds increases.

> Ascorbic acid + 0.5 O2 -Dehydroascorbic Acid + H2O (1)

$$\text{Dehydration} \text{acccbic Acid} + \text{GSH} \rightleftharpoons \text{Ascorchloric acid} + \text{GSSG} \tag{2}$$

Although many other chemicals (KIO3, KbrO3, chlorine, etc.) and oxidizing bacteria have been used to modify wheat flour functionality, the food industry is moving towards restricting the use of chemicals, and therefore the need for non-chemical treatment methods increases. Ozone is an example of an oxidizing agent, acceptable for use in the food industry [29]. Mei et al. [30] and Desouky et al. [31] investigated the effects of ozone on both wheat flour and grain functionality and suggested it as a good alternative to chemicals for improving wheat flour characteristics. The key features that are responsible for the superiority of ozone over commonly used chemicals are its high oxidation potential and the fact that no residues are left after the treatment [32].

Non-thermal plasma (NTP) has emerged as a new and effective method in food processing that retains food quality better than conventional methods [33]. Corona discharge (CD) and dielectric barrier discharge (DBD) are two forms of atmospheric non-thermal plasmas that can produce abundant amounts of ozone together with some other active species [34]. In contrast to ozone generators, DBD and CD can produce ozone in situ and create a ground for a direct reaction between reactive oxygen species and wheat proteins. Plasma is a an overall neutral gas, mainly composed of positively and negatively charged particles, neutrons, ions and particles that are heavy and with high energies [35]. Plasma is the fourth state of matter and is abundantly present in the universe. It was first discovered in the 18th century by William Crookes and named by Irving Langmuir and Levy Tonks in 1929 [36]. Based on electron temperature and thermodynamic equilibrium, plasma can be classified as high-temperature (thermal) or low-temperature (non-thermal) plasma [37]. Ionization degree may vary from partial ionization, in the case of NTP, to full ionization, in the case of thermal plasma [38].

Thermal plasmas with high electron densities (>1022 m−3) and temperatures (4000 to 20,000 K) can be produced by electric discharge to reach high current flow through a fully ionized gas [39]. Electron temperature in NTP is much higher compared to the temperature of the bulk gas molecules, leading to a much lower temperature of the overall plasma, in the range of 30–50 ◦C [7]. NTP is thus quite suitable for the treatment of living cells, tissues and heat-sensitive materials [38].

Numerous researchers ([4,34,40–45] have studied the interaction of DBD plasma with wheat flour and have found that ozone and active species (H•, O•, OH•, HO2 •) generated during gas ionization can promote structural changes in starch and protein macromolecules (e.g., promoting cross-linking, depolymerization between molecules and the formation of new functional groups) similar to the changes induced by the use of chemicals. Mishra et al. [46] studied the influence of NTP treatment on wheat flour structural properties and determined an increase in the elastic and viscous modulus of the prepared dough. Modifications to biological characteristics and surface properties were confirmed by Bahrami et al. [34]. They reported no change in the total aerobic bacterial count and non-starch lipids. No visible change in the total protein amount was reported; however, there was a trend towards higher-molecular weight fractions, which resulted in thicker dough.

NTP, which is produced in a gas phase, is rich in high-energy species and the contact between the surfaces of flour particles and plasma species is crucial for its structural changes. The reactor design for flour treatment with NTP is of vital importance, as it influences the uniform exposure to plasma species and thus allows for optimal effects of plasma treatment in terms of flour functionality [47].

This study introduces a novel concept of a cylindrical, rotational reactor for wheat flour treatment with in situ produced NTP. Ease of scaling up was considered as a key factor during the development of the concept. The research focus was on defining the optimal operating conditions (e.g., the minimal treatment time) to reach the maximum dough strength, as well as understanding the influence of reactor capacity and the influence of environmental air temperature change (often variable from day to day) on the efficiency of the NTP treatment process.

#### **2. Materials and Methods**

Germany's wheat flour type 550 (equivalent to all-purpose flour) and type 1050 (equivalent to high-gluten flour) were used in this study to investigate the effects of nonthermal plasma. The main difference between the two flour types lies in their protein contents. Flour analysis demonstrates protein contents of 11.10 g and 12.68 g per 100 g of flour in the types 550 and 1050, respectively. A change in the hydration properties and the microstructure of wheat flour was determined before and after the NTP treatment. The effects of NTP treatment in terms of change in dough strength were analyzed in detail by means of rheological analysis.

#### *2.1. Experimental Setup*

The experimental setup for a batch treatment of flour samples using NTP, as shown in Figure 1, consists of three parts: (1) a plasma generator (HVG 80-3000, company Diener, Germany) [48]; (2) a cylindrical, rotational reactor, for the treatment of flour samples with DBD plasma; and (3) a control and data acquisition system (DAQ) system.

**Figure 1.** Experimental setup for Non-thermal plasma (NTP) treatment of wheat flour.

DBD plasma type was produced under atmospheric conditions using a high-voltage plasma generator (power: 120 W, frequency: 80 kHz, voltage: 7 kV). Air was used as plasma gas. The reactor has a capacity between 50 and 500 g of flour. It is designed as a rotational cylindrical vessel, made of stainless steel, and contains two concentric electrodes (Figure 2): (1) an outer (ground) electrode, which is in the form of a hollow, rotating cylinder (*D* = 150 mm, *L* = 300 mm, wall thickness = 2 mm) made of stainless steel and mounted axially on a rotating shaft. A dielectric barrier (thickness = 2 mm), made of Plexiglas (Polymethyl-methacrylate, PMMA), was attached to the inner side of the outer electrode. To ensure a uniform mixing during the NTP treatment, baffles (made of plastic, height = 30 mm and thickness = 3 mm) were placed on the inner wall of the outer cylinder. The outer electrode is connected to a motor, in order to provide its rotation; and (2) an inner (high-voltage) electrode (*D* = 50 mm, made of stainless steel), which is stationary and is positioned coaxially to the outer electrode. In order to reduce the electric power needed to induce DBD, a row of thin pins (made of aluminum, height = 43 mm and thickness = 5 mm) are attached to the high-voltage electrode, with a discharge gap of 2 mm between the pin tips and the dielectric layer (Figure 2). The reactor is closed with a cap from one side, and the other side is left open to the atmosphere.

**Figure 2.** Test reactor for dielectric barrier discharge (DBD) plasma flour interaction.

At the beginning of each experiment, flour is added to the reactor and partially fills the annular gap between the electrodes. During the treatment, the reactor rotates at a constant speed of 10 RPM. Although the reactor is open to the environment, a low rotational speed was selected to avoid the formation of flour clouds that might lead to an explosive atmosphere. Since plasma-generated reactive species, including ozone, have very short half-lives, the described configuration enables a direct contact between these species and the target flour. After the treatment, flour samples were analyzed using a set of analytical techniques which will be described further in the text.

#### *2.2. Dough Preparation*

Doughs were prepared from untreated and treated flour samples in order to identify and compare the changes induced by the plasma treatment.

A standard approach to keep flour stable at room temperature for the purpose of packing and storing is to keep its moisture content at 14%. This standard moisture content may vary when flour is exposed to air, depending on its humidity. A compensation for this change in moisture content was used to modify the basic recipe. The correct amounts of flour and water for dough sample preparation were determined according to Equations (3) and (4), respectively.

$$\text{Flow mass (corrected)} = (100 - 14) \times M / (100 - x) \tag{3}$$

#### Water mass (corrected) = (WAM × *M*)+(*M* − corrected flour mass) (4)

where *M* (g) is the initial amount of flour (250 g), *x* (%) is the amount of moisture in flour (for this flour batch, determined to be 13.4%) and WAM (%) is the water-absorption capacity, for the default moisture content of 14%. Flour, water (25 ◦C) and salt (1 g) were mixed in a dough preparation machine (MUM48A1, Bosch). After 10 min of kneading, the dough was stored in a plastic bag to avoid drying and left to rest for 30 min at 25 ◦C. Afterwards, rheological analysis was performed to determine the dough's visco-elastic properties.

#### *2.3. Analysis of Flour and Dough*

#### 2.3.1. Flour Hydration Properties

Water plays an important role in wheat dough strength due to the ability of protein matrixes to absorb and retain bound, capillary and physically entrapped water against gravity [49]. At lower hydration levels, water bonds flour proteins and starch granules via hydrogen bonds. Free water phase starts to appear at higher hydration levels (water fraction from 0.23 to 0.35), where chemical reactions (i.e., development of gluten matrix) occur during the dough development process [50]. Hence, small changes in both bonded and free water levels result in considerable variation in dough visco-elastic properties.

For this reason, the water-holding capacity (WHC) and the water-binding capacity (WBC) of untreated (control) and treated flour samples were measured and compared. The WHC is the amount of water retained by flour particles without any stress conditions. For this purpose, 1 ± 0.05 g wheat flour was mixed with 10 mL of deionized water and left for 24 h. Afterwards, the supernatant was filtered and the wet wheat flour sample was weighed again. The WBC is the amount of water retained by flour after centrifugation. The samples were prepared according to the WHC procedure, and after 24 h centrifuged at 2000 revolutions for 10 min. Both WHC and WBC were measured according to the procedure described by Chaple et al. [41].

#### 2.3.2. Dough Rheology

Rheological testing is a method commonly employed in the analysis of food-related substances; it is used to study the deformation and flow behavior (i.e., visco-elastic behavior) of test samples. Frequency and amplitude sweeps are two common practices used to study visco-elastic behavior. The former shows the influence of angular frequency on viscoelastic properties, while keeping the deformation amplitude constant. It is usually used to study frequency and time-dependent behavior within a non-destructive deformation range of materials. The latter demonstrates the influence of stress amplitude (at a constant frequency) on dough rheological properties.

The current investigation is based on an amplitude sweep study, by means of which G (storage modulus), G" (loss modulus), the linear visco-elastic region (LVE) and the yield and flow point can be determined. A rheometer (UDS-200, Anton Paar, Germany) [51] was used for the rheological study. A dough sample (10 g) was placed between the probe (MP31) and the bottom plate of the rheometer. Sandpaper (No. 60) was used on the probe and on the bottom plate, to avoid wall slip during the analysis. The frequency was set to 0.5 Hz and the amplitude gamma was set from 0.001 to 200% log to observe the visco-elastic region. The total number of measurement points was 40 and the time interval between each point was 20 s. Storage and loss moduli were plotted as functions of deformation on a log–log graph to study dough behavior.

#### 2.3.3. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is a technique that allows analysis of the microstructures of wheat dough samples. The development of a wheat dough microstructure is a complex process which depends on time, the addition of water and the input of mixing energy. The microstructure and the rheological properties of a dough are connected to the properties of the individual flour components, their behavior after the addition of water, the free water phase and the mobility of water [52].

The microstructural changes in the wheat flours and the respective doughs were followed by SEM (Auriga, Carl-Zeiss, Jena, Germany). The analysis was performed with an accelerating voltage of 1 kV and a working distance of 5 mm. Prior to SEM analysis, both flour and dough samples were placed in an oven at 30 ◦C for 24 h and then cooled in a desiccator in order to remove moisture from the samples without destroying their structures.

#### *2.4. Experimental Procedure*

In the scope of this research, the influence of process parameters, i.e., treatment time, treated flour amount and air temperature, on changes in wheat flour and dough was determined and quantified. In the first set of experiments, the influence of treatment time (1, 3 and 5 min) on dough functional properties was determined. For this purpose, wheat flour samples with a mass of 50 g were treated by NTP. In the second set of experiments, the effect of the treatment reactor design (i.e., reactor volume vs. mass of the treated flour) on dough strength was investigated. The reactor was filled with flour (50, 150, 250, 350 and 450 g) and treated for 3 min. The third set of experiments included the effects of NTP, produced at different air temperatures (10, 20, 30 and 40 ◦C), on dough strength (flour amount: 50 g, treatment time: 3 min).

#### **3. Results and Discussion**

*3.1. Effects of Non-Thermal Plasma Treatment Time on Wheat Flour and Dough Properties* 3.1.1. Hydration Properties of Flour

The variation in the moisture content of wheat flour samples with treatment time (1, 3 and 5 min) is shown in Figure 3. It was determined five times per sample, based on which the standard deviation was calculated. The obtained results showed a general trend towards a decrease in moisture content with respect to treatment duration. A low moisture content (i.e., 12–15%) keeps the flour stable at room temperature. A decrease in the moisture content of wheat flour is also connected to change in physically "bounded" water amount and has a decisive influence on rheological, physical and chemical properties. There are certain indications in the literature that a lowering of moisture content results in mechanical rigidity and the complex viscosity of a dough [53,54], which characteristics are directly related to the stronger visco-elastic properties of wheat dough.

**Figure 3.** Change in the moisture content of wheat flour (type 550) as a function of NTP treatment time.

The dependence of the WHC on treatment time (1, 3 and 5 min), as shown in Figure 4a, was determined five times per sample, based on which the standard deviation was calculated. The WHC of the untreated sample was 168% ± 5% water retained per one gram of flour, which increased slowly with treatment time to reach its maximal value of 173% ± 4%, corresponding to 3 min of plasma treatment time. This corresponds to the finding shown in Figure 3, i.e., that as the moisture content decreases with treatment time, the water-holding ability of the flour increases. Still, no firm conclusion could be drawn at this point, as the standard deviation of this measurement was high compared to the observed change in WHC values.

The WBC dependence on treatment time, shown in Figure 4b, was also determined using five measurements per sample. The WBC level for untreated flour was measured to be 73% ± 3%. The maximum increase, just under 80% ± 1%, was observed for the NTP treatment of 3 min. Longer treatment resulted in decrease in WBC. Higher hydration values (WHC, WBC) were observed in the case of the untreated type 1050 flour (92% ± 1%) due to its higher protein content. However, no significant differences in the tendencies relating to hydration properties between the flour types 550 and 1050 were observed; thus, these were not separately presented.

Both the WHC and WBC of the tested wheat samples (types 550 and 1050) increased with plasma treatment time. The treatment time of three minutes resulted in a maximum enhancement (7% ± 3%) of the WBC of type 550 flour. It is postulated that this increase in WHC can be due to the hydrolytic depolymerisation of starch [41]. In addition, the surface modifications (increase in surface energy due to the addition or exchange of functional groups [55,56]) of flour particles due to the plasma treatment could be another important factor that influences the hydration properties of flour. It has also been reported in the

literature that plasma treatment results in a decrease in water contact angle, leading to higher water permeability, which results in an increased affinity of flour particles towards water [57].

**Figure 4.** Change in the water-holding (**a**) and water-binding (**b**) capacity of wheat flour (type 550) as a function of NTP treatment time.

#### 3.1.2. Visco-Elastic Properties of Dough

Changes in the functional properties of doughs produced with untreated and treated wheat flour were tested as a function of flour treatment duration. These properties were determined by rheological measurements, where the dependence of the storage and loss moduli (G and G") of doughs were plotted against the induced deformation. The modulus G represents the elastic properties and the modulus G" represents the viscous properties of a wheat dough. These two parameters, together with their combination, contribute to the functional properties of wheat dough, defining it as a visco-elastic material.

The change in the G and G" values of doughs made from the two flour types (550 and 1050) as a function of deformation and treatment time on a logarithmic scale are presented in Figure 5a,b.

A rheological behavior diagram for wheat dough has three parts: (1) a linear viscoelastic region, where the elastic properties prevail. This lies between zero deformation and the deformation at which the yield point (i.e., the end of the elastic behavior and the start of plastic behavior) has been reached. This is the point at which the strain corresponds to a ~5% drop in the storage modulus [58]; (2) a plastic region, where the increasing deformation starts to permanently destroy the dough (or protein–starch) structure and cannot be regained. This is the region between the yield point and the flow point (i.e., the threshold of shear stress, above which solid material will start to flow); and (3) the region above the flow point, where a solid behaves as a liquid, since the deformation potential is so huge that all the forces responsible for holding the material structure are overruled.

Figure 5a,b shows that G is higher than G", as is characteristic of visco-elastic materials. The experimental results demonstrate that the changes caused by the NTP treatment of flour, reflected by increases in the G and G" values of the formed dough, initially increased as the treatment time was prolonged. This increase reached its maximum value for the treatment time of 3 min for both flour types, after which the values of G and G" sank.

Figure 5a and Table 1 demonstrate that the NTP treatment showed its maximum potential for the type 550 flour, with a two-fold increase in the visco-elastic properties (G = 85–130% and G" = 76–111%) for the treatment duration of 3 min. This finding corresponds to the maximum change in hydration properties of flour with the three minutes treatment, as shown in Figure 4. On the other hand, the moduli of the untreated high-protein flour (type 1050) were already significantly higher compared to those of the flour type 550, i.e., 200–400% for G and 200–330% for G". Thus, the corresponding

maximum changes in G and G" in the plasma-treated samples of type 1050 were low to mild (G = 0–28% and G" = 0–16%) (Figure 5b and Table 1). This finding also confirms the limited enhancement of the hydration properties of the type 1050 flour.

**Figure 5.** Change in the storage (G ) and loss (G") moduli of wheat dough as a function of NTP treatment time: (**a**) for the flour type 550; (**b**) for the flour type 1050.

**Table 1.** Increase in G and G" for the doughs produced with NTP-treated flour samples (presented as minimum-to-maximum increase) in comparison to doughs produced with untreated flour samples) as a function of treatment time.

Analysis of variance (ANOVA) [59] was used in order to determine significant statistical differences between the samples. The obtained results for the type 550 flour (*p* < 0.05) (provided in the Supplementary Materials S1) confirmed the existence of significant statistical differences between untreated and treated samples, i.e., the effect of non-thermal plasma treatment on flour properties was statistically significant. However, in the case of type 1050, the obtained *p*-value was larger than 0.05. This implies that the effect of NTP treatment on type 1050 flour was not statistically significant, i.e., the influence of treatment cannot be established.

A general trend towards an increase in the yield points of the doughs produced with plasma-treated flour samples can be noticed, representing an increase in the linear viscoelastic (LVE) region. Increased elastic properties further indicate an increase in dough strength. The results presented in Figure 5a demonstrate that the flow point value increased by 51% and the yield point value by 75% when the flour type 550 was treated for 3 min in comparison to the untreated (control) sample. Chittrakorn et al. [60] explained an increase in dough strength in terms of an increase in the number of disulphide bonds within the protein matrix in the dough, i.e., the higher the number of disulphide bonds, the higher the storage and loss moduli. The properties of the dough made from flour type 1050 (Figure 5b) showed similar qualitative behaviors under the same plasma conditions; however, the quantitative change was much lower than for that made with type 550 flour.

Some other authors ([53,61,62]) have suggested that change in visco-elastic behavior could be connected to the hydration properties of wheat flour. The changes in moisture content, WHC and WBC, as shown in Figures 3 and 4, reflect the change in bonded and free water content levels, which directly affect gluten–starch and starch–starch bonding through hydrogen bonds. These bindings, enhanced by the plasma treatment, started to dominate the dough characteristics, making the treated samples stronger than the control samples.

Based on the presented results, it can be concluded that a plasma treatment time of 5 min will result in decreased structural strength of wheat dough, which could be connected to the weakening of disulphide bonds in the protein matrix and to the decrease in the WHC and WBC of wheat flour. Further, it can be concluded that NTP-induced intensification of visco-elastic properties is inversely proportional to the amount of protein present in the flour, i.e., with a higher protein content, the change in the visco-elastic properties of dough due to NTP is lower.

#### 3.1.3. Wheat Dough Microstructure

Bechtel et al. [63] studied dough development with light and transmission electron microscopy and revealed that starch granules in dough are usually bigger particles, having an average size of 10–20 μm, while the protein components are in the form of sheets and fibrils which surround the starch particles. The gluten (protein) network structure is affected by gluten quantity, quality and the number of intermolecular disulphide bonds. Similar SEM analyses of starch and protein structures can be found in the literature [52,63,64].

The SEM images of doughs with untreated and treated (1, 3 and 5 min treatment time) flours (type 550) are shown in Figure 6. As claimed above, proteins create a smooth structure that surrounds the round starch particles. Further, two classes of starch granules can be noticed which differ with respect to their average diameters. It has been reported that type A starch granules (with a flattened shape and about 25 microns in diameter) contribute to malting yield, while type B granules (spherical in shape and about 6 microns in diameter) are responsible for water absorption due to their large surface-to-volume ratios [65].

**Figure 6.** SEM Images of the wheat dough samples prepared with untreated and plasma-treated (1 min, 3 min, 5 min) flour under (**a**) low and (**b**) high magnifications (corresponding to 1000× and 2000× magnifications in the current SEM set-up). White arrows—gluten network; yellow arrows—changes to starch particles.

In the case of the control sample (Figure 6a), the gluten matrix was non-homogeneous, i.e., it was a discontinuous network in which large and small fragments of gluten could be observed (indicated by arrows). Starch granules are covered with thin gluten layers. However, the starch granules were not fully embedded in the protein matrix, which corresponds to the findings of Bajic et al. [66,67]. In comparison to that prepared with untreated flour, the SEM images of the dough prepared with plasma-treated flour (Figure 6a,b) showed a well-developed gluten matrix network in which starch particles were uniformly embedded, i.e., a well-developed gluten matrix could be observed. This resulted in a stronger dough, consistent with the rheological analysis presented in Figure 5.

The SEM images for increased treatment times showed only minor differences in the developed gluten network characteristics. However, when the treatment time was equal to 5 min, damage to starch granules was observed (Figure 6a,b), which led to the weakening of gluten networks. This negatively affects the storage and loss moduli of wheat dough [68,69], as confirmed by the results presented in Figure 5.

Comparison of the results presented in Figures 5 and 6 indicated an optimal plasma treatment time of 3 min to enhance dough rheological properties. If this time is exceeded, negative effects might occur as a result of the NTP treatment, leading to the weakening of dough networks.

SEM images for type 1050 samples are visually similar with respect to gluten and protein particles. However, no significant differences between the developed gluten networks of the NTP-treated samples were observed.

#### *3.2. Effects of Non-Thermal Plasma-Treatment Reactor Filling on Wheat Flour and Dough Properties*

As a next step, the role of flour mass in the plasma reactor on changes in flour properties was investigated. The reactor was filled with different amounts (50, 150, 250, 350 and 450 g) of wheat flour (type 550 and 1050), which are equivalent to 2, 6, 10, 13 and 17% occupancy of the reactor volume (flour density ρ = 593 kg/m3 [70]).

#### 3.2.1. Hydration Properties of Flour

Similar to the findings for moisture content, presented in Figure 3, the moisture content also remained close to 14% with varying flour masses and therefore the results are not discussed further here.

Figure 7a,b shows the dependence of WHC and WBC on wheat flour mass (type 550). Even though a stronger effect of NTP in a less full reactor would be expected, a general trend of increase in WHC and WBC values with increase in flour mass was observed. The WHC and WBC values generally increased for dough prepared with treated flour, with the maximal increase for the flour mass of 150 g, i.e., 6% filling of the reactor volume.

**Figure 7.** Effects of reactor filling on the WHC (**a**) and WBC (**b**) values of untreated and NTP-treated flour (type 550; treatment time: 3 min).

The above observation could be attributed to the fact that, during the flour treatment, a proportion of the flour particles stick to the reactor wall and to the HV pins. According to Chaple et al. [41], sample surface area exposed to plasma treatment is an important factor that influences the hydration properties of flours. At a lower flour mass (50 g, equivalent to 2% of reactor volume), a bigger share of the treated flour mass is immobilized and thus the treatment is not uniform. At a moderate mass (150–350 g, i.e., 6–13% of reactor volume), a smaller proportion of flour particles are 'glued' to the reactor surfaces and thus a more intensive mixing takes place, contributing to more effective contact between the plasma species and the flour particles. At higher mass (450 g, i.e., 17% of reactor volume), the contact between the flour mass and the plasma species decreases, hence contributing to a decrease in hydration properties. In the case of type 1050, the change in hydration properties lay within the standard deviation range, hence no conclusion could be drawn and the results are not shown here.

#### 3.2.2. Visco-Elastic Properties of Dough

Equivalent to the analysis shown in Figure 5 and Table 1, Figure 8 and Table 2 describe the influence of flour mass (i.e., reactor filling) on wheat dough visco-elastic properties. Independent of the treated flour quantity, the results demonstrate an increase in viscoelastic properties of doughs made with treated flour samples in comparison to doughs prepared with untreated wheat flour.

**Figure 8.** Change in the storage (G ) and loss (G") moduli of wheat dough as a function of NTP treatment at different reactor fillings (50, 150, 250, 350 and 450g, at 10 RPM): (**a**) type 550; (**b**) type 1050.

**Table 2.** Increase in the G and G" values of the doughs produced with plasma-treated flour samples (presented as minimum-to-maximum increase in comparison to doughs produced with untreated flour samples) as a function of reactor filling (i.e., flour mass).

The experimental results demonstrate that when a too small (50 g) or a too large (450 g) amount of flour is placed inside the treatment reactor, the rheological properties of the formed dough are only mildly increased. G and G" reach their maximum values with a flour mass of 150 g and then slowly decline again. Such a limitation is not observed in the case of rheological property enhancement using chemicals, such as Vitamin C or ascrobic acid, L-Cysteine and Azodicarbonamide (ADA) [71–73]. A few milligrams of such chemicals (e.g., 10–20 mg of ADA) per kilogram of wheat flour is enough to improve wheat flour properties [74]. Similar to the findings in Figure 5, the results shown in Figure 8 also confirm the previous findings about the limited influence of NTP treatment on the properties of dough made with flour type 1050. This can be connected to the changes in WHC and WBC values shown in Figure 7. It has also been reported that chemical improvers, e.g., vitamin C, also generally improve the visco-elasticity of low-gluten flour as compared with high-gluten flour [75]. Statistical analysis again confirmed *p*-values less than 0.05 in the case of type 550 and greater than 0.05 in the case of type 1050.

The presented results demonstrate the significance of flour particle–plasma species contact, which depends on the percentage of the reactor volume that is filled with flour. This factor can significantly limit the effect of NTP on flour functional properties. In order to achieve better effects of NTP treatment, the threshold for the reactor volume in this set-up is in a range of 6–13%.

#### *3.3. Effects of Environmental Temperature during NTP Treatment on Dough Properties*

Further, the effect of environmental air temperature on the NTP treatment of flour was investigated. A lower voltage demand for plasma formation and an earlier air breakdown due to increased air temperature was reported in the literature [76–78].

A flour sample (type 550, mass 50 g) was treated in the rotational reactor. The air temperature in the plasma reactor chamber was increased using an air heater in 10 ◦C steps from 10 ◦C to 40 ◦C while keeping the relative humidity at 50% ± 10%. The experimental results, shown in Figure 9, demonstrate the dependence of the visco-elastic properties of wheat dough produced at various air temperatures on the non-thermal plasma treatment. Statistical analysis (ANOVA) confirmed the existence of significant statistical differences between the untreated and treated samples, with *p*-values obtained less than 0.05. Independent of air temperature, the results demonstrated an increase in the visco-elastic properties of the dough produced with NTP-treated flour.

**Figure 9.** Change in the storage (G ) and loss (G") moduli of wheat dough as a function of environmental (air) temperature.

The results presented in Table 3 demonstrate the change (in percentage) in visco-elastic properties (G and G") of wheat doughs produced with plasma-treated flour samples at air temperatures between 10 ◦C and 40 ◦C in comparison to the control sample. When NTP was generated at an air temperature up to 20 ◦C, the visco-elastic properties of dough increased maximally between 25 and 53%. When the air temperature increased to 30 ◦C, the change in visco-elastic properties almost doubled. A further increase in air temperature (i.e., 40 ◦C) did not further affect the values of visco-elastic properties significantly.

**Table 3.** Increase in G and G" values for the doughs produced with plasma-treated flour samples (presented as minimum-to-maximum increase in comparison to doughs produced with untreated flour samples) as a function of environmental temperature.

The breakdown voltage demand is strongly influenced by the gas temperature, as it is inversely proportional to the temperature of the gas, as shown in Equation (5) [78]:

$$\mathbf{U} = (\mathbf{T}\_{\mathbf{r}} / \mathbf{T}\_{\mathbf{g}}) \times \mathbf{U}\_{\mathbf{o}} \tag{5}$$

where U (V) and Uo (V) are the breakdown voltage at the desired gas temperature and gas breakdown voltage at room temperature, respectively; Tr is the ambient room temperature (300 K); and Tg is the gas temperature.

If the temperature of the gas is increased, the breakdown voltage demand decreases and vice versa. It has been reported that air warmed to a certain temperature impairs its insulating power and makes it dielectrically weak, resulting in the formation of the arc at longer distances [79]. This means that if the discharge gap and supply voltage is constant (as is the case here; i.e., if a 2 mm discharge gap and a constant maximum voltage of 7 kV is supplied to the system), then at lower temperatures (in this case, an air temperature of 10 ◦C) the gas is partially ionized, leading to the production of a limited number of active species. This limit is overruled by the increase in the gas temperature (in this case, 30 ◦C), as at that constant voltage and discharge gap, the gas is more easily ionized (decrease in air dielectric strength) due to higher temperatures. Further increase in temperature leads to a decrease in breakdown voltage demand; however, the number of active species stays constant, as the gas is already fully ionized at lower temperatures. This was the reason why a strong increase in visco-elastic properties in wheat flour was noticed as the air temperature was increased from 10 ◦C to 30 ◦C. However, no further increase in visco-elastic properties was detected when the air temperature was increased to 40 ◦C.

#### **4. Conclusions**

The key findings of the presented research are:

NTP treatment has the potential to replace the use of chemicals in order to enhance the functional properties of wheat dough. However, as an outlook of this work, it is vital to investigate the effects of NTP on flour and dough structural network attributes, e.g., proteins, starch, microorganisms, color, etc., before further implementation of the treatment. Future scaling, involving such considerations as the processing efficiency, cost and control of the NTP process, could also be challenging. In the next phase of this research, the aim is to further study the effects of NTP on wheat network structural attributes in more detail.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/app12167997/s1, manuscript-supplementary.

**Author Contributions:** Conceptualization, M.J.K., A.Z.-R. and V.J.; methodology, M.J.K., A.Z.-R. and V.J.; software, M.J.K.; validation, M.J.K.; formal analysis, M.J.K. and A.Z.-R.; investigation, M.J.K.; resources, A.D. and V.J.; writing—original draft preparation, M.J.K., A.Z.-R. and V.J.; writing—review and editing, M.J.K., A.Z.-R. and V.J.; visualization, M.J.K., A.Z.-R. and V.J.; supervision, V.J. and A.D.; project administration, A.Z.-R.; funding acquisition, V.J. and A.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the German Federal Ministry of Economy and Energy (BMWi) through the Forschungskreis der Ernährungsindustrie e. V.—FEI program (grant number: 20629N).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We acknowledge financial support from Deutsche Forschungsgemeinschaft and Friedrich-Alexander-Universität Erlangen-Nürnberg within the funding programme "Open Access Publication Funding". Furthermore, we express our gratitude to the company Diener for support in the coupling of the plasma generation device and the treatment reactor.

**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results.

#### **References**

## *Article* **SDBD Flexible Plasma Actuator with Ag-Ink Electrodes: Experimental Assessment**

**Viktoras Papadimas 1,†, Christos Doudesis 1,†, Panagiotis Svarnas 1,\*, Polycarpos K. Papadopoulos 2, George P. Vafakos 2 and Panayiotis Vafeas 3**

**Abstract:** In the present work, a single dielectric barrier discharge (SDBD)-based actuator is developed and experimentally tested by means of various diagnostic techniques. Flexible dielectric barriers and conductive paint electrodes are used, making the design concept applicable to surfaces of different aerodynamic profiles. A technical drawing of the actuator is given in detail. The plasma is sustained by audio frequency sinusoidal high voltage, while it is probed electrically and optically. The consumed electric power is measured, and the optical emission spectrum is recorded in the ultraviolet–near infrared (UV–NIR) range. High-resolution spectroscopy provides molecular rotational distributions, which are treated appropriately to evaluate the gas temperature. The plasma-induced flow field is spatiotemporally surveyed with pitot-like tube and schlieren imaging. Briefly, the actuator consumes a mean power less than 10 W and shows a fair stability over one day, the average temperature of the gas above its surface is close to 400 K, and the fluid speed rises to 4.5 m s−1. A long, thin layer (less than 1.5 mm) of laminar flow is unveiled on the actuator surface. This thin layer is interfaced with an outspread turbulent flow field, which occupies a centimeter-scale area. Molecular nitrogen-positive ions appear to be part of the charged heavy species in the generated filamentary discharge, which can transfer energy and momentum to the surrounding air molecules.

**Keywords:** atmospheric pressure plasma; DBD; actuator; pitot tube; schlieren imaging; UV–NIR OES

#### **1. Introduction**

When there is relative movement between a fluid and a solid surface, a boundary layer is formed. This is the layer of fluid in the immediate vicinity of the bounding surface where the effects of viscosity are significant, and it can be either laminar or turbulent. Simplified, the laminar boundary corresponds to a smooth flow, whereas the turbulent boundary layer contains eddies. At the same time, the solid surface experiences a force of resistance in the fluid due to frictional and pressure forces: the "drag force". Practically, this is the component of the surface force parallel to the flow direction (the other component of the surface force that is perpendicular to the oncoming flow direction is the "lift force"). A laminar boundary is associated with less skin friction than a turbulent one, but it tends to break down more suddenly. In the case of a full-size body (e.g., a wing surface), as the flow develops along its surface, the initial laminar boundary layer becomes thicker (and thus less stable) as the flow continues back from the leading edge, and at a certain distance, the laminar boundary layer breaks down and transitions into a turbulent one. It becomes apparent that any spatial delay of this transition is advantageous since the surface experiences lower drag. Furthermore, turbulent flow is a source of aerodynamic

**Citation:** Papadimas, V.; Doudesis, C.; Svarnas, P.; Papadopoulos, P.K.; Vafakos, G.P.; Vafeas, P. SDBD Flexible Plasma Actuator with Ag-Ink Electrodes: Experimental Assessment. *Appl. Sci.* **2021**, *11*, 11930. https:// doi.org/10.3390/app112411930

Academic Editor: Bogdan-George Rusu

Received: 25 November 2021 Accepted: 13 December 2021 Published: 15 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

noise generated from the unsteady nature of this kind of flow. Finally, apart from the laminar-to-turbulent flow transition and the aerodynamic noise, a third important effect due to excessive momentum loss near the solid surface in a boundary layer trying to move downstream against increasing pressure (adverse pressure gradient) is the boundary layer detachment/separation from the surface (into a wake).

The tremendous technological and financial impact of the possibility of controlling the abovementioned flow field effects, e.g., in the aerospace industry and the aircraft market, has been recognized since decades and many methods for flow manipulation (e.g., lift enhancement and drag reduction) have been developed. Most of them refer to efficient mechanical devices, which however may be complicated, heavy, massive sources of noise and vibrations, or maintenance critical, thus partially offsetting the active airflow control value. As a counterweight, units based on atmospheric-pressure, low-temperature plasmas of electrical discharges have been devised for active airflow control. These are the plasma actuators. Two plausible principal physical mechanisms have been proposed on the operation of the plasma actuators. Either an electrohydrodynamic body force provokes collisions between ionic and neutral species, leading thus to the production of so-called electric or ionic wind [1,2], or electric energy is instantly transformed into heat energy, leading to the formation of moving blast waves responsible for large-scale flow field structures [3,4].

Excellent reviews on plasma actuators exist [5,6], gathering the fundamental concepts and the state-of-the-art in the field. We typically deal with surface electrical discharges exploiting unique features from the dielectric barrier discharge (DBD) [7]. This includes arc transition prevention due to the dielectric barrier, which in turn results in a low gas temperature, high engineerability, responsivity within the sub-microsecond time scale, absence of moving components, implicit conversion of electric power into a moving fluid, long background of studies and applications in various interdisciplinary scientific fields [7–9], etc. The great number of reports published over the last decade on the topic of plasma actuators and the ongoing research has a twofold meaning. First, it proves the importance and the potentiality of these devices, whereas second, it states that many issues have not yet been met and further challenges still exist (e.g., energy conversion efficiency factor enhancement). Numerous studies focus on the geometry-configuration optimization [10–12], the dielectric barrier specifications [12,13], the material endurance [13,14], the role of the driving voltage characteristics [12,15–17], the boundary layer separation control [18,19], the laminar-to-turbulent transition delay [20], etc., just to name a few.

As a contribution to the field, the present study is devoted to the implementation of a particular single dielectric barrier discharge (SDBD) [21] plasma actuator and its test under sinusoidal high voltage. Kapton® polyimide films are used as the dielectric barrier due to their ability to be mounted onto curved aerodynamic surfaces. On the other hand, silver (Ag) conductive paint is selected for the formation of the electrodes due to its provided feasibility to design electrodes of every shape with negligible thickness on a variety of surfaces. Both choices make the actuator fully flexible, and this is the main claim of the present communication. The actuator is probed by means of electrical and optical techniques. Electrical measurements are conducted to investigate the time evolution of the power consumption and the endurance of the actuator, over the time window of a day. High-resolution optical emission spectroscopy (OES) provides an insight into the emitting species of the plasma as well as an estimation of the temperature of the air above the actuator. Then, pitot-like tube and schlieren imaging give access to the induced fluid speed and the pattern of the relevant flow field, respectively. Eventually, the overall experimental results allow for a brief discussion on the physical mechanisms governing the aerodynamic specifications of this flexible plasma actuator.

#### **2. Experimental Setup**

Figure 1 shows the experimental setup, including the plasma actuator itself and the diagnostics employed in the present work. The actuator design is illustrated in detail in

Figure 2. It is an SDBD-based one, with the dielectric barrier to be a stack of Kapton® polyimide films. The patterns of the electrodes are formed on the polyimide films by means of silver ink (resistivity 100 μΩ cm) and stencils. The grounded electrode is encapsulated within a two-component epoxy resin.

**Figure 1.** Conceptual diagram of the experimental setup. 1: signal generator; 2: audio power amplifier; 3: oscilloscope; 4: current monitor; 5: ferrite core, step up transformer; 6: high-voltage probe; 7: SDBD actuator; 7a: painted electrodes; 7b: flexible dielectric barrier; 7c: epoxy resin; 8: pitot-like tube on x–y–z, linear, micro-translation stage; 9: pitot-like tube hardware and interface; 10: optical fiber; 11: optical matcher; 12: monochromator; 13,14: photo-electron multiplier tube; 15: data acquisition unit; 16: personal computer.

**Figure 2.** Design of the plasma actuator. (**a**) Top view depicting the high-voltage-driven electrode details. The inset defines the coordinate system that is used hereafter. The system origin coincides with the actuator surface (z = 0), the edge of the discharge gap toward the grounded electrode side (x = 0), and the middle of the grounded electrode length (y = 0). (**b**) Bottom view depicting the grounded electrode details. The stack of Kapton® polyimide films has a total thickness of 0.425 mm, whereas the epoxy resin (No\_7c in Figure 1) is not shown for simplicity purposes.

The high voltage is generated by amplifying a reference sinusoidal signal (10 kHz) and supplying the primary winding of a specially designed, ferrite core, step-up transformer. The output high voltage, v(t), is permanently monitored with a passive probe (Tektronix; P6015; DC-75 MHz). The DBD current, i(t), is recorded with a wideband current transformer (Pearson Electronics; 6585; 400 Hz–200 MHz). The wavelength- and space-integrated light, emitted from the plasma, is collected with a high-grade fused silica optical fiber (Newport; 77576; 260–2200 nm), and then, it becomes time-resolved with a photo-electron multiplier tube (PMT; Hamamatsu R928; 185–900 nm). All waveforms are recorded by a four-channel, wideband, digital oscilloscope (LeCroy; WaveRunner 44Xi-A; 400 MHz; 5 GSamples s−1).

The mean electric power consumed by the actuator is calculated by integrating the product [v(t:t + 5T) × i(t:t + 5T)] over five sequential voltage periods, i.e., 5T, and by dividing the result by this factor. The process is repeated for five independent [v(t:t + 5T) × i(t:t + 5T)] products. Two such measurement sets are realized on different dates. Hence, in total, 10 instantaneous power waveforms (5T duration each) are considered for extracting mean values and standard deviations (error bars in the graphs) of the electric power.

The time- and space-integrated light is guided by the abovementioned optical fiber to a monochromator (Jobin Yvon; THR 1000; 170–750 nm; 2400 grooves mm−1) and the optical emission spectrum is thus identified. The wavelength calibration of this unit is carried out with an Hg (Ar) pencil-style lamp (Newport; 6035). Special attention is paid to calibrating the relative spectral efficiency of the entire optical system. This is accomplished with a quartz-tungsten-halogen lamp (Newport; 6334NS; 250 W) operated at 3400 K. The rotational temperature of principal excited molecular species is estimated by fitting their experimental rotational distributions to the corresponding theoretical ones, using a labbuilt software [22]. In addition, a 0.25 m imaging spectrograph (Newport; MS260i) is used for exploring wider wavelength ranges. It is equipped with two motorized gratings: one holographic (2400 grooves mm−1; blaze wavelength 250 nm; 180–700 nm) and one ruled (600 grooves mm−1; blaze wavelength 400 nm; 250–1300 nm). The photodetector of this spectrograph is a linear CCD array (200–1100 nm). Optical matchers are used in both cases (monochromator and spectrograph) for optimal alignment of the fiber image.

The speed of the plasma-induced fluid is recorded in real time with a pitot-like tube (glass capillary tube; Øout = 1.3 mm; Øin = 0.8 mm) and appropriate hardware (Trotec; TA400). Precise, point-specific measurements are allowed due to the tube mounting on a x–y–z linear micro-translation stage.

Finally, flow field patterns are visualized by means of schlieren imaging. The main parts of the system are provided by Edmund Optics®. It contains a high-power green light emitting diode, two aluminized first surface spherical mirrors (152.4 mm in diameter, 1524 mm focal length and 1/8 wave surface accuracy) overcoated with silicon monoxide, and two Foucault knife edge testers mounted on separate micro-translator stages (one horizontal and one vertical). The components are arranged in a Z–type configuration. A camera (Nikon; D3300) is added to the system enabling photographic records. The camera is equipped with a zoom lens (Canon; FD 70–210 mm). The settings for the schlieren photos are ISO 400, exposure time 1/4000 s, and f-number 4. The settings for the conventional photos of the plasma are ISO 6400, f-number 5.6, and 1/30 s at 14 kVpp and 1/40 s at 16 kVpp ("kVpp" stands for kV peak-to-peak).

#### **3. Results and Discussion**

Figure 3a–c provide representative oscillograms of the actuator driving voltage, DBD current, and plasma emission integrated over ultraviolet-near infrared (UV–NIR) wavelengths, respectively. Clearly, the main ionization and excitation processes are related to the rising part of the driving voltage, whereas much weaker current and emission are observed during the falling part. This fact has been mentioned and studied extensively in other DBD-based setups, showing that the propagation of cathode- and anode-directed streamers is involved [23].

**Figure 3.** Representative waveforms of (**a**) the high voltage, (**b**) DBD current, and (**c**) wavelengthintegrated optical emission over two periods. The projection of the optical fiber acceptance cone on the actuator surface corresponds to a circle of 12.5 mm in radius, centered on the point (x = 0, y = 0, z = 0).

During the rising part of the voltage, primary electrons propagate towards the anode, promoting ionization and excitation processes. The qualitative similarity between the current and emission impulses (Figure 3b,c) is underlined. At the same time, the resultant heavy positive ions are accelerated towards the dielectric-covered cathode, transferring momentum and energy to the surrounding air molecules, leading eventually to the flow field patterns that are illustrated below. On the other hand, during the falling part of the voltage, we probably deal with anode-directed streamers. According to this assumption, since the exposed electrode is now negatively driven, electrons are repealed towards the dielectric-covered electrode. They are following the field lines and diverging from each other giving a diffused and weaker excitation.

The impulsive form of the current (Figure 3b) and the optical emission (Figure 3c) are in accordance with the conventional photographs of the DBD pattern shown in Figure 4. The induced plasma is associated with filamentary DBDs. The number and the electron density of the filaments along the flat electrodes increase for higher voltage amplitude, but a homogeneous glow discharge is never reached with the voltage amplitudes used here. It is considered that the electric field is quite elevated due to the electrode design (thickness of a few micrometers only). Thus, important fluid perturbations are anticipated because of electrohydrodynamic forces [2,24–26]. Schlieren images are in line with this statement (discussed later).

Based on the above observations, it becomes apparent that the power is principally delivered to the actuator during the positive cycle of the driving voltage. Figure 5 shows that, under the present experimental conditions, the actuator consumes a mean power of less than 10 W. An increase at the voltage amplitude of just 2 kVpp leads to power doubling (Figure 5a versus Figure 5b). However, the consumption remains practically unchanged over at least 24 h of operation (accumulative). The latter is an implication of a quite reliable actuator.

**Figure 4.** Representative conventional photographs of the DBD developed between the Ag-ink electrodes of the actuator. The filamentary nature of the discharge is manifested: (**a**) 14 kVpp. (**b**) 16 kVpp. The scale is the same in both photos (the plasma visible pattern has a length of about 50 mm).

**Figure 5.** Electrical mean power consumed by a newly constructed actuator during the first 24 h of its operation (accumulative): (**a**) 14 kVpp and (**b**) 16 kVpp. Solid symbols: mean values following 10 sets of measurements (see Section 2 for details). Error bars: standard deviations derived from the 10 sets. Lines: linear fitting of the mean values.

However, despite the consistency of the actuator in terms of power consumption, both dielectric and electrode degradation are observed as the operation time accumulates. Figure 6 demonstrates this aging effect, as it is exhibited on the top surface of the actuator. The photo in Figure 6b refers to 24 h of accumulative operation and the worst-case stress scenario (i.e., 16 kVpp). The actuator of Figure 6b remains functional, but the quantification of its material degradation and its possible efficiency deterioration might be the task of another study.

A better insight into the species contributing to the plasma-induced flow field may be gained by the OES spectrum of Figure 7a. Although a much wider wavelength range has been examined (see Section 2 for grating specifications), only N2(SPS) and N+ 2 (FNS), shown in Figure 7a, are detectable. The identification is organized in Table 1. Interestingly, with regard to ionic species, the nitrogen molecular ion is solely observable, and despite its weak relative spectral emission, its role in the flow field formation is emphasized. Otherwise, the spectrum is mainly populated by excited nitrogen molecules, whereas species such as NOγ, OH, and N2(FPS) are not detected.

**Figure 6.** External top view of a newly constructed actuator (**a**) before and (**b**) after operation during 24 h (accumulative time) at 16 kVpp.

Furthermore, high-resolution spectra can give the patterns of the rotational distributions of excited molecules, which thus serve as probe molecules for the gas temperature evaluation. In Figure 7b, the rotational distribution of the N2 (SPS) species (337.13 nm in Table 1) is presented, being determined both experimentally and numerically. The numerical fitting of the latter to the former implies a gas temperature up to around 410 K at the maximum applied voltage (16 kVpp). Further experiments have given values around 318 K at 12 kVpp and 336 K at 14 kVpp. Apart from the positive ions, the role of this rather elevated temperature in the flow field pattern should also be considered. This remark becomes more critical if one considers the local temperature, which is expected to be higher than the average one measured here. The term "local" means the temperature within specific elementary volumes of the plasma, contrary to the integrated emissive volume probed here (see the caption of Figure 7). In the frame of this work, the relative contribution of each factor, i.e., ions versus temperature, to the flow field modifications cannot be conclusive, but this topic remains challenging (see the further discussion below based on the schlieren imaging results).

**Figure 7.** (**a**) Optical emission spectroscopy wide scan of the SDBD plasma (resolution 50 pm). For temporal evolution of the emission, see Figure 3c. (**b**) N2(SPS; v = 0 − v = 0) rotational distribution (resolution 1 pm). Solid symbols: experimental points. Line: theoretical rotational distribution, at 410 K, fitted to the experimental data. In both frames, the projection of the optical fiber acceptance cone on the actuator surface corresponds to a circle of 12.5 mm in radius, centered on the point (x = 0, y = 0, z = 0).

**Table 1.** Identification of the numbered bands of the spectrum given in Figure 7a.

Figure 8 presents the point-specific speed of the fluid over the actuator surface (z = 0.75 mm; y = 0 mm) at various positions far away from the discharge gap (x = 8, 18, and 24 mm) versus the time. These graphs unveil the variation in the speed that may take place as a function of the space coordinates and the time elapsed. Speed fluctuations are highly pronounced at longer distances from the discharge gap and they almost fade as the gap is approached (sequence Figure 8c → Figure 8b → Figure 8a). It is thus suggested that a transition from laminar to turbulent flow takes place along the x direction, even if we remain close to the surface of the actuator.

Based on time-resolved speed graphs such as those of Figure 8, the mean speed is calculated from each 300 s graph, and the 2D distribution (xz-plane) of the gas mean speed is mapped as in Figure 9 (y = 0). As regards the fluid speed close to the actuator surface (z = 0.75 mm; solid symbols in graphs), an increased speed is measured close to the discharge gap (x = 0 mm), whereas it decays monotonously along longer distances (up to x = 24 mm). This trend persists independently of the applied voltage amplitude. However, at 16 kVpp (Figure 9b), a slightly higher speed is achieved compared with that at 14 kVpp (Figure 9a). On the other hand, an abrupt decrease in the speed is observed at an almost double distance from the actuator surface (z = 1.4 mm; open symbols in graphs), and this is more noticeable when the actuator is driven by a lower voltage (Figure 9a). Another remarkable fact is the increased values of the standard deviations (error bars) calculated in the case of z = 1.4 mm compared with those found for the measurements closer to the surface (z = 0.75 mm). This fact should not be attributed to experimental uncertainties but, as it is mentioned above, it is rather related to the transition from a laminar flow layer to a turbulent phase. The subtleties of this assumption are considered below along with the schlieren imaging results.

**Figure 8.** Real-time recordings of the point-specific, "instantaneous" speed (integration time 1 s) over 300 s time intervals. Two independent sets are obtained at x = 8 mm, y = 0 mm, z = 0.75 mm (**a1**,**a2**); x = 18 mm, y = 0 mm, z = 0.75 mm (**b1**,**b2**); and x = 24 mm, y = 0 mm, z = 0.75 mm (**c1**,**c2**). A newly constructed actuator is used for each set (first set: **a1**–**c1**; second set: **a2**–**c2**).

**Figure 9.** Mean speed along the *x*-axis at two different positions over the actuator surface. Solid symbols: y = 0 mm and z = 0.75 mm. Open symbols: y = 0 mm and z = 1.4 mm. Driving voltage: (**a**) 14 kVpp and (**b**) 16 kVpp. Mean values and standard deviations are derived from two sets of measurements. Four newly constructed actuators are used (first and second: 14 kVpp/z = 0.75 mm/z = 1.4 mm; third and fourth: 16 kVpp/z = 0.75 mm/z = 1.4 mm).

The vertical (i.e., along the *z*-axis) variation in the mean speed for two representative positions behind the discharge gap is plotted in Figure 10. In both cases, i.e., x = 4 mm and x = 12 mm, as the actuator surface is approached, the fluid speed increases constantly, whereas it becomes negligible at a vertical distance z = 1.5–2 mm. Comparing Figure 10a,b, the turbulent nature of the flow as we move behind the discharge gap is mirrored in the increasing standard deviation values, similarly to what is implied by the data in Figure 8.

**Figure 10.** Mean speed along the *z*-axis at two different positions behind the discharge gap. (**a**) x = 4 mm and y = 0 mm; (**b**) x = 12 mm and y = 0 mm. Mean values and standard deviations are derived from two sets of measurements. Two newly constructed actuators are used (first: 16 kVpp/x = 4 mm/x = 12 mm; second: 16 kVpp/x = 4 mm/x = 12 mm).

A direct comparison of the above measured parameters with the corresponding values appearing in the bibliography does not make sense since an identical plasma actuator operating at the same conditions has not been evaluated earlier. Nevertheless, we can appose indicative information from experiments on SDBD plasma actuators in terms of fluid speed, electrical features, electrode dimensions, etc. For an actuator with both electrodes exposed (Al tapes of 200 mm length, 5 mm width, and 0.1 mm thickness), a 2 mm PMMA barrier, an electrode gap equal to 5 mm, and a sinusoidal (700 Hz) driving voltage at 40 kVpp, a maximum speed of about 4.5 m s−1 has been noted [27]. In the same work, an electrode width of 20 mm, an electrode gap of 0 mm, a frequency of 1.5 kHz, and a voltage of 40 kVpp led to a maximum speed of about 6.5 m s−1. In another case [4], where a 300 mm long actuator consisting of 0.1 mm thickness Kapton® barrier, 12.7 mm wide covered grounded electrode, and 6.35 mm wide exposed high-voltage electrode were used, the time-averaged consumed power was reported to be less than 0.3 W mm−1 when the actuator was driven by short nanosecond pulses at frequencies less than 3 kHz and typical peak voltage at 16 kV. The velocity field generated in still air gave ensemble-averaged speed values up to 0.5 m s−1, 20 mm downstream of the discharge for repetition frequency 2 kHz.  For a 24 mm wide insulated lower electrode and 16 kVpp triangular (5 kHz) high voltage, a maximum fluid speed between 2.5 and 3 m s−1 was recorded.

Regarding the present actuator, the above speed plots provide a reasonable picture of the flow field pattern within the 2D space. This picture is validated by the schlieren images of Figure 11. It is noticed that schlieren images, unavoidably, correspond to sideon snapshots (xz-plane). This may point to an integration of optical events along the Y

direction and their projection onto the xz-plane. However, following this angle, three different regions and regimes of flow field are readily distinguished. The first region (1) refers to the area above the high-voltage-driven electrode. This area is associated with an eddy-like flow field, which appears concentrated on the electrode edge adjacent to the discharge gap. The second region (2) is the interelectrode one, i.e., the gap itself, where a laminar flow well bounded onto the actuator surface is established. The thickness of this layer is approximately 1 mm. The third region (3) is the extended area above the grounded electrode, where the boundary laminar layer continues to propagate onto the surface. Nonetheless, it becomes progressively thicker along the *x*-axis, and a clear transition to intense perturbated (turbulent) flow field is observed along the *z*-axis. In addition, it tends to bend upwards as it approaches the end of the grounded electrode, resembling the separation effect. The perturbated field occupies more and more space as the distance along the *x*- or/and *z*-axis increases. This result is reflected in the speed and standard deviation values discussed above. The distinction of the three areas holds true for both voltage amplitudes in Figure 11. Nevertheless, the 16 kVpp case promotes more intense perturbations.

**Figure 11.** Representative flow field patterns recorded by means of schlieren imaging for two different voltage amplitudes: (**a**) 14 kVpp and (**b**) 16 kVpp. A newly constructed reactor is used. The three white bullets (insets) refer to indicative coordinates (x, y, z) in millimeters, common to (**a**,**b**). They mark off the image scale and facilitate the discussion in the text. The coordinates are given with respect to the Cartesian system defined in Figure 2.

Lastly, the flow field perturbations induced by the actuator propagate to long distances, even behind the grounded electrode. The snapshots of Figure 11 unveil perturbations far away from (i) the grounded electrode left edge (up to about 60 mm on the *x*-axis) and (ii) the electrode plane (up to about 20 mm on the *z*-axis). Such an intense directed motion of neutrals should be attributed to collisions with ions drifting into high local electric fields. The extremely thin electrodes were painted on the dielectric point to electric field enhancement as a dominant factor determining the effectiveness of momentum coupling into the surrounding air [28]. The role of temperature remains ambiguous even after the present results. However, similar electrohydrodynamic and thermal effects have been extensively studied by our group numerically [24–26,29], and the adaptation of the available numerical models to the present actuator case is in progress.

#### **4. Conclusions**

An SDBD-based plasma actuator having flexible a dielectric barrier and electrodes made of conductive ink was implemented and tested. Being driven by sinusoidal high voltage at 10 kHz, the actuator consumed up to 10 W of electric power and yielded a fluid mean speed of up to 4.5 m s−1. Molecular nitrogen ions were produced during the rising part of the driving voltage, originating from air ionization due to the propagation of cathode-directed streamers. These ionic species were considered potential agents for the flow field establishment. This consisted of (i) a thin laminar flow layer, better developed closer to the actuator surface and the discharge gap, and (ii) an extended turbulent structure, which prevailed as being moved away from the surface and the gap. The average temperature of the neutral species over the actuator was found to be around 410 K at the maximum power level. Consequently, although the fluid directed speeds were presumably accredited to an ionic wind effect, the role of the temperature in the flow field formation remained debatable.

**Author Contributions:** V.P. and C.D. contributed equally: conceptualization; investigation; methodology; software; data curation; writing—original draft preparation; writing—review and editing; formal analysis; validation; visualization. P.S.: project administration; funding acquisition; supervision; resources; writing—original draft preparation; writing—review and editing; validation. P.K.P.: resources; validation; writing—review and editing. G.P.V.: resources; investigation. P.V.: validation; writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available from the corresponding author upon request. The data are not publicly available due to privacy.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Plasma Gas Temperature Control Performance of Metal 3D-Printed Multi-Gas Temperature-Controllable Plasma Jet**

**Yuma Suenaga 1, Toshihiro Takamatsu 2,3,\*, Toshiki Aizawa 1, Shohei Moriya 1, Yuriko Matsumura 4, Atsuo Iwasawa 4 and Akitoshi Okino 1**

**Abstract:** The aim of the study was to design and build a multi-gas temperature-controllable plasma jet that can control the gas temperature of plasmas with various gas species, and evaluated its temperature control performance. In this device, a fluid at an arbitrary controlled temperature is circulated through the plasma jet body. The gas exchanges heat with the plasma jet body to control the plasma temperature. Based on this concept, a complex-shaped plasma jet with two channels in the plasma jet body, a temperature control fluid (TCF) channel, and a gas channel was designed. The temperature control performance of nitrogen gas was evaluated using computational fluid dynamics analysis, which found that the gas temperature changed proportionally to the TCF temperature. The designed plasma jet body was fabricated using metal 3D-printer technology. Using the fabricated plasma jet body, stable plasmas of argon, oxygen, carbon dioxide, and nitrogen were generated. By varying the plasma jet body temperature from −30 ◦C to 90 ◦C, the gas temperature was successfully controlled linearly in the range of 29–85 ◦C for all plasma gas species. This is expected to further expand the range of applications of atmospheric low temperature plasma and to improve the plasma treatment effect.

**Keywords:** atmospheric plasma; metal 3D printing; temperature-controllable plasma gas

#### **1. Introduction**

Plasma has been actively applied in industrial fields, such as semiconductor manufacturing and fluorescent lighting. For example, in the field of surface treatment, plasma is used to harden the surface of materials [1] and surface modification [2]. These surface treatments are techniques that change the properties of solid surfaces using the action of the ions and electrons generated by plasma, for which low-pressure plasma has been used. Low-pressure plasmas are generated at low pressures of 1/1000 atm or less, which enables uniform surface treatment; however, this has limitations in terms of the types of treatment objects and throughput.

In recent years, it has become possible to stably generate low-temperature plasmas under atmospheric pressure; therefore, these plasmas have been applied in various fields such as medicine, industry, and chemical analysis. For example, in the medical field, they are used for disinfection [3–5], hemostasis [6], and wound healing [7], and they are used for surface modification in the industrial field [8]. In the field of chemical analysis, they have been utilized for surface adhesion analysis [9] and on-site analysis [10]. To increase

**Citation:** Suenaga, Y.; Takamatsu, T.; Aizawa, T.; Moriya, S.; Matsumura, Y.; Iwasawa, A.; Okino, A. Plasma Gas Temperature Control Performance of Metal 3D-Printed Multi-Gas Temperature-Controllable Plasma Jet. *Appl. Sci.* **2021**, *11*, 11686. https:// doi.org/10.3390/app112411686

Academic Editor: Bogdan-George Rusu

Received: 15 November 2021 Accepted: 6 December 2021 Published: 9 December 2021

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