however, due to the presence of the mws effect (or the mws polarization) a mixture containing at least one conducting phase will exhibit this relaxation at much lower frequencies. the mws effect depends on several factors such as volume fraction of each phase, phase orientation, conductivity and other mixture parameters. wagner formula for dilute mixture and bruggeman formula for dense mixtures are among the most notable formulation of effective dielectric constant. hanai's formulation of complex dielectric constant, an extension of bruggeman formula of effective dielectric constant, is instrumental in analyzing mws effect for complex dielectric constant. hanai's formula for complex dielectric writes as from left, reconstructed images of the flow model, conducting phase and nonconducting phase. ( ε 1 ∗ − ε 2 ∗ ε 1 ∗ − ε ∗ ) 3 ε ∗ ε 2 ∗ = 1 ( 1 − ϕ ) 3 ^-\varepsilon _^^-\varepsilon ^\right)^^= where ε 1 ∗ ^ , ε 2 ∗ ^ , and ε ∗ are the complex effective permittivity of the dispersed phase, continuous phase, and mixture, respectively. ϕ is the volume fraction of the dispersed phase. knowing that a mixture will exhibit dielectric relaxation due to the mws effect, this additional measuring dimension can be exploited to decompose multiphase flows when at least one of the phases is conducting. the figure to the right shows the reconstructed images of the flow model, conducting phase, and non-conducting phases extracted by exploited mws effect from experimental data. ecvt velocimetry normalized sensitivity distribution, sensitivity gradient between a pair of electrodes, reconstructed velocity profile when the spheres are moved in a 3d profile, and in a 2d profile in plane. velocimetry refers to techniques used to measure velocity of fluids. the use of the sensitivity gradient enables the reconstruction of 3d velocity profiles using an ecvt sensor, which can readily provide information of fluid dynamics. the sensitivity gradient is defined as f = ∇ s = a ^ x ∂ s ∂ x + a ^ y ∂ s ∂ y + a ^ z ∂ s ∂ z _+_+_ where s is the sensitivity distribution of an ecvt sensor as shown to the right. upon application of the sensitivity gradient as described in, a 3d and 2d velocity profile corresponding to the figure above are shown in the figure to the right. the application of the sensitivity gradient provides significant improvement over more traditional (cross-correlation based) velocimetry, exhibiting better image quality and requiring less computational time. another advantage of the sensitivity gradient based velocimetry is its compatibility with conventional image reconstruction algorithms used in ecvt. advantages modular the basic requirements of ecvt sensors are simple and can therefore be very modular in design. ecvt sensors require only conductive electrodes which are electrically isolated from one another and are also not shorted through the medium being inspected by the ecvt sensor. additionally there must be a way to excite and detect signal to and from each electrode. the lack of constraints on the sensor design allow it to be made out of a variety of materials and take a plethora of forms including flexible walled, high temperature performance, high pressure performance, thin walled, elbowed and flat sensors. with the addition of aecvt technology, sensor electrode configuration becomes modular as well without the need to fabricate new sensors. safe ecvt is low energy, low frequency, and non-radioactive, making it safe to employ in any situation where toxic waste, high voltage, or electromagnetic radiation is a concern. the low energy nature of the technology also makes it suitable for remote locations where power is in short supply. in many occasions, a simple solar powered battery may prove sufficient enough to power an ecvt device. scalable ecvt operates on very large wavelengths, typically using frequencies below 10 mhz to excite the electrodes. these long wavelengths allow the technology to operate under the quasi-electrostatic regime. as long as the diameter of the sensor is much smaller than the length of the wave, these assumptions hold valid. for instance, when exciting with 2 mhz ac signal, the wavelength is 149.9 meters. sensor diameters are typically designed well below this limit. additionally, capacitance strength, c , scales proportionally according to electrode area, a , and distance between the plates, d , or the diameter of the sensor. so as a sensor diameter becomes larger, if the plate area scales accordingly, any given sensor design can easily be scaled up or down with minimal effect on the signal strength. c ∝ a d low cost & profile compared to other sensing and imaging equipment such as gamma radiation, x-ray, or mri machines, ecvt remains relatively cheap to manufacture and operate. part of this quality of the technology is due to its low energy emissions which do not require any additional mechanisms of containing waste or insulating high power outputs. adding to the low cost is the availability of a wide variety of materials to fabricate a sensor. electronics can also be placed remotely from the sensor itself which allows for standard environment electronics to be utilized for data acquisition even when the sensor is subjected to extreme temperatures or other conditions which typically make it difficult to employ electronic instrumentation. high temporal resolution (fast) in general terms, the method of data acquisition used alongside ecvt is very fast. data can be sampled from the sensor at several thousand times per second depending on the number of plate pairs in the sensor design and the analog design of the data acquisition system (i.e. clock speed, parallel circuitry, etc.). the potential for collecting data very quickly makes the technology very attractive to industries which have processes that occur very quickly or transport at high speeds. this is a great contrast to mri which has high spatial resolution but often very poor temporal resolution. challenges for spatial resolution in ecvt as noted above, spatial resolution is a fundamental challenge in ect/ecvt. the spatial resolution is limited by the soft-field nature of ect/ecvt and the fact that the interrogating electric field in ect/ecvt is quasi-static in nature. the latter property implies that the potential distribution between the plates is a solution of laplace equation. as a consequence, there cannot be any relative minima or maxima for the potential distribution between the plates and hence no focal spots can be produced. in order to increase spatial resolution, two basic strategies can be pursued. the first strategy consists of enriching the measurement data. this can be done by (a) adaptive acquisitions with synthetic electrodes, (b) spatio-temporal sampling using additional measurements obtained when objects are in different positions inside the sensor, (c) multi-frequency operation to exploit permittivity variations with frequency due to the mws effect, and (d) combining ect/ecvt with other sensing modalities, either based on the same hardware (such as dcpt) or on additional hardware (such as microwave tomography). the second strategy to increase spatial resolution consists in the development of multi-stage image reconstruction that incorporate a priori information and training data sets, and spatial adaptivity. applications multi-phase flow multi-phase flow refers to simultaneous flow of materials of different physical states or chemical compositions, and is heavily involved in petroleum, chemical and biochemical industries. in the past, ecvt has been extensively tested in a wide range of multi-phase flow systems in laboratory as well as industrial settings. ecvt's unique ability to obtain real-time non-invasive spatial visualization of systems with complex geometries under different temperature and pressure conditions at relatively low costs renders it favorable for both fundamental fluid mechanics research and applications in large-scale processing industries. recent research efforts in exploring these two aspects are summarized below. gas-solid illustration of the cfb reactor (left), ecvt sensor configuration at the bend (middle), and reconstructed images of the solids holdup distribution in the bend (right). gas-solid fluidized bed is a typical gas-solid flow system, and has been widely employed in chemical industries due to its superior heat and mass transfer and solid transport and handling. ecvt has been successfully applied to gas-solid fluidized bed systems for system properties measurements and dynamic behaviors visualization. an example is the study of choking phenomenon in a 0.1 m id gas-solid circulating fluidized bed with a 12-channel cylindrical ecvt sensor, where the formation of slug during transition to choking is clearly recorded by ecvt. another experiment studies the bubbling gas-solid fluidization in a 0.05 id column, where the solid holdup, bubble shape and frequency obtained from ecvt are validated with mri measurements. the flexibility of the ecvt sensor geometry also enables it for imaging of bend, tapering and other non-uniform sections of gas-solid flow reactors. for example, a horizontal gas jet penetrating into a cylindrical gas-solid fluidized bed can be imaged with a modified ecvt sensor, and information such as penetration length and width of the jet as well as jet coalescence behavior with the bubbles in the fluidized bed can be obtained from ecvt. another example is ecvt imaging of the riser and bend of a gas-solid circulating fluidized bed (cfb). a core-annulus flow structure in both the riser and the bend and a solid accumulation in the horizontal section of the bend are identified from quantitative ecvt images. gas-liquid images of the bubble plumes from ecvt (top) and the actual column (bottom). gas-liquid bubble column is a typical gas-liquid flow system that is widely used in petrochemical and biochemical processes. the bubbling flow phenomena have been extensively researched with computational fluid dynamic methods as well as traditional invasive measurement techniques. ecvt possesses the unique ability to obtain real-time quantitative visualization of an entire gas-liquid flow field. an example is the study of the dynamics of spiral bubble plumes in bubble columns. ecvt is shown to be able to capture the spiral motion of bubble plumes, the structures of large scale liquid vortices and gas holdup distributions. another example of the application of ecvt in gas-liquid systems is the study of a cyclonic gas-liquid separator, where a gas-liquid mixture enters a horizontal column tangentially and creates a swirling flow field where gas and liquid is separated by centrifugal force. ecvt successfully captures the liquid distribution inside the vessel and the off-centered gas core drifting phenomenon. the quantitative results match mechanistic models. gas-liquid-solid the trickle bed reactor (tbr) is a typical three-phase gas-liquid-solid system, and has applications in petroleum, petrochemical, biochemical, electrochemical and water treatment industries. in a tbr, gas and liquid flow downward concurrently through packed solid materials. depending on the gas and liquid flow rates, tbr can have different flow regimes, including trickling flow, pulsating flow and dispersed-bubble flow. ecvt has been successfully used to image the turbulent pulsating flow in a tbr, and detailed pulse structure and pulse velocity can be obtained from ecvt. combustion (high temperature and flame) slug velocities at varying ug-umf for various temperatures, 25°c, 300°c, 400°c, and 650°c.most of the gas-solid flow systems in chemical industries operate at elevated temperatures for optimal reaction kinetics. under such harsh conditions, many laboratory measurement techniques are no longer available. however, ecvt has the potential for high-temperature applications due to its simple and robust design and non-invasive nature, which allows for insulating materials to be imbedded in the sensor for heat-resistance. currently the high-temperature ecvt technology is under rapid development and research efforts are being made to address engineering issues associated with high temperatures. ecvt has been utilized in environments of high temperatures up to 650 °c to image and characterize fluidized beds under high temperatures such as those used in fluidized bed reactors, fluid catalytic cracking and fluidized bed combustion. the application of this technology to high temperature fluidized beds has allowed the in-depth analysis of how temperature affects flow behavior in the beds. for instance, in a slugging fluidized bed with large column height to column diameter ratio with geldart group d particles, increasing temperature up to 650 °c can change the density and viscosity of the gas, but has negligible effect on slugging behavior such as slug velocity and frequency. non-destructive testing (ndt) in the infrastructure inspection industry, it is desirable to use equipment that inspects embedded components non-invasively. issues such as corroded steel, water penetration, and air voids are often embedded within concrete or other solid members. here, non-destructive testing (ndt) methods must be used to avoid compromising the integrity of the structure. ecvt has