*4.2. Metabolic Profiling*

The sample preparation, extract analysis, metabolite identification and quantification were performed at Wuhan MetWare Biotechnology Co., Ltd. (www.metware.cn) following their standard procedures and previously described by Zhang et al. [48]

### *4.3. Metabolite Data Analysis*

Before the data analysis, quality control (QC) analysis was conducted to confirm the reliability of the data. The QC sample was prepared by the mixture of sample extracts and inserted into every five samples to monitor the changes in repeated analyses. Data matrices with the intensity of the metabolite features from the 30 samples were uploaded to the Analyst 1.6.1 software (AB SCIEX, Ontario, Canada) for statistical analyses. The supervised multivariate method, partial least squares-discriminant analysis (PLS-DA), was used to maximize the metabolome differences between the pair of samples. The relative importance of each metabolite to the PLS-DA model was checked using the parameter called variable importance in projection (VIP). Metabolites with VIP ≥ 1 and fold change ≥ 2 or fold change ≤ 0.5 were considered as differential metabolites for group discrimination [48].

#### *4.4. RNA Extraction, cDNA Library Construction, and Transcriptome Sequencing*

Total RNAs were extracted using Spin Column Plant total RNA Purification Kit following the manufacturer0 s protocol (Sangon Biotech, Shanghai, China). Purity of the extracted RNAs was assessed on 1% agarose gels followed by NanoPhotometer spectrophotometer (IMPLEN, Los Angeles, CA, USA). RNA quantification was performed using Qubit RNA Assay Kit in Qubit 2.0 Flurometer (Life Technologies, Carlsbad, CA, USA). Next, RNA integrity was checked by the RNA Nano 6000 Assay Kit of the Agilent Bioanalyzer 2100 system (Agilent Technologies, Santa Clara, CA, USA).

Sequencing libraries was created using NEB Next Ultra RNA Library Prep Kit following manufacturer0 s instructions. The index codes were added to each sample. Briefly, the mRNA was purified from 3 µg total RNA of each of three replicate using poly-T oligo-attached magnetic beads and then broken into short fragments to synthesize first strand cDNA. The second strand cDNA synthesis was subsequently performed using DNA Polymerase I and RNase H. PCR was carried out with Phusion High Fidelity DNA polymerase using universal PCR primers and index (×) primer. Finally, six paired-end cDNA libraries with an insert size of 300 bp were constructed for transcriptome sequencing and sequenced on Illumina HiSeq 2500 platform (Illumina Inc., San Diego, USA) by Biomarker Technology Corporation (www.biomarker.com.cn). The raw RNAseq data are submitted at: www.ncbi.nlm.nih.gov/bioproject/PRJNA558197.

#### *4.5. De novo Assembly, Functional Annotation, Classification and Metabolic Pathway Analysis*

The clean reads were retrieved after trimming adapter sequences, removal of low quality (containing > 50% bases with a Phred quality score < 15) and reads with unknown nucleotides (more than 1% ambiguous residues N) using the FastQC tool (http://www.bioinformatics.babraham.ac.uk/ projects/fastqc/). The high quality reads from all the 30 libraries were de novo assembled into transcripts using Trinity (Version r20140717, [62]) by employing paired-end method [63]. Next, the transcripts were realigned to construct unigenes. The assembled unigenes were then annotated by searching against various databases such as Kyoto Encyclopedia of Genes and Genomes (KEGG) [64], Gene Ontology (GO) [65], Clusters of Orthologous Groups (COG) [66], PfAM, Swissprot [67], egNOG [68], NR [69], euKaryotic Orthologous Groups (KOG) [70] using BLAST [71] with a threshold of E-value < 1.0 × 10−5 .

The software KOBAS2.0 [72] was employed to get the unigene KEGG orthology. The analogs of the unigene amino acid sequences were searched against the Pfam database [73] using HMMER tool [74] with a threshold of E-value < 1.0 × 10−10. The sequenced reads were compared with the unigene library using Bowtie [75], and the level of expression was estimated in combination with RSEM [76]. The gene expression level was determined according to the fragments per kilobase of exon per million fragments mapped (FPKM).

#### *4.6. Di*ff*erential Expression and Enrichment Analysis*

The read count was normalized and EdgeR Bioconductor package [77] was used to determine the differential expression genes (DEGs) between the two varieties at each developmental stage with the fold change > 2 [44] and false discovery rate (FDR) correction set at *p* < 0.01. GO enrichment analysis was performed using the topGO method based on the wallenius noncentral hypergeometric distribution with *p* < 0.05 [78]. KEGG pathway enrichment analysis of the DEGs was done using KOBAS2.0 [72]. The FDR correction was employed (*p* < 0.05) to reduce false positive prediction of enriched GO terms and KEGG pathways.

## *4.7. SNP Analysis*

The reads and unigene sequences of each sample were compared using the software STAR [79] and the single nucleotide polymorphism (SNP) was identified through the pipeline (SNP Calling) for RNA-Seq by GATK2 [80]. Raw vcf files were filtered with GATK standard filter method and other parameters (clusterWindowSize: 35; MQ0 ≥ 4 and (MQ0/(1.0\*DP)) > 0.1; QUAL < 10; QUAL < 30.0 or QD < 5.0 or HRun > 5), and only SNPs with distance > 5 were retained.

## *4.8. Gene Expression Using Quantitative Real Time-PCR*

The qRT-PCR was performed on RNA extracted from root samples of both varieties at the five developmental stages as described by Dossa et al. [81] using the *Actin* gene as the internal control. Specific primer pairs of 15 selected genes were designed using the Primer Premier 5.0 [82] (Table S4). The qRT-PCR was conducted on a Roche Lightcyler® 480 instrument using the SYBR Green Master Mix (Vazyme), according to the manufacturer's protocol. Each reaction was performed using a 20 µL mixture containing 10 µL of 2 × ChamQ SYBR qPCR Master Mix, 6 µL of nuclease-free water, 1 µL of each primer (10 mM), and 2 µL of 4-fold diluted cDNA. All of the reactions were run in 96-well plates and each cDNA was analyzed in triplicate. The following cycling profile was used: 95 ◦C for 30 s, followed by 40 cycles of 95 ◦C/10 s, 60 ◦C/30 s. Data are presented as relative transcript level based on the 2−∆∆Ct method [83].


**3. Results and Discussion**

Initially, the chemical structure of fabricated silica nanoparticles was examined with the help of an FT-IR spectroscopy (Perkin Elmer, Waltham, MA, USA) as shown in Figure 2. The silica nanoparticles exhibited a distinctive absorption peak at 1000–1100 cm−1, which was ascribed to a stretching vibration of Si–O–Si bonding. Another distinctive absorption peak also appeared at 1630 cm−1, which was allocated to a bending vibration by way of H–O–H in the water molecules. An additional absorption peak was further detected at 3400 cm−1, which was due to a stretching vibration of the hydroxyl group, confirming the existence of hydroxyl groups in the silica surface. Figure 2 also showed the FT-IR spectrum of vulcanized natural rubber filled with 3 phr silica composite. It is seen that the asymmetrical stretching vibration of Si–O–Si and the hydroxyl of silica in the NR/SiO2 composite appeared at 1088 cm−1 and 3403 cm−1, respectively.

**Figure 2.** FT-IR spectra of fabricated nanosilica and natural rubber (NR)/3 phr silica composite.

Regarding the crystalline structure of the samples, the powder XRD pattern in Figure 3 indicated a broad peak at 22◦ of 2θ, revealing the amorphous nature of the nanosilica particles. Furthermore, the TEM image of nanosilica in Figure 4 showed its diameter of 40–60 nm.

**Figure 3.** XRD pattern of nanosilica.

**Figure 4.** TEM of nanosilica.

Table 2 lists the main curing characteristics of the fabricated NR compounds that were obtained from the cure curves (Figure 5), in which both t2 and t90 increased with increasing silica nanoparticle content in the NR. In general, the minimum torque (ML) in Figure 5 is associated with a shear viscosity of the blend, while the maximum torque (MH) has a relation to the elastic stiffness of vulcanized samples. The results implied that with addition of nanosilica, both ML and MH values increased, meaning that the viscosity or stiffness of vulcanized NR increased with the presence of nanosilica particles. The difference between maximum torque and minimum torque, (MH-ML), which was obtained from the dynamic shear modulus test, corresponds indirectly to the crosslinking density of the vulcanization. The results indicated that the (MH-ML) exhibited the same tendency to the maximum torque and increased with increasing nanosilica loading as a result of increased cross-linking of the NR with the presence of nanosilica particles.

Figure 6 presents typical stress-strain curves of the nanosilica-reinforced NR composite materials. All samples exhibited the deformation-forced crystallization characteristics with the rapidly raised sharp slopes when the strain reached more than 1500% [36,43,44]. Meanwhile, the tensile stress and slope curves increased with increased contents of silica and reached the optimal properties at 3 phr of silica filled NR. The details of the mechanical characteristics are provided in Table 3.

**Table 2.** Curing properties of the silica/rubber compounds.

**Figure 5.** Cure curves of the investigated rubber compounds taken at 150 ◦C (black line—M0; red line—M1; blue line—M2; pink line—M3; olive line—M4; green line—M5).

**Figure 6.** (**a**) Typical stress-strain of cured samples (Black line—M0; red line–M1; blue line—M2; pink line—M3; olive line—M4; orange line—M5). (**b**) The bottom figure is the one expanded in the initial stage;

**Table 3.** Mechanical properties of cured NR/silica compounds.

The tensile strength exhibited the decreasing trend when the silica content exceeded 3 phr. Although the elongation at break decreased with increased silica concentration, the hardness of the NR showed the same trend with the tensile strength. Both changes in the tensile strength and elongation at break for different nanosilica loadings have a relationship with a cross-linking density of the cured NR composites. That of the NR increased with silica concentration, resulting in an increase of the tensile strength and a decrease of the elongation at break due to the decreased slippage among the molecular chains. The H-bond between silica particles and rubber chain prevented the mobility and slippage of the rubber chain. On the other hand, the fact that tensile strength only increased with silica contents up to 3 phr may be due to the agglomeration of nanosilica at a higher content. The crosslinking densities of samples with name M0, M1, M2, M3, M4, M5 were 7.82 × 105, 8.06 × 105, 8.19 × 105, 8.22 × 105, 8.36 × 105 and 8.45 × 105 mol/cm3, respectively. The trend of crosslinking density is the same as with the trend of MH and hardness. The hardness also increased with increased silica content. The incremental increase of crosslinking density resulted in the decreasing of elongation at break because of the prevention of the slip between each NR molecule. In addition, more energy was needed to break the linkage between each rubber molecule as the result of increased tensile strength.

The SEM image results of the tensile fractured surface of the NR composites both in the absence and presence of 3 phr of nanosilica were presented in Figure 7. These results indicated that the fractured surface of the 3 phr silica-filled NR was observed to be rougher in comparison with the pristine sample. Therefore, extra energy was required compared to the case of the smooth fractured surface of the pristine NR. This result is in agreement with the tensile results shown in Figure 6.

**Figure 7.** SEM images of the fracture surface of pristine natural rubber and 3 phr silica filled natural rubber.

The TEM images of both the natural rubber and 3 phr silica filled natural rubber are shown in Figure 8. The pristine NR sample had no silica particles, while the silica particles existed in the modified NR with nano scale from 20–60 nm.

**Figure 8.** TEM images of pristine natural rubber (**a**) and 3 phr silica filled natural rubber (**b**).

The thermal degradation of both the pristine NR and the 3 phr nanosilica filled NR composites was examined in terms of the weight loss (%) as a function of temperature, as shown in Figure 9.

**Figure 9.** TGA of cured natural rubber (NR) (black dot line) and 3 phr of nanosilica filled natural rubber (red dot line).

Thermal degradation of the NR can be explained via various processes such as chain-scission of the polymers, and breakage of the cross-linked portion. The silica filled NR composite exhibited a 2.5% higher decomposition temperature in comparison with the pristine NR because of the existence of H-bonds between silica particles and rubber chains with higher thermal stability. The thermal degradation of both pristine and silica filled NR occurs through the degradation of isoprene units at around 368 ◦C. The presence of nanosilica induced the higher residues of nanocomposite when compared with pristine NR due to the presence of inorganic filler, which is more thermally stable than NR. The TGA results demonstrate that the nanosilica filled NR showed only a very slightly higher thermal stability than that for the NR. The initial temperature decomposition, the maximum decomposition temperature, and residue of silica modified NR and pure NR were 268.2 ◦C, 368.2 ◦C, 1.1% and 266.8 ◦C, 366.3 ◦C, 3.7%, respectively. The entire thermal degradation of the nanocomposite can be explained by the two-step process. First of all, the rubber chains and cross-linking were deteriorated into smaller parts. In the second step, the smaller parts in the first step continuously degraded into volatile products and disappeared. The residual char was higher for the nanosilica-filled sample.

Figure 10 shows the XRD patterns of vulcanized pure NR and NR/3 phr silica nanocomposite. The broad diffraction peak around 20◦ is the noncrystalline structure of NR, while the diffraction peaks

between 30◦–50◦ are assigned to ZnO particles in the vulcanizates. None of the samples show obvious characteristic peaks of graphite or silica, indicating that silica particles are homogeneously dispersed in the rubber matrix. These results indicated that the silica did not keep the amorphous structure when embedded in natural rubber.

**Figure 10.** XRD spectra of vulcanized NR and NR/3 phr silica.

#### **4. Conclusions**

In this study, amorphous silica nanoparticles (between the sizes of 40–60 nm) were extracted from hexafluorosilicic acid waste produced by the Vietnamese fertilizer industry via a precipitation process. The production and utilization of the nanosilica by-product could become a reliable and sustainable solution for dealing with waste water from fertilizer plants environmentally as well as economically, regarding the waste water treatment. The resulting silica nanoparticles were then adopted as the filler for NR. The tensile strength of 3 phr silica-added NR nanocomposites increased by 20.6% compared to that of pristine NR. The elongation at break decreased with increased filler content and the hardness of the filled sample increased with increasing nanosilica content. The filled sample also exhibited better thermal resistance than the pristine sample due to the presence of nanosilica.

**Author Contributions:** Data curation, C.M.V., B.X.K.; formal analysis, C.M.V.; investigation, V.-H.N.; validation, H.J.C.; writing—original draft, C.M.V.; writing—review and editing, H.J.C.

**Funding:** This work was funded by Vietnam National Foundation for Science and Technology Development (grant number: 104.02-2017.15). One of the authors (H.J.C.) was partially supported by National Research Foundation of Korea (2018R1A4A1025169).

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


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Myers, J.; Moore, J.; Ojha, M.; Johnston, K. & Ethier, C. (2001). Factors influencing blood flow

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**4. Five basic and stable features**

In this section five basic features will be described, that can help us to distinguish leadership abilities that were mentioned in the Introduction. Usually it is not possible to predict with certainty who will fit a leadership position (there are too many unknown criteria that should be met) but it is often possible to predict who will probably not be able to manage the leadership in a satisfactory way. Thus, only indications of poor leadership will be focused.

#### **4.1. Trust in others**

*Trust in others*—an important part of the IWM—is shaped early during responsive caregiving. The term trust in others (which originates from Bowlby [34] and regards infants) has in adults a wider meaning than just to trust other persons. In adults, a more suitable term could be to *understand others' intentions* [18, 19]. Having that ability—or feature as it is called here—facilitates trust in others.

#### *4.1.1. Understanding is not necessary for interaction*

For our successful interaction with others, a good understanding of their intentions may appear to be a prerequisite. However, it is also possible to interact successfully without really understanding much of others' intentions. As long as the own actions are relevant responses to others' behaviors, the interaction is perceived as successful, whether others' intentions are understood or not. So, normally, relying on an adequate repertoire of schemas may work quite satisfactory. In fact, quite often, those who are perceived as markedly socially competent put an effort on such an attitude because they have a poor early development [18, 42] (they want to "belong") and because they have a deficient understanding of others' minds [4, 6, 19].

Although social competence is very important in situations when it is required, it should never be a major merit for a leader. When social competence is a major attribute of a person, we can be pretty sure that this person has been carried far in her/his career without much of other essential merits. If so, this is an indication of a deceitful conduct, and we can assume that it is also a cover for a number of severe deficiencies.

#### *4.1.2. Cooperation is not a matter of course*

For a person who has a difficulty to understand others' intentions, a fear for negative impact (such as being used, undervalued or criticized) is not well balanced by an insight in others' true—and often positive—intentions. For this person, selfless cooperation for mutual profit is not a matter of course [43]. Often, other persons' kind offers to help are perceived as derogating criticisms [44] and other persons' humble requests for help are perceived as unjustified and stressful demands [16]. When such a person collaborates or helps others, often it is only to get credit for the positive attitude, and it is not an altruistic effort to accomplish a good result for the benefit of all or to unselfishly help others [16, 45].

### **4.2. Trust in self**

the scope of this chapter. However, for the purpose of identifying detrimental leaders, we can

In this section five basic features will be described, that can help us to distinguish leadership abilities that were mentioned in the Introduction. Usually it is not possible to predict with certainty who will fit a leadership position (there are too many unknown criteria that should be met) but it is often possible to predict who will probably not be able to manage the leader-

*Trust in others*—an important part of the IWM—is shaped early during responsive caregiving. The term trust in others (which originates from Bowlby [34] and regards infants) has in adults a wider meaning than just to trust other persons. In adults, a more suitable term could be to *understand others' intentions* [18, 19]. Having that ability—or feature as it is called here—facilitates trust in others.

For our successful interaction with others, a good understanding of their intentions may appear to be a prerequisite. However, it is also possible to interact successfully without really understanding much of others' intentions. As long as the own actions are relevant responses to others' behaviors, the interaction is perceived as successful, whether others' intentions are understood or not. So, normally, relying on an adequate repertoire of schemas may work quite satisfactory. In fact, quite often, those who are perceived as markedly socially competent put an effort on such an attitude because they have a poor early development [18, 42] (they want to "belong") and because they have a deficient understanding of others' minds [4, 6, 19]. Although social competence is very important in situations when it is required, it should never be a major merit for a leader. When social competence is a major attribute of a person, we can be pretty sure that this person has been carried far in her/his career without much of other essential merits. If so, this is an indication of a deceitful conduct, and we can assume

For a person who has a difficulty to understand others' intentions, a fear for negative impact (such as being used, undervalued or criticized) is not well balanced by an insight in others' true—and often positive—intentions. For this person, selfless cooperation for mutual profit is not a matter of course [43]. Often, other persons' kind offers to help are perceived as derogating criticisms [44] and other persons' humble requests for help are perceived as unjustified and stressful demands [16]. When such a person collaborates or helps others, often it is only to get credit for the positive attitude, and it is not an altruistic effort to accomplish a good result

ship in a satisfactory way. Thus, only indications of poor leadership will be focused.

use the following five basic features quite well.

*4.1.1. Understanding is not necessary for interaction*

that it is also a cover for a number of severe deficiencies.

for the benefit of all or to unselfishly help others [16, 45].

*4.1.2. Cooperation is not a matter of course*

**4. Five basic and stable features**

**4.1. Trust in others**

106 Leadership

*Trust in self* is initially built up by feedback from the caregiver [18], both positive and negative, both verbal and nonverbal. In this is included the picture of the self, abilities, personal significance and rights that the parents usually imprint [32]. Later on, also perceived success and failure are important factors.

#### *4.2.1. Dependence on others' opinions*

For a person who has a poor early development, the understanding of self is too minimal to be of much help, and the opinions from other persons will become more important. S/he will spend much efforts to assure that the desires for self-worth and social "belonging" are met [16, 18]. During the years when the youngster is very keen on adjusting to the social environment where s/he wants to be accepted, this person is extra vulnerable since the externally perceived impressions are not well balanced by an internal guide of reference. The result may be an oversensitive person who is "streamlined" to fit whatever is required: performance, looks, language use, and so on and who may become a high performing, insensitive, and intolerant young adult [41].

This kind of (over-)sensitivity (which is a lack of self-worth and resilience) should not be mistaken for the sensitivity that comes with an ability to be receptive/observant and to understand another person. The first is a self-centered attitude and an indication of poor early development, whereas the latter emerges from a genuine interest and an insight in other persons minds and is commonly not a clearly exposed ability.

#### *4.2.2. High self-esteem or inflated self-esteem?*

A young person who has a poor early development often is anxious to be accepted, or rather, to be appreciated, and is prone to do what is required to reach that goal. The appreciation (n.b., often in a rather narrow area) may give this person a sense of high self-esteem, which, as discussed in more detail by Baumeister et al. [40], should not be mistaken for a promise of a generally high performance. It may in fact be an indication of the opposite.

#### *4.2.3. Conflict management*

For a person who has a poor early development it is more difficult to manage conflicting situations (regarding moral issues as well as personal conflicts) as external impressions are not well balanced by a firm self-worth [16].

#### **4.3. Flexibility**

In adolescents and adults3 *flexibility* can be said to be the cognitive capacity to simultaneously handle and change between alternatives in values, plans and actions. Inherent in this is also the ability to handle uncertainty, as choosing among alternatives implies uncertainty [36].

3 In infants, flexibility is a free will to explore the environment [28] and an ability "flexibly to change focus between toys and the parent" [20].

#### *4.3.1. Flexibility, a demanding task for the brain*

Handling complexity often requires that you have more than one thoughts in mind at the same time. Although the brain is running multiple processes simultaneously all the time (vision, hearing, balance, etc.), having several conscious thoughts in mind requires something extra [26]. Flexibility may be described in terms of how much complexity that conscious brain activities involve, in the following ways:

*4.4.1. The emergence of truthfulness*

tress or when other things get a higher priority.

• Obvious lying or bluffing.

*4.4.2. Habitual untruthfulness*

Untruthfulness may be manifest in a few slightly different ways:

nection to reality – which is not any better in a leader position.

leadership abilities s/he can be assumed to have.

*4.4.3. Untruthfulness, the reliability aspect*

• Having a hidden agenda, that is, intending to do something else than what you say.

that you have "a better value" than people who do not have or do the same.

• Idealization, that is, describing something (primarily your own state) as better than it is. • Exposing prestigious objects or other circumstances (title, relations, lifestyle, etc.) depicting

Concerning persons who have an inflated self-concept, if they really believe in their selfconcept, maybe they are not really lying or bluffing but instead they display a deficient con-

Being habitually untruthful means that the reality is not more important to the person than the fiction that s/he presents, or the consequences of that fiction. Everybody is not habitually untruthful [15], and it is also a matter of degree; the more untruthful s/he is—the less general

To most people, obvious lying is not acceptable. However, there are forms of untruthfulness that, among many people, are not only accepted but even seen as positive, as a merit. A very common example of this is when somebody pretends to be "bigger" or better in some respect, displayed in body language and appearance, in possessing prestigious items, or engagement in prestigious contexts. Strictly, these actions are meant to betray the observer (although many people do not even perceive it as betrayal), and they are accordingly untruthful acts.

When a person is habitually untruthful, s/he puts the priority of the own advantages over the priority of others (the family, company and society), particularly so, if the untruthfulness

A child that is 3 years old does not understand untruthfulness, and an alternative to truth is not comprehensible. In psychology, this is called the false belief principle [32]. This means that untruthfulness does not exist to the child during the very important time when the system is developed that will guide the person during the rest of life. The child who gets a rich early development will accordingly get a firm belief that truth is fundamental. The child who got a poor early development and was not able to build a conception of the world and self until after 4 years of age, was aware of untruthfulness by then. This child built its conception of the world with both truth and fiction/lies as feasible options that can be used as they were needed. Although a truthful attitude and behavior may be strongly influenced by parents and other factors (school, church, neighbors, friends, and society) during childhood and adolescence, there seems to emerge a particular intrinsic truthfulness from a rich early development. An important difference is that the later learned truthfulness may be easily suppressed by dis-

Knowledge in Neuroscience Can Help Us Avoid Underperforming Leaders

http://dx.doi.org/10.5772/intechopen.78564

109

A person who got a poor early development typically does not seem to have much of this ability (else than in a narrow area of knowledge). This person also often has rather a shortsighted approach to planning and decisions [16, 44, 46].

#### *4.3.2. Dependence on schemas*

The inflexible person prefers to do what is customary in the social (or work) environment where s/he wants to be accepted (or where s/he wants to be successful) and avoids actions that may risk desired acceptance or success. As an adult, this attitude may take the form of a dedication to social schemas that warrant a successful result [31, 47].

In an organization, this may show as a person who only does what important persons (or authorities) desire or permit. This often is a person who has a "perfect performance" or who is a "literal follower of the rules" (and who also often avoids uncertainty) and therefore never makes any mistakes. The lack of mistakes is often perceived by other persons as a sign of good judgment and skill, whereas it very well could be regarded as an indication that a good judgment in fact probably is missing.

It is usually important to follow the rules that apply but when following rules replace good judgment, then you might assume that good judgment actually is missing.

#### **4.4.  (However, children may very well experiment with untruthful statements before the perception of reality has matured).

#### *4.4.1. The emergence of truthfulness*

*4.3.1. Flexibility, a demanding task for the brain*

activities involve, in the following ways:

approach to planning and decisions [16, 44, 46].

in mind at the same time.

demanding.

108 Leadership

*4.3.2. Dependence on schemas*

judgment in fact probably is missing.

**4.4. Truthfulness**

Handling complexity often requires that you have more than one thoughts in mind at the same time. Although the brain is running multiple processes simultaneously all the time (vision, hearing, balance, etc.), having several conscious thoughts in mind requires something extra [26]. Flexibility may be described in terms of how much complexity that conscious brain

• When you compare the current state of things with an alternative state of things (to make a decision about, e.g., what is best), you might need to keep more than one thought or focus

• When we consider "cause and effect" connections or time dependent processes, we also often must have more than one conscious thought in mind or more than one potential focus. An application of this is a person's attitude to rules and their intentions: One can regard the intentions behind a rule as a cause, the rule as a means and when the rule is followed as an effect. To the inflexible person, such distant connections may seem too

A person who got a poor early development typically does not seem to have much of this ability (else than in a narrow area of knowledge). This person also often has rather a shortsighted

The inflexible person prefers to do what is customary in the social (or work) environment where s/he wants to be accepted (or where s/he wants to be successful) and avoids actions that may risk desired acceptance or success. As an adult, this attitude may take the form of a

In an organization, this may show as a person who only does what important persons (or authorities) desire or permit. This often is a person who has a "perfect performance" or who is a "literal follower of the rules" (and who also often avoids uncertainty) and therefore never makes any mistakes. The lack of mistakes is often perceived by other persons as a sign of good judgment and skill, whereas it very well could be regarded as an indication that a good

It is usually important to follow the rules that apply but when following rules replace good

In attachment research it has been well documented that adult persons with a secure attachment are inclined to tell the truth and that insecurely attached adult persons are prone to allow themselves to be untruthful [20, 29]. (However, children may very well experiment with

dedication to social schemas that warrant a successful result [31, 47].

judgment, then you might assume that good judgment actually is missing.

untruthful statements before the perception of reality has matured).

A child that is 3 years old does not understand untruthfulness, and an alternative to truth is not comprehensible. In psychology, this is called the false belief principle [32]. This means that untruthfulness does not exist to the child during the very important time when the system is developed that will guide the person during the rest of life. The child who gets a rich early development will accordingly get a firm belief that truth is fundamental. The child who got a poor early development and was not able to build a conception of the world and self until after 4 years of age, was aware of untruthfulness by then. This child built its conception of the world with both truth and fiction/lies as feasible options that can be used as they were needed.

Although a truthful attitude and behavior may be strongly influenced by parents and other factors (school, church, neighbors, friends, and society) during childhood and adolescence, there seems to emerge a particular intrinsic truthfulness from a rich early development. An important difference is that the later learned truthfulness may be easily suppressed by distress or when other things get a higher priority.

Untruthfulness may be manifest in a few slightly different ways:

Concerning persons who have an inflated self-concept, if they really believe in their selfconcept, maybe they are not really lying or bluffing but instead they display a deficient connection to reality – which is not any better in a leader position.

#### *4.4.2. Habitual untruthfulness*

Being habitually untruthful means that the reality is not more important to the person than the fiction that s/he presents, or the consequences of that fiction. Everybody is not habitually untruthful [15], and it is also a matter of degree; the more untruthful s/he is—the less general leadership abilities s/he can be assumed to have.

To most people, obvious lying is not acceptable. However, there are forms of untruthfulness that, among many people, are not only accepted but even seen as positive, as a merit. A very common example of this is when somebody pretends to be "bigger" or better in some respect, displayed in body language and appearance, in possessing prestigious items, or engagement in prestigious contexts. Strictly, these actions are meant to betray the observer (although many people do not even perceive it as betrayal), and they are accordingly untruthful acts.

#### *4.4.3. Untruthfulness, the reliability aspect*

When a person is habitually untruthful, s/he puts the priority of the own advantages over the priority of others (the family, company and society), particularly so, if the untruthfulness regards circumstances concerning his/her own person. Hence, if your subordinate, wife/husband or friend is untruthful to others, s/he also is prone to be untruthful to you.

more practical leadership contexts. To emphasize that a reference only regards the state of the art that the inferences are based on, and that the reference does not state a verified connection between early experiences and leadership, that reference is enclosed in an extra parentheses. The presented concept is, to my knowledge, not published before, and there are few or no studies that have studied the following inferred connections. Consequently, there is little or no strictly scientific evidence for these examples. Even so, they may be better alternatives than some of the rather unfounded concepts that are used today. This section can also be seen as a

Knowledge in Neuroscience Can Help Us Avoid Underperforming Leaders

http://dx.doi.org/10.5772/intechopen.78564

111

There are a few researchers who have touched upon an analysis of the background of inferior leadership; examples are Adorno et al. [15], Argyris et al. [50], Fors Brandebo et al. [51], Friedman et al. [44], Heckman [52], McCrae et al. [13] and Rubenowitz [53]. Of these, Argyris et al. have published the most complete analysis when they characterized and compared

There are a number of typical behaviors and attitudes that signify particular unproductive or detrimental mental states. It would certainly have been nice to list them here as a help for the reader. However, doing so would impede the value of them as predictors, as those who are interested in success without effort would misuse this knowledge. The following are

The person with a rich early development does normally not present much problems due to dishonesty, self-centered decisions or lack of responsibility [16, 50]. The problems they bring as leaders are often due to deficiencies that do not regard stable features, which means that they usually can be improved. Common to these persons are a low need to display prestige (so they often have a "low profile" and are not very "visible") [15, 40, 53], they do not strive for power for power's own sake (so they may lack an impressing career record) [15, 53], they may have a "democratic" stance in their leadership [51] (which in authoritarian cultures often is seen as a weak leadership) [50], they dare to do things their own way (they are often not streamlined) [15, 51], they may venture into uncertain areas or assume difficult undertakings (with a risk for substandard results, as perceived by outsiders) [15], and they do not skillfully disguise their shortcomings or aggressively deny their faults [15, 47, 50]. To many people, they do not seem as suitable for a leadership position—which may be a great mistake as it is among those that you will find the high potentials [15, 50]. Hence, this is a very diverse group

Person (1,1) (rich early development and rich life management support) may seem as the perfect choice for a leader position but that depends on what flavor of life management support that has been supplied. Although they often may have a high basic potential for successful leadership, they may have other priorities in life. As leaders, they are normally not connected

*Model I* vs. *Model II Theory-in-use* and *Single* vs. *Double Loop Learning*.

that cannot be distinguished by a simple template judgment [20].

to problems and they are accordingly not within the focus of this chapter.

collection of suggestions for future research.

*5.1.1. My best tips should not be exposed in public*

examples of general behaviors that you may see.

*5.2.1. The person with a rich early development*

**5.2. A few general characteristics**

#### **4.5. Intrinsic sense of responsibility**

Our sense of responsibility has two components: one intrinsic and one extrinsic. The *intrinsic sense of responsibility* is believed to emerge from the same location in brain as some of the IWM do [48]. How strong it is can be assumed to depend on how rich the early development was and hence how strong and dominant the early created control system is. The extrinsic responsibility emerges from the parts in our brain that are dedicated to reading, writing, counting and other abilities controlled by rules [48] and can be assumed to have been shaped at later stages of development than the intrinsic sense of responsibility. The balance between intrinsic and extrinsic depends on the balance of influences during childhood and adolescence.

#### *4.5.1. Requirements for responsibility*

Winter and Barenbaum [49] published a scoring system for responsibility, built on five descriptors: # 1 Moral standard. # 2 Obligation. # 3 Concern for others. # 4 Concern about consequences. # 5 Self-judgment.

Hence, if these criteria are not met—that is, if the scores are low—it may be reasonable to assume that the intrinsic sense of responsibility of the individual is impaired. In persons who have a poor early development, two of these criteria, # 3 Concern for others and # 5 Selfjudgment commonly are not met [20]. Therefore, persons who have a poor early development can generally be assumed to have an impaired sense of responsibility.

#### **4.6. The basic features indicate leadership abilities**

The general leadership abilities may be indicated by the basic features, as follows: (Note however, that also the influences from relevant life management support on the development of abilities are important [17, 32, 33].) The ability to spontaneously cooperate with and understand other persons is related to the feature trust in others (i.e., to understand others' intentions) [18, 19]. The ability to have integrity in conflicts is related to the feature trust in self [16]. The ability to handle complexity and uncertainty is related to the feature flexibility [15, 16]. The abilities to have a good moral judgment [17] and to distinguish between appearance and reality [20] are related to a combination of all the five basic features.


**1. Introduction**

The amount of energy requirement has increased as part of a rapidly growing population, which directly causes several problems in terms of environmental sustainability and an economic crisis. The deficiency in conventional energy resources such as petroleum and natural gases has been a struggle for the sustainable development goals.

Several proven alternative energy resources have been developed to partially replace the main energy resources. Lignocellulosic biomass represents its potential for producing various value-added products including biofuels, biochemicals, and biomaterials via a biorefinery [1–4]. The utilization of lignocellulosic material as a renewable carbon source for high value-added products has been developed over the last decade [5]. The lignocellulosic biomass is typically composed of three major natural biomolecules including cellulose

**Citation:** Phojaroen, J.; Jiradechakorn, T.; Kirdponpattara, S.; Sriariyanun, M.; Junthip, J.; Chuetor, S. Performance Evaluation of Combined Hydrothermal-Mechanical Pretreatment of Lignocellulosic Biomass for Enzymatic Enhancement. *Polymers* **2022**, *14*, 2313. https:// doi.org/10.3390/polym14122313

Academic Editor: Domenico Acierno

Received: 11 May 2022 Accepted: 3 June 2022 Published: 8 June 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/).

hemicellulose and lignin, which are associated together with complex chemical bonding to form a recalcitrant structure [6,7]. This structural complexity and recalcitrance make the proper lignocellulosic valorization become more difficult and have a low conversion yield [8–10]. However, the scientific breakthrough and research on lignocellulosic valorization have been emphasized to be more applicable on a large scale in order to reduce the environmental concerns. To maximize the use of lignocellulosic biomass, one of the most important processes in a biorefinery is the pretreatment process, which is responsible for altering and deconstructing the complex structure for the release of active components to be converted into active component biomolecules for further applications. The pretreatment process mostly affects the physical and chemical barriers of the lignocellulosic structure for its enhancement [11,12].

The existing pretreatment technologies have been divided into four categories including physical, chemical, physicochemical, and biological pretreatment. The physical pretreatment consists of alleviating the physical properties (crystallinity, degree of polymerization, and porosity) of the material by using several technologies such as mechanical size reduction and irradiation. The chemical pretreatment is mostly the use of acid and alkaline pretreatments, which are typically responsible for breaking down the chemical bonding in the lignocellulosic structure. Moreover, ionic liquid and deep eutectic solvents (DESs) have recently been developed for lignocellulosic pretreatment, which significantly enhances the delignification. The physicochemical pretreatment mostly consists of the steam explosion and liquid hot water, and the biological pretreatment consists of mainly using the enzymes of fungi for the removal of lignin [1,7].

However, these pretreatment technologies possess drawbacks such as high energy consumption, undesired waste generation, high installation costs, and long duration. The pretreatment process basically counts for 40% of the total capital cost of lignocellulosic valorization because of the high energy consumption, high chemical usage, and undesired waste generation [2,13]. Even though the existing pretreatment technologies lead to the enhanced utilization of lignocellulosic material, the single pretreatment process is inadequate to cover all of the biorefinery processes in terms of the technological, economic, and environmental considerations. Moreover, the existing biorefinery process is still developing because of the high investment cost, in particular, the pretreatment process, which generally counts for 30–40% of the total investment cost.

The recent challenges in developing the proper biorefinery processes include the reduction in investment costs, no undesired waste generation, no inhibitors for biological conversion, and a comparable production yield. Several studies have revealed that the combination of pretreatment has been effective in various aspects such as high active component recovery, less waste generation, economic feasibility, and environmentallyfriendly processes.

Hydrothermal pretreatment presents an industrial potential for integration into the biorefinery process. It consists of hemicellulose solubilization and lignin removal via the autohydrolysis reaction of water at high temperature and pressure, and in particular, this process does not require the addition and recovery of chemicals different from water, and it can be said that hydrothermal processes are environmentally-friendly fragmentation processes [14,15]. Autohydrolysis is used to pretreat the lignocellulosic material to disrupt the linkage between cellulose-hemicellulose and lignin, which could enhance the enzymatic hydrolysis. Several cellulosic enzymes such as cellulase provide an effective saccharification, in particular, cellulose [16].

The hydrothermal pretreatment consists of using the liquid state of water at high pressure and temperature to form an H3O+ for hemicellulose solubilization and partial lignin removal [17]. The hydrothermal pretreatment is typically operated at an elevated temperature between 140–200 ◦C and high pressure in the range of 10–20 bar in a Parr reactor [18,19]. A number of studies on hydrothermal pretreatment indicated its efficiency to solubilize the hemicellulose component by approximately 60–80% compared to the untreated samples [20,21]. Furthermore, the hydrothermal pretreatment, along with the chemical catalysts, could achieve a high lignin removal for further enzymatic accessibility.

On the other hand, one of the most efficient physical pretreatments is mechanical size reduction, which can tackle the physical properties including cellulose crystallinity and the degree of polymerization of lignocellulosic material for further processes [22–24]. However, the mechanical size reduction process typically needs a high energy input to break down the internal structure, and this technology has its own limitations with regard to integration into the biorefinery process.

To overcome these aforementioned limitations of pretreatment technology, a combination of hydrothermal and mechanical pretreatment is an effective and promising strategy in an integrated biorefinery for the production of biofuels, biochemicals, and biomaterials [25]. The hydrothermal pretreatment, followed by mechanical size reduction, leads to the solubilization of the hemicellulose and partial lignin removal for enzymatic enhancement as well as making the material softer and more porous. Moreover, the combined process affects the crystallinity of the cellulose to be more amorphous, thus increasing the reactivity of the particles in subsequent processing [26]. This current study investigated the effect of a combined hydrothermal-mechanical pretreatment of corncob on its performance on the production of fermentable sugars. The combined process was evaluated in terms of energy efficiency and waste generation compared to its efficiency in enhancing the production yield of fermentable sugars. Two established indicators, energy efficiency and waste generation, can be used as the crucial factor for the further techno-economic analysis of the process. The process integration could define the holistic bioethanol production from corncob material. This research could lead to further development, application management, and valorization of agricultural byproducts to create a bio-circular economy and to decrease CO2 emissions, which would consequently slow down global warming.

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

#### *2.1. Raw Material Preparation*

Corncob (CC) was obtained from farmers in Maetha District, Lampang Province, Thailand. Samples were oven-dried until the moisture content in the samples was approximately 8–10%. Samples were coarsely ground by knife milling to obtain the particle size of approximately 2–4 mm and subjected to a sieve with a screen size of 1 mm. The samples were stocked in a Ziploc plastic bag at room temperature for the further pretreatment process.

### *2.2. Hydrothermal Pretreatment*

CC was pretreated by using a hydrothermal technique without a chemical catalyst. A sample of 30 g CC was placed in a stainless-steel Parr reactor 1 L. This pretreatment was conducted by using distilled water boiled under a pressure of 20 bar and a temperature between 140–180 ◦C for two different durations (30 and 60 min) and a rotation speed of 200 rpm [26]. The solid fraction was collected and washed with distilled water for neutralization and oven-dried at 60 ◦C to obtain a constant weight. The dried CC was stocked at room temperature for the mechanical size reduction process.

#### *2.3. Mechanical Pretreatment*

In this study, the CC samples hydrothermally pretreated at three different temperatures and two different durations were used to conduct the mechanical size reduction. The CC samples were ground by using centrifugal milling using a 0.25 mm screen size. The centrifugal milling was equipped with a digital wattmeter to measure the intensity and voltage during the grinding process.

In the case of the specific energy consumption calculation due to the mechanical size reduction, this can be solved with Equation (1)

$$E\_{sp} = \int\_{t\_0}^{t} (P\_t - P\_{t\_0})dt / m \tag{1}$$

where *Esp* is the total specific energy consumption (kWh/kg); *Pt* is the power in watt consumed at time *t*; *Pt0* is the average power consumption in Watt under empty conditions (without biomass); and m is the mass in kg of material to be ground.

#### *2.4. Monosaccharide Concentration Analysis*

The monosaccharide concentration including glucose, xylose, and arabinose of the biomass was determined using the standard method for biomass analysis provided by the National Renewable Energy Laboratory (NREL), Golden, CO, USA [27]. The concentration of monosaccharides such as glucose and xylose in the soluble fraction was measured using high-performance liquid chromatography (CTO-10AS VP, Shimadzu, Kyoto, Japan) equipped with an Aminex HPX-87 H column (Bio-Rad Laboratories, Inc., Hercules, CA, USA). The column temperature was 65 ◦C with 0.005 M of sulfuric acid as the mobile phase at a flow rate of 0.5 mL/min. Monosaccharides of an analytical grade with a known concentration were used as the standards [12].

### *2.5. Enzymatic Saccharification*

Enzymatic hydrolysis of the untreated and pretreated CC was performed by using commercial enzymes Cellulast CTec2 (Novozymes, Belgrave, Denmark). The reaction (5 mL total volume) contained 5% of the solid biomass sample (on a dry weight basis) with 10 FPU/g enzyme loading in 50 mM of sodium acetate buffer, pH 5 adjusted with acetic acid. Sodium azide was added at the end of the experiment to inhibit microbial growth. The reaction was incubated at 50 ◦C for 72 h with 200 rpm agitation [12]. The experiment was performed in triplicate. The amount of monosaccharide concentration was quantified by HPLC as described above. Moreover, the enzymatic efficiency was calculated as:

$$\text{Enzyme Efficiency (\%)} = \frac{\text{The amount of glucose (kg)}}{\text{The amount of cellulose (kg)}} \times 100\tag{2}$$

#### *2.6. Energy Efficiency and Waste Generation Evaluation*

Energy efficiency was used to evaluate the performance of an integrated hydrothermalmechanical pretreatment. It was defined by the ratio of output and input, where the input is the total energy consumption and the output is the total fermentable sugars released by enzymatic hydrolysis. The energy efficiency was calculated according to Chuetor et al. [12] as:

$$\text{Energy Efficiency} \left(\frac{\text{kg}}{\text{kWh}}\right) = \frac{\text{Total amount of permeate sugar} \left(\text{kg}\right)}{\text{Total energy consumption (kWh)}} \tag{3}$$

Waste generation was used to investigate the undesired waste generated during the pretreatment process, which was defined by the ratio of total waste generation and the total amount of fermentable sugars obtained from enzymatic hydrolysis. The waste generation was calculated as:

$$\text{Waste generation} \left(\frac{\text{kg}}{\text{kg}}\right) = \frac{\text{Total amount of waste generation (kg)}}{\text{Total amount of fermentation} \left(\text{kg}\right)}\tag{4}$$

where the total waste is calculated by the difference between the total reactant (biomass + water) and the total fermentable sugars.

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

*3.1. Effect of Temperature and Duration on Enzymatic Hydrolysis Efficiency*

Hydrothermal pretreatment of CC was conducted at different temperatures ranging from 140–180 ◦C with two different durations (30 and 60 min). Figure 1 illustrates the variation of the biochemical compositions, cellulose, hemicellulose, lignin, and others of each sample. It was noticed that the cellulose content relatively increased with the increasing temperature whereas the hemicellulose content decreased in a solid fraction. The

maximum cellulose content was 52.90%, obtained by CC 180 ◦C at 60 min, which corresponded to a 100% increase compared to the native CC. Furthermore, Figure 1 indicates that the hemicellulose content trended to decrease significantly because of its solubilization during hydrothermal pretreatment. Imman et al. revealed that the hydrothermal pretreatment of corncob was effectively caused by the solubilization of hemicellulose, which was due to the hydronium ions being responsible for breaking down the cellulose– hemicellulose–lignin linkages [28,29]. Concerning the lignin content, it still remained in the solid fraction and was hard to remove during hydrothermal pretreatment, which would certainly restrict the enzymatic hydrolysis of CC due to the lignin content and its derived inhibition impact [30,31].

**Figure 1.** The biochemical composition and enzymatic hydrolysis efficiency of corncob pretreated by the hydrothermal pretreatment.

From the aforementioned results in the biochemical composition, it interestingly noticed that the hydrothermal pretreatment was effective for hemicellulose solubilization and cellulose enrichment, which was subsequently favorable for further enzymatic hydrolysis [32,33]. Figure 1 also shows that the enzymatic efficiency increased with the temperature. The highest enzymatic efficiency was 75.30% obtained by CC at 180 ◦C at 30 min, which corresponded to a 216% increase compared to a native CC. This large augmentation of enzymatic hydrolysis efficiency was due to the hemicellulose solubilization during hydrothermal pretreatment [34].

#### *3.2. Evolution of Energy Consumption during the Combined Pretreatment Process*

In this study, the CC was pretreated hydrothermally in the Parr reactor at three different temperatures (145, 165, and 180 ◦C) and two different durations (30 and 60 min), followed by mechanical size refining, and evaluated for energy consumption and enzymatic hydrolysis. The combined hydrothermal-mechanical pretreatment led to a decrease in the particle size, which subsequently increased the specific surface area for further enzymatic digestibility. Table 1 shows the amount of energy used in each process of the combined pretreatment method. The total energy consumption during the hydrothermal pretreatment increased when the temperature increased, while the energy consumption during the grinding process decreased because the CC structure had been destroyed due to the hydrothermal pretreatment process before.

**Table 1.** The energy consumption of corncob pretreated by combined pretreatment at different conditions.

The highest total energy consumption was obtained from CC pretreated for 30 min at 180 ◦C, which corresponded to 17.8 kWh/kg of biomass. It was approximately 66% of the addition of energy compared to the lowest total energy consumption that was obtained by CC pretreated for 60 min at 165 ◦C, which corresponded to 10.7 kWh/kg of biomass. Interestingly, the essential energy consumption was due to the energy-consuming during hydrothermal pretreatment, which was in the range of 6.5–16.5 kWh/kg biomass. This indicates that the energy consumption of the Parr reactor increased with the increased temperature. Concerning the energy consumption due to the mechanical size reduction, the energy consumption decreased with the increased temperature that caused the structural changes during pretreatment [35,36].

#### *3.3. Effect of Pretreatments on Enzymatic Hydrolysis*

The effect of pretreatment duration on the enzymatic hydrolysis was evaluated. In this study, three different temperatures and two different durations were used to investigate the evolution of fermentable sugar concentration after pretreatment. The total fermentable sugar content was calculated to evaluate the performance of a combined hydrothermal-mechanical process. Figure 2 illustrates that when the temperature increased, the amount of fermentable sugar increased (T180 ◦C > T165 ◦C > T140 ◦C), respectively. In the case of pretreatment duration, the longer the duration in the reactor, the more fermentable sugars were released (D60 min > D30 min). These indicate that the temperature and duration of the pretreatment affected the internal structural deconstruction of the corncob to enzyme accessibility.

**Figure 2.** The total fermentable sugars from the enzymatic hydrolysis of different pretreatment conditions.

The highest fermentable sugar was 0.488 kg/kg of biomass obtained from CC pretreated at 180 ◦C for 30 min, which corresponded to a 356.07% increase in the fermentable sugar compared to the control (0.107 kg/kg of biomass). This increase in fermentable sugar was due to the structural deconstruction of the biomass and the removal of the hemicellulose and lignin. The high amount of fermentable sugars obtained by enzymatic hydrolysis was related to the enzymatic efficiency, as seen in Figure 1. Several studies have suggested that the augmentation of fermentable sugar concentration was due to the structural alteration through the combined pretreatment process, which is typically an important stage for lignocellulosic valorization.

### *3.4. Evaluation of Energy Efficiency and Waste Generation of Combined Hydrothermal-Mechanical Pretreatment*

To evaluate the performance of the combined hydrothermal-mechanical pretreatment, the relationship between the total energy consumption and total fermentable sugars was evaluated through the estimation of the energy efficiency. The energy efficiency was used to evaluate the performance of this developed combined pretreatment to provide fermentable sugars. Energy efficiency is defined as the ratio of the total product (fermentable sugars) to the total energy consumption consumed during pretreatment. Table 2 indicates that the highest energy efficiency was obtained by the CC pretreated at 165 ◦C for 60 min, which corresponded to 0.041 kg of product/kWh. The effect of temperature on energy efficiency showed that the elevated temperature provided the high energy efficiency that was due to the high fermentable sugars obtained from enzymatic hydrolysis. On the other hand, the effect of pretreatment duration revealed that the longer duration released more fermentable sugars. These obtained results were relatively associated with the obtained results of the biochemical compositions and enzymatic efficiency, as seen in Table 1.

**Table 2.** The comparison of the energy efficiency and waste generation at different pretreatment conditions.

Beyond sustainability, the combined hydrothermal-mechanical pretreatment has also been investigated for waste generation during pretreatment. The waste generation represents the total amount of waste generated during the pretreatment process, which is calculated from the ratio of the total waste generated and total fermentable sugars. The obtained results showed that the minimum waste generation was 7.21 kg of waste/kg product obtained from the CC pretreated at 180 ◦C at 30 min, as seen in Table 2. The waste generation was the total waste generated in solid and liquid form in the process. To compare different pretreatment conditions, the waste generation was varied between 7.2–26.7 kg of waste/kg of fermentable sugars.

#### **4. Conclusions**

The development of an economically environmentally-friendly pretreatment technology for integration into a biorefinery is important for industrial-scale production. The combined hydrothermal-mechanical pretreatment is potentially an alternative solution process for lignocellulosic valorization. The integrated hydrothermal and mechanical pretreatment could achieve the structural alteration of lignocellulosic material for enzymatic enhancement. The combined pretreatment provided a high fermentable sugar concentration compared to the untreated material. The maximum fermentable sugar was 0.488 kg/kg

of biomass obtained at 180 ◦C at 30 min, which was due to the hemicellulose solubilization via the hydrothermal pretreatment and the increase in the specific surface area via the mechanical size reduction. The energy efficiency was used to evaluate the performance of the combined pretreatment, which could be used for further economic analysis. The highest energy efficiency was 0.0283 kg of fermentable sugars/kWh. On the other hand, the waste generation was investigated in terms of the environmental impacts of the process. The lowest waste generation was 7.21 kg of waste/kg product obtained at 180 ◦C at 30 min. The aforementioned results showed that the combined hydrothermal-mechanical pretreatment provided a high fermentable sugar concentration, which consequently enhanced further biofuel production.

These obtained results suggest that the combination of hydrothermal-mechanical pretreatment could be a promising pretreatment technology in terms of energy efficiency and an environmentally-friendly process for lignocellulosic valorization.

**Author Contributions:** Investigation, resources, formal analysis, J.P.; Investigation, resources, formal analysis, T.J.; Writing—review and editing, S.K.; Writing—review and editing, M.S.; Writing—review and editing, J.J.; Conceptualization, methodology, validation, investigation, writing—original draft preparation, writing—review and editing, visualization, supervision, funding acquisition, S.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research obtained financial support from the A New Researcher Scholarship of CSTS, NSTDA, Thailand under contract NO. JRA-CO-2565-16402-TH.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This study did not report any data.

**Acknowledgments:** The authors also thank the Department of Chemical Engineering, KMUTNB, and TGGS for the facilities support. We would also like to thank the BIOTEC-JGSEE Integrative Biorefinery Laboratory (IBL) for the supporting equipment used in the experiments.

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

### **References**


**1. Introduction**

Surface microtexturing has gradually attracted attention due to its good hydrophobicity and drag reduction properties, which has become an effective way to improve the performance of new products and reduce energy consumption. Since Hamilton and Halavit [1] first proposed to process the microstructure on the surface of the friction pair to obtain the dynamic pressure effect, many scholars have carried out research in this area. The effects of texture morphology, geometric parameters, texture distribution and environmental factors on the lubricating performance were investigated by simulation and experiment. Rahmani et al. [2] combined the Reynolds equation and surface discontinuity of the hydrostatic bearing to establish a model and studied the influence of various forms of surface texture on the friction and carrying capacity of the hydrostatic bearing and believed that the form of texture has a significant impact on the bearing lubrication performance; waist triangle, right triangle and rectangular microtextures significantly improved the lubricating performance. Sinano˘glu et al., used experimental methods to verify that the

**Citation:** Shang, Y.; Cheng, K.; Bai, Q.; Chen, S. Drag Reduction Analysis of the Hydrostatic Bearing with Surface Micro Textures. *Appl. Sci.* **2022**, *12*, 10831. https://doi.org/ 10.3390/app122110831

Academic Editors: Josep Maria Bergadà and Gabriel Bugeda Castelltort

Received: 9 August 2022 Accepted: 17 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 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/).

trapezoid has a higher load-carrying capacity than the zigzag texture, and the maximum pressure value is reduced by 36% compared to the smooth journal. In terms of pit-like textures, Tala-Ighil et al. [3] analyzed the effects of spherical, cylindrical and parallelepiped textures on the minimum oil film thickness, maximum pressure, axial flow velocity and friction force on the shaft diameter surface under the condition of stable flow field by using the finite element method, and the cylindrical texture performance improved more. Wang [4] found through experiments that with a reasonable design of geometric shapes and distributions, micropits can obtain nearly 2.5 times the carrying capacity of smooth surfaces.

Rahmani's [2] research shows that texture height has a greater impact on tribological properties than texture area ratio. Through experimental comparison, Sinano˘glu [5] found that reducing the texture height and inclination would reduce the LCC. Cupillard [6] applied numerical methods to study the effect of microgrooves on the axial direction of the bearing pad wall on the lubricity of radial hydrostatic bearings when cavitation effects were considered. It shows that the microgrooves can increase the minimum oil film thickness and reduce the frictional resistance under the condition of small depth and low load (eccentricity *ε* < 0.15); under the condition of large depth and high load (*ε* > 0.15), the microgrooves can reduce the friction force, but simultaneously, the thickness of the oil film is reduced. In terms of thrust hydrostatic bearing research, Wang [4] used the load capacity/friction coefficient transition point to measure the performance of the bearing, and studied the effects of the ratio of the area of the crater, the depth of the crater and the diameter of the crater in the transition point of the LCC and obtained the best result. Zhang [7] optimized the geometrical parameters of the fan-shaped straight groove texture on the bearing surface and considered that the microtexture had different optimal parameters under different constraints and objectives. He analyzed the effect of groove texture on the fluid thrust lubrication mechanism of thrust washers and found that the width and number of grooves have optimal values for load carrying capacity.

Regarding the location of texture distribution, Cupillard [6] simulation found that the microtexture model arranged in the maximum pressure region can reduce friction and oil film thickness compared with the smooth wall model. Liu Hongbin's numerical study found that there is a reasonable density and distribution for the oil film LCC. Brizmer [8,9] found in the micropit texture research that the global texture model cannot effectively improve the LCC, but it is suitable for mechanical seals.

Hamilton [1] proposed the "lubrication effect of microbumps", which believes that the microbumps on the surface hinder the movement of the fluid. When the fluid flows through the microbumps, the pressure rises rapidly and cavitation causes the pressure asymmetry, resulting in additional LCC. Cupillard's [10,11] numerical analysis of textured slider bearings found a "reflow phenomenon". When the texture depth increases to a certain value, backflow will occur, and the backflow will reduce the pressure gradient and LCC. Lo [12] found that the pit-textured surface lubricating fluid penetrates into the surrounding area to reduce friction and wear. Wang experimentally verified this phenomenon and summarized it as "secondary lubrication effect". Fowell [4] discovered the phenomenon of "inlet suction" when studying low-convergence textured wall bearings: unlike the classical shear force that entrains the lubricating oil to form hydrodynamic pressure, the phenomenon is that the textured area at the bearing inlet produces a lower pressure than the outside air. The high pressure causes the lubricating oil to be squeezed into the bearing, thereby increasing the LCC of the bearing. Tauviqirrahman et al. [13] believed that the surface microstructure affects the slip length of the fluid–solid surface, the journal bearings lubricated by Newtonian fluid and non-Newtonian fluid are compared, and it is concluded that non-Newtonian fluid is more effective in improving the load-carrying capacity in the case of high eccentricity.

Numerous studies have shown that applying a reasonable surface texture can lead to better tribological performance than smooth surfaces, but some issues still need to be investigated [14]. Whether there are some special textures that can achieve the best results performance in any situation; how different calculation models used affect the accuracy of the calculation results.

### **2. Analysis of Smooth-Walled Radial Bearing Ignoring Cavitation Effects**

### *2.1. Governing Equations*

In the process of high-speed rotation of hydrostatic bearings, the oil film of journal bearings will form negative pressure under the action of oil wedge. This cavitation phenomenon has been considered by a large number of scholars, and the most common is the half-Sommerfeld method. However, it is more practical to explain this phenomenon through gas–liquid two-phase flow [13,15]. Therefore, the conservation of momentum and energy are formulated as follows:

Mass conservation equation:

$$
\nabla \cdot \vec{\nabla} = 0 \tag{1}
$$

Momentum equation:

$$
\rho \frac{D\vec{V}}{Dt} = -\nabla p + \rho \vec{\text{g}} + \mu \nabla^2 \vec{V} \tag{2}
$$

We can see that the momentum equation is related to the density, viscosity and the volume velocity of the micro element, extending Equation (2):

$$\frac{\partial}{\partial \mathbf{x}\_{j}} (\rho u\_{i} u\_{j}) = -\frac{\partial p}{\partial \mathbf{x}\_{i}} + \frac{\partial}{\partial \mathbf{x}\_{j}} \left( \mu \left( \frac{\partial u\_{i}}{\partial \mathbf{x}\_{j}} + \frac{\partial u\_{j}}{\partial \mathbf{x}\_{i}} \right) \right) + \frac{\partial}{\partial \mathbf{x}\_{j}} \left( -\rho \overline{u\_{i}^{\prime} u\_{j}^{\prime}} \right) \tag{3}$$

Energy equation:

$$\frac{\partial}{\partial t}(\rho E) + \nabla \cdot [\vec{v}(\rho E + p)] = \nabla \cdot \left[k\_{eff} \nabla T\right] + \mathcal{S}\_h \tag{4}$$

The two-dimensional schematic diagram of a radial hydrostatic bearing, shown in Figure 1.

**Figure 1.** Schematic diagram of a radial hydrostatic bearing.

The main parameters of oil are: density 810 kg/m3 ; specific heat capacity 2000 J/(kg · K); thermal conductivity 0.37 W/(m · K); dynamic viscosity of lubricating oil is 3.85 × 10−3 Pa · s (temperature *T* = 293 K). Analysis parameters are shown in Table 1.

**Table 1.** Calculation parameters for radial hydrostatic bearing.

According to the Reynolds number calculation formula:

$$Re = \frac{\rho v d}{\mu} \tag{5}$$

where *d* is length scale, and *v* is reference velocity. Based on the maximum oil film thickness, let *d* = 2*h*max, the Reynolds number calculated as about 75, which is much lower than the range of 1000 to 2000, which can generate turbulent flow. Therefore, this problem can be calculated according to the laminar flow theory, and the obtained pressure distribution in the oil film is shown in Figure 2.

**Figure 2.** Pressure distribution of oil film on the journal hydrostatic bearing: oil pressure (MPa).

Based on the maximum oil film thickness, the Reynolds number is calculated as about 75, which is much lower than the range of 1000 to 2000, which can generate a turbulent flow. Therefore, this problem can be calculated according to the laminar flow theory, and the obtained pressure distribution in the oil film is shown in Figure 2.

In order to verify mesh independence, three different proportions of mesh number are compared with the FEM method. As shown in Table 2.

**Table 2.** Mesh independence verification.

It can be seen that without considering the cavitation effect, the minimum pressure may be negative pressure, which is obviously inconsistent with the actual situation. Therefore, the cavitation effect must be considered to obtain reasonable results. At the same time, the number of the mesh has little influence on the calculation results, which proves the independence of the mesh.

The process of generating, compressing, expanding and collapsing of tiny bubbles in a liquid is called cavitation. Several scholars have made outstanding contributions to the mathematical theory of cavitation effects. It was first predicted by the physicist Reynolds in 1873 that the high-speed relative motion between the propeller and the water would create a vacuum cavity that affects the performance of the propeller during the travel of the ship. Later, S.W. Barnaby et al. [16]. However, cavitation is not the only disadvantage. Along with the cavitation process, the bubbles generated by the fluid and the moving wall reduce the macroscopic shear force during the movement. For example, supercavitation

technology is used to generate cavitation on the surface of the torpedo, so that the resistance of the torpedo can be reduced by 90% [17], among which the "full cavitation model" was studied by Singhal et al. It accounts for phase change, bubble dynamics, turbulent pressure fluctuations, and noncondensable gases. It is able to take into account multiphase flows or flows with multiphase transport of matter, the effect of sliding velocity between liquid and gas phases, as well as the thermal effects and compressibility of liquid and gas phases, establishing the Rayleigh–Plessset equation, also known as the Porisky equation, which is also a widely used cavitation model [18].

Since the negative pressure problem in the diverging wedge region needs to be solved in combination with the cavitation phenomenon, the bearing oil film model is divided into two simplified slider bearings, the convergent wedge and the diverging wedge, which are analyzed separately.

With the multiphase cavitation modeling approach, a basic two-phase cavitation model consists of using the standard viscous flow equations governing the transport of mixture phases. In cavitation, the liquid–vapor mass transfer equation has the following form:

$$\frac{\partial}{\partial t}(a\rho\_{\upsilon}) + \nabla \cdot (a\rho\_{\upsilon}\vec{v}\_{\upsilon}) = R\_{\varepsilon} - R\_{\varepsilon} \tag{6}$$

In the equations mentioned above, *t* is time, *α* is volume fraction, *ρv* is vapor density, ~*vv* is vapor velocity, *Re* and *Rc* are mass transfer source terms connect to the growth and collapse of the vapor bubbles. For different cavitation models, the definition of *α* or other assumptions are different, resulting in different calculation methods of *Re* and *Rc*. However, several cavitation models can be written as similar formulas with different calculation methods of coefficients *Cvap* and *Ccond*.

$$\begin{cases} \begin{array}{c} \mathcal{R}\_{\varepsilon} = \mathcal{C}\_{vap} \rho\_{v} \rho\_{l} \sqrt{\frac{2}{3} \frac{(p\_{v} - p)}{\rho\_{l}}}\\ \mathcal{R}\_{\varepsilon} = \mathcal{C}\_{cond} \rho\_{v} \rho\_{l} \sqrt{\frac{2}{3} \frac{(p - p\_{v})}{\rho\_{l}}} \end{array} \end{cases} \tag{7}$$

The saturation pressure *pv* is corrected by an estimation of the local values of the turbulent pressure fluctuations:

$$p\_{\upsilon} = p\_{\text{sat}} + \frac{1}{2}(0.39\rho k) \tag{8}$$

where *k* is kinetic energy.

### *2.2. Method Selection for Cavitation Effect Calculation*

To account for cavitation in oil film CFD calculations, a multiphase flow model needs to be enabled. There are three commonly used multiphase flow models, namely the VOF model, the mixture model and the Eulerian model, and their characteristics are as follows.

For the bearing oil film model where the gas phase and liquid phase are mixed and the gas phase volume ratio is much greater than 10%, the VOF model mainly used to calculate the stratified flow or free surface flow is obviously not suitable. Therefore, the calculation model needs to compare the mixture model and the Eulerian model. Pull the model to make trade-offs. The selection of the mixture model and Eulerian model is mainly based on the following principles.

For the bearing oil film model with micro-texture on the wall surface, the flow field is more complicated. In order to improve the stability of the calculation, the mixture model is used to calculate the cavitation phenomenon.

### *2.3. Simulation of Smooth-Walled Radial Bearing Considering Cavitation Effects*

The pressure distribution of the smooth wall radial bearing calculate d in this paper is singular because cavitation effect is not considered. Here, the model considering cavitation effect is adopted, and the calculated pressure distribution is shown in Figure 3. The analysis results show that the results after considering the cavitation effect are more in line with the real situation, and the maximum pressure also increased.

**Figure 3.** Oil film pressure distribution considering cavitation effect: Oil pressure (MPa).

Figure 4 shows the cloud diagram of gas phase volume fraction distribution. The dark blue area indicates that the gas phase volume fraction is 0, that is, this area only contains oil, while other warm tone areas indicate that this area is a cavitation area. Obviously, the cavitation area is distributed in the bearing divergence wedge area. From the volume fraction of the gas phase in the cavitation area, it can be seen that the volume fraction of the gas phase increases gradually from the minimum oil film thickness to the maximum oil film thickness along the circumference. The area with less volume fraction of the gas phase corresponds to the initial stage of the cavitation. With the rotation of the journal, the cavitation is brought to the area with large oil wedge thickness and developed, and finally collapses and disappears in the area near the maximum oil film thickness.

The results show that the maximum velocity of the gas phase and the liquid phase are almost the same, and about 70% of the bubbles are distributed in units with a gas volume fraction of less than 9.71%.

### **3. Analysis of lUbrication Characteristics of Converging Wedge Slider Bearing Surface Texture**

### *3.1. Pressure Distribution Simulation of Non-Textured Convergent Wedge Slide Bearing*

The slider bearing is a simplification of the convergent area and the divergent area of the radial hydrostatic bearing, so that the calculation can be more refined. The size of the slider bearing is selected as shown in Figure 5.

**Figure 5.** Schematic diagram of slider bearing dimensions.

The lubricating oil is Mobil Velocite oil No 2, and the oil temperature is 293.15 K. The moving speed of the upper wall (Moving Wall) is *v* = 20 m/s, and the lower wall (state wall) is stationary. In order to ignore the influence of the two side walls facing the flow, the periodic boundary conditions are set in the analysis. The processing of periodic boundary conditions from the finite element analysis theory has the characteristics of accelerating the convergence, so it will not adversely affect the analysis results.

For the untextured smooth surface, applying CFD analysis, the flow velocity variation along the oil film thickness direction at the inlet position, the center position and the outlet position is obtained as shown in Figure 6.

In the figure, *x* = 5 mm is the entrance position, *x* = 0 mm is the center position, and *x* = −5 mm is the exit position.

**Figure 6.** Velocity distribution along the thickness of the oil film.

*3.2. Simulation of Convergence Wedge-Slide Bearing Pressure Distribution on the Streamwise and Transverse Textured Surfaces*

Figure 7 shows two microtexture models (streamwise and transverse) selected to analyze the effects of texture orientation on bearing performance.

**Figure 7.** Schematic diagram of slider bearing microtexture.

The micro-texture topography only considers the rectangular cross-section, and the micro-texture is uniformly distributed, with a micro-textured surface in the whole area on the state wall.

The dimensionless depth *ho* is defined as the ratio of the texture depth to the minimum oil film thickness. The minimum oil film thickness is set to 10 µm and the texture groove width is 50 µm. The effect of the dimensionless depth on the carrying capacity is calculated. As shown in Figure 8.

Figure 8 shows that the dimensionless LCC decreases with the increase of the dimensionless depth. The transverse texture has better LCC than the streamwise texture. With the increasing dimensionless texture depth, the dimensionless relative LCC of the transverse texture shows a decreasing trend, but it is not a rapid linear decrease like the streamwise texture, but is basically stable at around 0.78, that is, the transverse texture decreases. Compared with the smooth surface, the LCC of the structural model is reduced by about 22%.

**Figure 8.** Effects of dimensionless depth of microtexture on carrying capacity.

The variation of shear force on the fixed wall and moving wall of the streamwise and transverse microtextures with the dimensionless texture depth is obtained through analysis. Figure 9 shows the variation trend of the dimensionless shear force of the streamwise texture model.

**Figure 9.** Influence of dimensionless depth of microtexture on wall shear force in streamwise texture model.

Figure 10 shows that compared with the smooth model without texture, the wall shear force of the streamwise textured model is significantly reduced. The shear force of the moving wall is slightly lower than that of the fixed textured wall. When the dimensionless texture depth is 1, the dimensionless shear force of the upper plate wall is close to 0.8, that is, the drag reduction rate is about 20%.

Figure 10 shows the variation trend of dimensionless shear force of the transeverse texture model.

**Figure 10.** Influence of dimensionless depth of microtexture on wall shear force in transverse texture model.

Figure 10 shows that when the transverse micro-textured slider bearing is applied, the shear force between the moving wall and the fixed wall is quite different. The dimensionless depth is about 0.6 close to the minimum value, and then the shear force increases gradually. The dimensionless shear force of the moving wall is basically between 0.93 and 0.94. Usually, the drag reduction of hydrostatic bearings has no meaning for fixed walls, so the drag reduction characteristics reflected by the transverse texture are not obvious.

### *3.3. Lubrication Characteristic Analysis of Local Texture Model*

No matter what texture type is used, the above analysis shows that the LCC is lower than that of the untextured smooth wall model, which obviously does not meet the lubrication goals of improving LCC and reducing wall friction. Therefore, the loss caused by changing the full texture model needs to be considered.

First of all, it is determined that only suitable microtextures are produced in the required bearing parts, the length of the slider bearing is set as *L*, and two parameters *α* and *β* are defined to describe the size and entrance distance of the microtextures. The length of the microtexture is *l* = *α* × *L* and the entrance distance *d* = *β* × *L*. Combining *α* and *β*, the wall shear force and pressure distribution of the slider bearing were analyzed. Among them, the dimensionless shear force on the wall surface of the streamwise texture is shown in Table 3.

**Table 3.** Dimensionless shear forces for the streamwise local texture state wall.

The analysis shows that with the same value of *α*, the texture distribution on the inlet side can obtain a higher drag reduction rate.

Figure 11 calculates the pressure distribution of the smooth surface, transverse textured and streamwise textured slider bearings for setting *α* = 0.1, *β* = 0.9.

**Figure 11.** Influence of local texture on oil film pressure distribution.

It can be seen from the figure that the introduction of microtexture not only affects the small area of the texture distribution, but also affects the size and shape of the pressure distribution of the entire bearing. However, the maximum load-carrying capacity of a smooth bearing cannot be reached regardless of the distribution.

### **4. Analysis of Bearing Lubrication Characteristics under Cavitation Effect**

*4.1. Analysis of Lubrication Characteristics of Divergent Wedge Slider Bearing Based on Full Texture Model*

In the analysis of a convergent wedge texture model, CFD calculation can be completed only by using a pressure-based solver. For the divergent wedge texture model, the influence of cavitation should be considered.

Figure 12 shows the distribution of oil film pressure on the moving wall before and after considering the cavitation effect when the streamwise dimensionless texture depth is 0.5. The abscissa −5 mm point in the figure corresponds to the oil film inlet, and +5 mm corresponds to the oil film outlet. The figure on the left shows the situation without considering the cavitation effect. The oil film pressure distribution is similar to a parabola. When the *x*-axis coordinate is about −1 mm, there is a minimum pressure of −0.9 MPa (absolute pressure); the picture on the right shows the case considering the cavitation effect. On the moving wall, the oil film pressure in the middle of the wall is equal to the cavitation pressure. There is a sudden change in the pressure value at the inlet and outlet of the oil film, and the oil film pressure at the inlet. The strength first increases to 0.038 MPa and then decreases to the cavitation pressure, while the oil film pressure at the outlet surges from the cavitation pressure to about 0.082 MPa. The upper part of the left figure shows the pressure position of cavitation pressure, showing the huge difference between considering and not considering cavitation.

According to the previous analysis, the arrangement of the micro-texture at the inlet will lead to the reduction of LCC, while at the outlet, due to the consideration of cavitation effect, the negative pressure strength obtained without considering cavitation effect is corrected, which improves the LCC. Therefore, it can be imagined that if the cavitation effect is strengthened by using a micro-texture at the exit, the LCC may be improved. Figure 13 shows the corresponding dimensionless bearing capacities from the analysis of different dimensionless texture depths for a fully textured diverging wedge slider bearing.

**Figure 12.** Pressure distribution on moving wall before and after considering cavitation effect in streamwise texture.

**Figure 13.** The influence of the depth of streamwise texture on the dimensionless LCC.

Figure 13 shows that changing the depth of the texture has no obvious effect on improving the LCC, but it usually does not reduce the LCC. Figure 14 shows the dimensionless shear force obtained by changing the dimensionless texture depth analysis.

**Figure 14.** The effect of the depth of streamwise texture on the dimensionless shear force.

It can be seen from Figure 14 that increasing the texture depth is very effective in reducing the wall shear force, and its maximum drag reduction rate is likely to reach 22%.

In order to compare the two different directions of micro texture used in the previous convergent wedge, the oil film pressure analysis, dimensionless LCC and dimensionless shear force analysis of the divergent wedge with a transverse micro-texture are carried out. The characteristics of oil film pressure are almost the same as those in the streamwise micro-texture pressure distribution in Figure 12, but the LCC and shear force are quite different. Figure 15 shows the dimensionless LCC obtained by changing the dimensionless texture depth analysis.

**Figure 15.** Influence of transverse texture depth on dimensionless LCC.

Figure 15 shows that the LCC of the transverse texture model is greater than that of the smooth model. The dimensionless LCC first increases rapidly with increasing dimensionless texture depth, and when the dimensionless texture depth is 0.6, the maximum dimensionless LCC is 1.18, and then the dimensionless LCC decreases slowly.

The variation of transverse wall dimensionless shear force with dimensionless texture depth is shown in Figure 16.

**Figure 16.** Influence of transverse texture depth on dimensionless shear force.

Transverse texture can effectively reduce the dimensionless shear force on the fixed wall, that is, the shear force lower than that of the smooth wall model can be obtained. When the dimensionless texture depth is 0.6, the minimum dimensionless shear force can be obtained as about 0.85, and the corresponding drag reduction rate is about 12%. However, while reducing the shear force on the state wall, the transverse texture model increases the dimensionless shear force on the moving wall. For example, when the dimensionless texture depth is 0.6, the shear force increases by 6.5% compared with the smooth wall. For the divergent wedge, the increase of the LCC of the moving wall and the increase of the shear force are unfavorable factors for lubrication.

### *4.2. Simulation of Drag Reduction Effect of Local Texture Model of Divergent Wedge Slider*

Dimensionless parameters are defined in the same way as in the previous convergence wedge *α* and *β* to represent the length of the texture area and the distance from the exit, that is, when *β* at 0, the local texture is arranged at the exit. For the streamwise texture model, the dimensionless shear force of the moving wall obtained from the simulation analysis is consistent with *β*, and the relationship is shown in Figure 17.

**Figure 17.** The influence of the position of the streamwise texture on the dimensionless shear force on the moving wall.

Figure 17 shows that the texture arrangement at the oil film outlet of the divergent wedge can obtain a lower wall shear force, for example *α* = 0.3 and *β* at 0, the dimensionless wall shear force of the moving wall can be reduced to left and right, respectively. As the distance from the outlet increases, the shear force will increase, and the texture is arranged in an inappropriate area. Compared with the smooth surface, the shear force can be increased. For example, from the pressure distribution, if *α* ≤ 0.3, and if the texture is arranged at the entrance, it will be higher than the smooth wall by about 20%, because the texture area is in the pressure rise area of the oil film, and the corresponding texture boundary position just corresponds to the peak point.

By analyzing the hydrostatic block bearing with transverse local texture, the dimensionless shear force on the moving wall and *β*. The relationship is shown in Figure 18.

It can be seen from the figure that if the transverse micro-texture arrangement is adopted, it is possible to reduce the shear force only if it is arranged in a very narrow area at the outlet.

**Figure 18.** Influence of transverse texture position on dimensionless shear force on moving wall.