materials

Article Preparation of Edible Non-wettable Coating with Soybean for Repelling Liquid Foods with Little Residue

Tianyu Shen, Shumin Fan *, Yuanchao Li, Guangri Xu and Wenxiu Fan * School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, Henan 453003, China; [email protected] (T.S.); [email protected] (Y.L.); [email protected] (G.X.) * Correspondence: [email protected] (S.F.); [email protected] (W.F.)

 Received: 17 June 2020; Accepted: 20 July 2020; Published: 24 July 2020 

Abstract: Liquid food adhesion on containers has increased food waste and pollution, which could be effectively alleviated with a superhydrophobic surface. In this research, the superhydrophobic coating was fabricated with edible soybean wax on different substrates by a spraying method. The coated surface showed excellent superhydrophobicity due to its microstructure formed by self-roughening, which could repel a variety of viscous liquid food with the apparent contact angle of 159 2 . ± ◦ The coated surface was still liquid-repellent after hot water immersion (45 ◦C), abrasion test with sandpaper, water impact, finger touch and immersion into yogurt. The liquid-repellent coating with soybean wax, which is natural and green, is promising for application in the food industry to reduce waste.

Keywords: non-wettable coating; edible; soybean wax; residue

1. Introduction With the development of the economy, people’s living standards improved, especially in food demand. In daily life, viscous liquid food (such as yogurt, honey, milk, coffee, et al.) residue remain adhering to the container after drinking, which has given us a great deal of inconvenience and resulted in huge waste (up to 15% of liquid food products) [1]. Food adhesion problems could be effectively alleviated by using a superhydrophobic surface. Superhydrophobic surfaces have become a popular topic due to their application in oil/water separation [2–4], anti-corrosion [5], drag reduction [6], reproductive medicine and cryobiology [7] et al. Superhydrophobic surfaces, characterized by high apparent contact angles (> 150◦) and low contact angle hysteresis (the difference between the advancing and receding contact angles, which could lead to a low sliding angle), have tremendous practical applications, including self-cleaning and drag reduction. The non-wetting coatings could eliminate liquid food residue, avoiding the adhesion of viscous liquid to packaging material [8–14]. Superhydrophobic coatings are fabricated through surface texture and low solid surface energy [15] or modification of surface microstructure without using low surface-energy reagents [16,17]. Microscale structure, nanoscale structure and hierarchical structure turned out to be important for surface texture fabrication. So far, most of the studies to fabricate water-repellent surfaces use nanoparticles combined with fluorine-containing reagents. Vahabi et al. [18] prepared flexible non-wettable films by creating coatings of polyurethane and fluorinated silica particles on a substrate, which repelled varieties of liquids. Pan et al. [19] fabricated superhydrophobic stainless-steel wire meshes by creating coatings of cross-linked poly(dimethylsiloxane) and fluorodecyl polyhedral oligomeric silsequioxane. The fluorocarbon materials could decompose to perfluorooctanoic acid (PFOA), whose toxicity is biocumulative and persistent to humans. Thus, the fluorocarbon materials, classified as contaminants, are unfit for the preparation of edible non-wettable coatings.

Materials 2020, 13, 3308; doi:10.3390/ma13153308 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 11

Materials(PFOA),2020 whose, 13, 3308 toxicity is biocumulative and persistent to humans. Thus, the fluorocarbon materials,2 of 11 classified as contaminants, are unfit for the preparation of edible non-wettable coatings. To make a non-wettable coating on packaging material, the most important factor is safety: not allowingTo make toxicity a non-wettable when contacting coating food. on A packagingsuperhydrophobic material, surface the most could important be prepared factor by spraying is safety: nota mixture allowing of toxicitya non-polar when compound contacting food.and A superhydrophobic [20,21]. Wang surface et al. [22] could prepared be prepared water-repellent by spraying acoatings mixture using of a non-polarbeeswax and compound carnauba and wax. solvent The coatings [20,21]. were Wang non-wettable et al. [22] prepared towards water-repellentdifferent liquid coatingsfood, which using was edible and carnaubanatural. Liu wax. et Theal. [23] coatings created were liquid-repellent non-wettable coatings towards using different rice liquid bran food,wax and which candelilla was edible wax and by natural. a one step Liu etspraying al. [23] createdmethod liquid-repellent on polypropylene coatings (PP) using substrates, rice bran which wax andcould candelilla be used wax for byeliminating a one step sprayingliquid waste method of food on polypropylene containers. Zhang (PP) substrates, et al. [24] which fabricated could bea usedsuperhydrophobic for eliminating liquid surface waste with of food good containers. transparency Zhang and et stability al. [24] fabricatedproperties a via superhydrophobic mixture coating paperof beeswax surface and with carnauba good transparency wax. The wax and mixture stability was properties emulsified via mixtureand coated coating on a of paper beeswax surface, and carnaubafollowed by wax. annealing The wax at mixture different was temperatures. emulsified and Su coatedbmicrometer on a paper structure surface, was followed generated by annealing on the base at diofff erentmicrometer temperatures. spherical Submicrometer wax particles. structure Yang was generatedet al. [25] on theprepared base of micrometerthree kinds spherical of edible wax particles.superhydrophobic Yang et al. surfaces [25] prepared by fumigating three kinds lard, of ediblefood grade superhydrophobic paraffin and beeswax surfaces on by fumigatingcalcined Fe lard,foil. foodAll samples grade para showedffin and excellent beeswax superhydrophobicity on calcined Fe foil. All and samples effectively showed repelled excellent starch superhydrophobicity slurries and liquid andfoods eff withoutectively repelledobservable starch residues. slurries The and used liquid sample foods withoutcould be observable recovered residues. by re-fumigation. The used sample These couldresearches be recovered have promoted by re-fumigation. the preparation These of researches edible non-wettable have promoted materials, the preparation which should of edible be non-wettableenvironmentally materials, friendly whichand should should not be contai environmentallyn toxic chemicals friendly and unapproved and should additives. not contain toxic chemicalsSoybean and wax unapproved is made additives.from natural soybeans, which are abundant. Thus, the soybean wax is cheap,Soybean natural wax and isbiodegradable made from natural without soybeans, environm whichental pollution, are abundant. which Thus, has great the soybean advantages wax in is cheap,terms of natural health and and biodegradable environmental without protection environmental [26,27]. The pollution, main ingredient which has of greatsoybean advantages wax is intriacylglyceride terms of health composed and environmental of stearic acid. protection Herein, superhydrophobic [26,27]. The main ingredientcoatings were of soybeanfabricated wax with is triacylglyceridesoybean wax by spray composed coating of stearicof a wax acid. suspension Herein, in superhydrophobic ethanol. This low-cost coatings and weresimple fabricated non-wettable with soybeancoating exhibited wax by spray excellent coating liquid of a waxrepelling suspension properties in ethanol. against This a variety low-cost of andnon-Newtonian simple non-wettable viscous coatingfood liquids exhibited or hot excellent water solution liquid repellingon different properties food packaging against a materials variety of (glass, non-Newtonian paper, plastic viscous and foodceramics). liquids The or coating hot water provided solution excellent on different resistan foodce packagingto hot water materials immersion, (glass, sandpaper paper, plastic abrasion, and ceramics).water impact, The finger coating touching provided and excellent yogurt immersio resistancen. toThus, hot waterthe non-wettable immersion, coating sandpaper with abrasion,soybean waterwax could impact, be used finger to touchingreduce liquid and yogurtfood residue immersion. on different Thus, thesubstrates, non-wettable including coating glass with slide, soybean paper, waxplastic could and beceramic. used to reduce liquid food residue on different substrates, including glass slide, paper, plastic and ceramic. 2. Materials & Methods 2. Materials & Methods 2.1. Materials 2.1. Materials Ethanol was purchased from Tianjin Hengxing Chemical Reagent Manufacturing co, LTD, Ethanol was purchased from Tianjin Hengxing Chemical Reagent Manufacturing co, LTD, Tianjin, Tianjin, China. Soybean wax was purchased from Hubei Xinghe Chemical co, LTD, Hubei, China. China. Soybean wax was purchased from Hubei Xinghe Chemical co, LTD, Hubei, China. Milk, fruit Milk, fruit juice, Coca Cola, honey and black tea were purchased from local supermarkets, Xinxiang, juice, Coca Cola, honey and black tea were purchased from local supermarkets, Xinxiang, China. Glass China. Glass slides, paper, plastic and ceramic were available at local markets, Xinxiang, China. slides, paper, plastic and ceramic were available at local markets, Xinxiang, China. 2.2. Preparation of Soybean Wax Coating 2.2. Preparation of Soybean Wax Coating As shown in Figure 1, the soybean wax suspension was prepared thorough mixture of 1 g of As shown in Figure1, the soybean wax suspension was prepared thorough mixture of 1 g of soybean wax and 50 mL ethanol solution followed by heating at 65 ℃ for 3 min. The hot wax soybean wax and 50 mL ethanol solution followed by heating at 65 °C for 3 min. The hot wax suspension was sprayed (Flying Boat Glass Co. LTD, Yancheng, China) onto glass slides (Zhejiang suspension was sprayed (Flying Boat Glass Co. LTD, Yancheng, China) onto glass slides (Zhejiang Pride Electric Appliance Co. LTD, Jinhua, China) with a distance of 50 cm. Finally, the coated surface Pride Electric Appliance Co. LTD, Jinhua, China) with a distance of 50 cm. Finally, the coated surface was dried at room temperature to obtain its superhydrophobic characteristics. was dried at room temperature to obtain its superhydrophobic characteristics.

Figure 1. Figure 1. IllustrationIllustration for preparation of a non-wettable surface.

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2.3. Characterization 2.3. Characterization The surface structure of the superhydrophobic coating was observed by a Quanta 200 scanning electronThe microscope surface structure (FEI, ofHillsb the superhydrophobicoro, OR, USA) operated coating at was an observedacceleration by avoltage Quanta of 200 20 scanningkV. The electronroughness microscope of the superhydrophobic (FEI, Hillsboro, coating OR, was USA) anal operatedyzed by GTK-16-0300 at an acceleration white light voltage interferometer of 20 kV. The(BRKR, roughness Billerica, ofMassachusetts, the superhydrophobic USA). The wettabili coatingty was of the analyzed superhydrophobic by GTK-16-0300 surface white was tested light interferometerwith an optical (BRKR, contact Billerica, angle measuring Massachusetts, instrument USA). (Shenzhen The wettability testing of equipment the superhydrophobic CO., LTD, surfaceShenzhen, was China, tested withTST-200H). an optical Apparent contact anglecontact measuring angles of instrument the superhydrophobic (Shenzhen testing coating equipment were CO.,measured LTD, Shenzhen,with a deionized China, TST-200H).water droplet Apparent (Shenzhe contactn testing angles equipment of the superhydrophobic CO., LTD, Shenzhen, coating China) were measuredof 10 μL on with a video a deionized optical water contact droplet angle (Shenzhen system to testing measure equipment the resistance CO., LTD, of Shenzhen, superhydrophobic China) of 10coatingµL on to a videodifferent optical liquid contact foods angle at room system temperatur to measuree. The the values resistance of the of superhydrophobicapparent contact angle coating and to ditheff erentsliding liquid angle foods were atdetermined room temperature. by averaging The values values measured of the apparent at 5 different contact points angle on and each the sample sliding anglesurface. were determined by averaging values measured at 5 different points on each sample surface. 3. Results 3. Results 3.1. Morphology of the Soybean Wax Coating 3.1. Morphology of the Soybean Wax Coating Glass is very common in our life and it is widely used as a liquid food packaging material. Glass is very common in our life and it is widely used as a liquid food packaging material. After After treated with soybean wax in ethanol, the surface was made up of irregular and discernible treated with soybean wax in ethanol, the surface was made up of irregular and discernible nanoscale nanoscale sheets of waxy crystals, which distributed neatly and tightly (Figure2a). At higher sheets of waxy crystals, which distributed neatly and tightly (Figure 2a). At higher magnifications, a magnifications, a flower-like micro-nano roughness structure (Figure2b) could be observed. The rapid flower-like micro-nano roughness structure (Figure 2b) could be observed. The rapid volatilization volatilization of the solvent resulted in the rough structure of the coating. of the solvent resulted in the rough structure of the coating.

Figure 2.2. (a) SEM image of the soybean wax coatingcoating andand ((b)) itsits higherhigher magnificationmagnification image.image

Futhermore, the the roughness roughness of of the the coating coating was was analyzed analyzed by white by white light lightinterferometer interferometer at room at roomtemperature. temperature. As shown As shown in Figure in Figure 3, the3 ,convex the convex hulls hulls showed showed micron micron fluctuation fluctuation structure. structure. The Thestaggered staggered convex convex structure structure and and uniform uniform distribution distribution of ofconvex convex hulls hulls on on the the surface surface facilitated facilitated its superhydrophobic characteristics.

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FigureFigure 3. 3. TheThe roughness roughness of of the the coating. coating. Figure 3. The roughness of the coating. 3.2.3.2. Influence Influence of of Surface Surface Density Density 3.2. Influence of Surface Density FigureFigure 4a4a shows the influence influence of surface density ( ρρ) )of of soybean soybean wax wax on on th thee values values of of apparent apparent Figure 4a shows the influence of surface density (ρ) of soybean wax on the values of apparent contactcontact angle angle and and sliding sliding angle. angle. The The water water dr dropletsoplets stick stick to to the the coating coating surface surface when when ρρ isis smaller smaller than than contact angle 2and sliding angle. The water droplets stick to the coating surface when ρ is smaller than 0.870.87 mg mg cm cm−2−. The. The maximum maximum apparent apparent contact contact angle angle of 159° of 159 and◦ theand minimum the minimum sliding sliding angle angleof 7° were of 7◦ 0.87 mg cm−2. The maximum apparent2 contact angle of 159°2 and the minimum sliding angle of 7° were obtainedwere obtained with ρ with of 1.74ρ of mg 1.74 cm mg−2. At cm −ρ =. At1.74ρ mg= 1.74 cm mg−2, the cm advancing− , the advancing and receding and receding contact contact angle were angle obtained with ρ of 1.74 mg cm−2. At ρ = 1.74 mg cm−2, the advancing and receding contact angle were measuredwere measured as 159° as and 159 154°,◦ and respectively, 154◦, respectively, which led which to low led contact to low angle contact hysteresis. angle hysteresis. The high apparent The high measured as 159° and 154°, respectively, which led to low contact angle hysteresis. The high apparent contactapparent angle contact and anglelow contact and low angle contact hysteresis angle hysteresiswere from were complete from completecoverage coverageof the coating of the and coating the contact angle and low contact angle hysteresis were from complete coverage of the coating and the desiredand the roughness. desired roughness. The increase The increasein surface in surfacedensity densityincreased increased the thickness the thickness of the ofsoybean the soybean wax desired roughness. The increase in surface density increased the thickness of the soybean wax coating,wax coating, which which could could not change not change the theroughness roughness significantly. significantly. Thus, Thus, the the valu valuee of of apparent apparent contact contact coating, which could not change the roughness significantly. Thus, the value of apparent contact angleangle and and sliding sliding angle angle basically basically tended tended to to be be stable stable when when ρρ continuedcontinued to to increase. increase. As As shown shown in in angle and sliding angle basically tended to be stable when ρ continued to increase. As shown in FigureFigure 4b,4b, the pristine glass substrates show a smooth surface. However, However, the hierarchical microscale Figure 4b, the pristine glass substrates show a smooth surface. However, the hierarchical microscale andand nanoscale nanoscale roughness roughness structure structure on on glass glass surfac surfacee after after soybean soybean wax wax coating coating treatment treatment was was formed formed and nanoscale roughness structure on glass surface after soybean wax coating treatment was formed (Figure(Figure 44c),c), which is the criticalcritical pointspoints forfor non-wettingnon-wetting [[28].28]. Consequently, Consequently, the the superhydrophobic superhydrophobic (Figure 4c), which is the critical points for non-wetting [28]. Consequently, the superhydrophobic coatingcoating with with soybean soybean wax wax was was fabricated fabricated with with sufficiently sufficiently high high surface surface density density of of the the wax. wax. coating with soybean wax was fabricated with sufficiently high surface density of the wax.

FigureFigure 4. 4. (a(a) )The The effect effect of of surface surface density density (ρ (ρ) )on on apparent apparent contact contact angle angle and and sliding sliding angle; angle; (b (b) )SEM SEM Figure 4. (a) The effect of surface density (ρ) on apparent contact angle−2 2 and sliding angle; (b) SEM imageimage of of pristine pristine glass glass and and (c (c) )the the coated coated glass glass with with ρρ ofof 1.74 1.74 mg mg cm cm−. . image of pristine glass and (c) the coated glass with ρ of 1.74 mg cm−2. 3.3.3.3. Repellency Repellency to to Viscous Viscous Liquid Liquid Food Food in in Daily Daily Life Life 3.3. Repellency to Viscous Liquid Food in Daily Life TheThe soybean soybean wax wax coating coating method method could could be be easily easily applied applied on on different different food food packaging packaging materials, materials, The soybean wax coating method could be easily applied on different food packaging materials, suchsuch as as glass, glass, paper, paper, plastic plastic and and ceramic ceramic substrat substrates.es. Figure Figure 5a5a shows shows the the photographs photographs of of Coca Coca Cola, Cola, such as glass, paper, plastic and ceramic substrates. Figure 5a shows the photographs of Coca Cola, blackblack tea, tea, yogurt, yogurt, honey honey and and water water on on different different so soybeanybean wax wax treated treated materials. materials. All All these these materials materials black tea, yogurt, honey and water on different soybean wax treated materials. All these materials

Materials 2020, 13, x FOR PEER REVIEW 5 of 11 Materials 2020, 13, 3308 5 of 11 becameMaterials 2020non-wetting, 13, x FOR PEERwith REVIEW spherical shape of various liquid food droplets on the coated substrates.5 of 11 Figure 5b shows the as-coated glass surface had apparent contact angles of 159°, 157°, 154°, 153°, 156° andbecamebecame 152°, non-wettingnon-wetting respectively withwith to sphericalsphericalwater, Coca shape Cola, of various juice, liquidhoney, food tea dropletsdrandoplets yogurt. on the The coated complex substrates. flow characteristicsFigureFigure5 5bb showsshows of thethenon-Newtonian as-coated glass glass fluids surface surface like hadhoney had apparent apparent and yogurt contact contact resulted angles angles in of a of159°, little 159 157°, bit◦, 157lower 154°,◦, 154 153°,apparent◦, 153 156°◦, contact156and◦ and152°, angle 152 respectively◦ than, respectively other tofood water, to liquids. water, Coca Coca The Cola, Cola,non- juNewtonianice, juice, honey, honey, fluids tea tea and andcould yogurt. yogurt. have aThe Thecertain complex “memory flowflow effect”characteristicscharacteristics and make ofof it non-Newtoniannon-Newtonian difficult to flow. fluidsfluids As shown likelike honeyhoney in Video andand S1, yogurtyogurt the water resultedresulted droplet inin aafalls littlelittle down bitbit lowerlower on the apparentapparent coating surfacecontactcontact of angleangle glass than thanand other itother bounced food food liquids. back liquids. to Thethe The air. non-Newtonian non-WhenNewtonian the water fluids columnfluids could could quickly have have a certainhit thea certain “memorycoating “memory surface, effect” waterandeffect” make droplets and it make diffi bouncedcult it difficult to flow. high to As and flow. shown rolled As inshown down Video inalong S1, Video the the water S1, surface the droplet water (Video droplet falls S2). down falls on down the coating on the surfacecoating ofsurface glass of and glass it bounced and it bounced back to back the air. to the When air.the Wh wateren the column water column quickly quickly hit the hit coating the coating surface, surface, water dropletswater droplets bounced bounced high and high rolled and downrolled alongdown thealong surface the surface (Video (Video S2). S2).

Figure 5. (a) Photographs of Coca Cola, black tea, yogurt, honey and water droplets on different coated materials and (b) the apparent contact angles and sliding angles of different liquid food on Figure 5. (a) Photographs of Coca Cola, black tea, yogurt, honey and water droplets on different coated coatedFigure glass. 5. (a ) Photographs of Coca Cola, black tea, yogurt, honey and water droplets on different materials and (b) the apparent contact angles and sliding angles of different liquid food on coated glass. coated materials and (b) the apparent contact angles and sliding angles of different liquid food on 3.4.3.4. Heat Heatcoated Resistance Resistance glass. TheThe heat resistance of the coated surface was st studiedudied by immersing the the treated glass in water 3.4. Heat Resistance heatedheated by by electric-heated electric-heated thermostatic thermostatic water water bath bath (Spring (Spring Instrument. Instrument. Co., Co., LTD, LTD, Jintan, Jintan, China) China) for for10 min.10 min. AfterThe After heat contacting contactingresistance with withof hotthe hot watercoated water from surface from 25 25 towas to 50 50 st°C,udied◦C, the the apparentby apparent immersing contact contact the angle angletreated was was glass determined determined in water everyeveryheated 5 5 by°C.◦C. electric-heated As shown in Figure thermostatic 66,, TheThe coatedwatercoated bath surfacesurf (Springace keptkept Instrument. superhydrophobicsuperhydrophobic Co., LTD, whenwhen Jintan, thethe China) waterwater for bathbath 10 increasedincreasedmin. After from contacting 25 °C◦C to to with 45 45 °C.◦ C.hot When When water the the from water water 25 temp temperatureto 50erature °C, the increased increased apparent to contact 50 °C◦C,, theangle the coated coated was surfacedetermined surface lost lost itsitsevery superhydrophobicity 5 °C. As shown in with withFigure apparent apparent 6, The coatedcontact surf angle ace lower kept superhydrophobic than 150150°.◦. The The soybean soybean when wax,the wax, water with with baththe the meltingmeltingincreased point point from of of 25 52 °C °C,◦ C,to is 45 a °C. kind When of phase the water change temp material.erature increasedThe partial to disappearance 50 °C, the coated of of surface the rough lost structurestructureits superhydrophobicity happened happened when when with the the temperature temperatureapparent contact was was close closeangle toto lower 50 °C,◦C, than which 150°. reduced The soybean the apparent wax, with contact the angleanglemelting of droplet point of on 52 the °C, surface. is a kind More More of phase research research change is is needed material. to to improve The partial the disappearancerobustness of soybean of the rough wax towardstowardsstructure hot happened water above when 50°C, 50 the◦C, temperaturewhich is an impo importantwas rtantclose factor to 50 for °C, its which practical reduced applications. the apparent contact angle of droplet on the surface. More research is needed to improve the robustness of soybean wax towards hot water above 50°C, which is an important factor for its practical applications.

Figure 6. The apparent contact angles with different temperatures of immersion water. Figure 6. The apparent contact angles with different temperatures of immersion water. 3.5. Robustness and Durability of the Coating Figure 6. The apparent contact angles with different temperatures of immersion water. The physical damage of viscous liquid food and external pressure could wear down the food packaging material. The physical damages of non-wettable coating contacting with viscous liquid is a

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3.5.3.5. RobustnessRobustness andand DuDurabilityrability ofof thethe CoatingCoating MaterialsTheThe2020 physicalphysical, 13, 3308 damagedamage ofof viscousviscous liquidliquid foodfood anandd externalexternal pressurepressure couldcould wearwear downdown thethe 6foodfood of 11 packagingpackaging material.material. TheThe physicalphysical damagesdamages ofof non-wettablenon-wettable coatingcoating contactingcontacting withwith viscousviscous liquidliquid isis a big challenge for large-scale application. Robustness still remains a major challenge for application abig big challenge challenge for for large-scale large-scale application. application. RobustnessRobustness stillstill remainsremains aa majormajor challengechallenge for application of superhydrophobic coating. Thus, the abrasion resistance of the coating was analyzed by placing of superhydrophobic coating. Thus, Thus, the the abrasion abrasion resistance resistance of of the coating was analyzed by placing the coated glass face down to a sandpaper with 1000 meshes under a weight of 100 g along a ruler thethe coated coated glass glass face face down down to to a a sandpaper with with 1000 1000 meshes under under a a weight weight of of 100 100 g g along a a ruler for a distance of 20 cm (Figure 7a,b), which was defined as one test cycle. The change of apparent forfor a distance of 20 cm (Figure7 7a,b),a,b), whichwhich waswas defineddefined asas oneone testtest cycle.cycle. TheThe changechange ofof apparentapparent contact angle and sliding angle after cycles of abrasion is shown in Figure 7c. The coating was still contact angle and sliding angle after cycles of abrasion is shown in Figure 77c.c. TheThe coatingcoating waswas stillstill superhydrophobic even after 10 cycles of abrasion. Then, the apparent contact angle decreased to less superhydrophobic even after 10 cycles of abrasion abrasion.. Then, Then, the the apparent apparent contact contact angle angle decreased decreased to less less than 150°. The photographs of yogurt and water droplets on the coated glass surface after 0, 10 and thanthan 150°. 150 . The The photographs photographs of of yogurt yogurt and and water water droplets droplets on on the the coated coated glass glass surface surface after after 0, 0, 10 10 and 12 cycles◦ of abrasion test are shown in Figure 8d–f. After 10 times of abrasion, the yogurt and water 12 cycles of abrasion test are shown in Figure8 8dd–f.–f. AfterAfter 1010 timestimes ofof abrasion,abrasion, thethe yogurtyogurt andand waterwater droplets could still maintain spherical shape on the surface. The coated glass was immersed into droplets couldcould stillstill maintain maintain spherical spherical shape shape on theon the surface. surface. The coatedThe coated glass glass was immersed was immersed into yogurt into yogurt to see its non-wettable property. The yogurt flowed down easily without wetting or yogurtto see its to non-wettable see its non-wettable property. Theproperty. yogurt The flowed yogurt down flowed easily down without easily wetting without or contaminating wetting or contaminating the surface after 10 times of abrasion (Figure 8b). However, the yogurt adhered to the contaminatingthe surface after the 10 surface times ofafter abrasion 10 times (Figure of abrasion8b). However, (Figure 8b). the However, yogurt adhered the yogurt to the adhered surface to due the surface due to loss of superhydrophobicity after 12 cycles of abrasion (Figure 8c). During the test, the surfaceto loss ofdue superhydrophobicity to loss of superhydrophobicity after 12 cycles after of 12 abrasion cycles of (Figureabrasion8c). (Figure During 8c). the During test, thethe surfacetest, the surface was abraded longitudinally. The physical abrasion led to removal of soybean wax and partial surfacewas abraded was abraded longitudinally. longitudinally. The physical The physical abrasion abra ledsion to removal led to removal of soybean of soybean wax and wax partial and partial loss of loss of surface roughness, which were key factors for its superhydrophobicity. losssurface of surface roughness, roughness, which werewhich key were factors key factors for its superhydrophobicity.for its superhydrophobicity.

FigureFigure 7. 7. ((aa,,bb)) One One abrasion abrasion test test cycle cycle and( and(cc)) the the change changechange of of apparent apparentapparent contact contact angle angleangle and andand sliding sliding angle angle afterafter cycles cycles of of abrasion. abrasion.

FigureFigure 8. 8. TheThe coatedcoated glassglassglass after afterafter immersion immeimmersionrsion into intointo yogurt yogurtyogurt after afterafter (a )((a 0;a)) (0;0;b )((b 10b)) 10 and10 andand (c) 12((cc)) cycles 1212 cyclescycles of abrasion ofof abrasionabrasion test; test;Photographstest; PhotographsPhotographs of yogurt ofof yogurtyogurt and water andand waterwater droplets dropletsdroplets on a coated onon aa coatedcoated glass surfaceglassglass surfacesurface after ( afterdafter) 0; ((edd))) 10 0;0; ( and(ee)) 1010 (f )andand 12 cycles ((ff)) 1212 cyclesofcycles abrasion ofof abrasionabrasion test. test.test.

Apart from sandpaper abrasion, the impact resistance of the soybean wax coated surface was evaluated by water dropping from a height of 30 cm (Figure9a). The apparent contact angle and sliding angle were measured every 10 drops. The change of apparent contact angle and sliding angle after water impact are shown in Figure9b. The coating maintained excellent water repellency after Materials 2020, 13, x FOR PEER REVIEW 7 of 11

MaterialsApart 2020, from13, x FOR sandpaper PEER REVIEW abrasion, the impact resistance of the soybean wax coated surface7 ofwas 11 evaluated by water dropping from a height of 30 cm (Figure 9a). The apparent contact angle and slidingApart angle from were sandpaper measured abrasion, every 10 thedrops. impact The resistancechange of apparentof the soybean contact wax angle coated and slidingsurface angle was evaluatedMaterialsafter water2020 by, impact13 water, 3308 aredropping shown fromin Figure a height 9b. The of 30coating cm (Figure maintained 9a). The excellent apparent water contact repellency angle7 afterand of 11 sliding70 drops angle with were excellent measured superhydrophobicity. every 10 drops. The Then, change the apparent of apparent contact contact angle angle decreased and sliding to less angle than after150°. water The impact impact resistance are shown of inthe Figure coated 9b. surface The coating was crucial maintained for its application excellent water in liquid repellency packaging. after 7070 drops drops with with excellent excellent superhydrophobicity. superhydrophobicity. Then, Then, th thee apparent apparent contact contact angle angle decreased decreased to to less less than than 150°.150◦. The The impact impact resistance resistance of of the the coated coated surface surface was was crucial crucial for for its its application application in liquid packaging.

Figure 9. (a) The device used for an impact resistance test; (b) the change of apparent contact angle Figureand sliding 9. (a) angle The device after water used forimpact. an impact resistance test; (b) the change of apparent contact angle and Figure 9. (a) The device used for an impact resistance test; (b) the change of apparent contact angle sliding angle after water impact. andThe sliding coated angle surface after was water pressed impact. with a finger to test its durability (Figure 10a). The direction of touchingThe coatedis marked surface with was a red pressed line in with Figure a finger 10b. The to test water its durability droplets maintained (Figure 10a). spherical The direction shape; ofthey touchingThe could coated roll is marked surfacerandomly with was on apressedred the line touched with in Figure a fingersurface. 10 b.to TheAstest shown waterits durability dropletsin Figure (Figure maintained 10c, 10a).the coated The spherical direction glass shape; was of touchingtheyimmersed could is into marked roll yogurt randomly with repeatedly. a onred the line touchedThe in Figuresuperhydroph surface. 10b. The Asobicity water shown of droplets the in Figuretreated maintained 10 surfacec, the coatedwasspherical studied glass shape; after was theyimmersedimmersion could into roll(Figure yogurtrandomly 10d). repeatedly. The on apparentthe touched The superhydrophobicitycontact surface. angl eAs decreased shown of in theand Figure treated the sliding 10c, surface the angle coated was increased studied glass afterwithwas immersedimmersionthe number into (Figure of yogurt immersion 10d). repeatedly. The times. apparent TheAfter superhydroph contact immersed angle into decreasedobicity yogurt of the andfor treated the20 times, sliding surface the angle wascoating increased studied remained after with immersion (Figure 10d). The apparent contact angle decreased and the sliding angle increased with thesuperhydrophobic. number of immersion The coated times. glass After was immersed immersed into into yogurt yogurt for for 20 times,24 h to the study coating the remained effect of the number of immersion times. After immersed into yogurt for 20 times, the coating remained superhydrophobic.immersion time on The the coatedsuperhydrophobicity glass was immersed of the into coating. yogurt forAfter 24 ha to24 study h immersion, the effect ofthe immersion apparent superhydrophobic. The coated glass was immersed into yogurt for 24 h to study the effect of timecontact on angle the superhydrophobicity of the coated surface ofwas the still coating. above After150°, ashowing 24 h immersion, excellent liquid-repellent the apparent contact properties. angle immersion time on the superhydrophobicity of the coating. After a 24 h immersion, the apparent ofTo thefurther coated evaluate surface the was durability still above of 150 the◦, coated showing surface, excellent the liquid-repellent glass treated with properties. soybean To wax further was contact angle of the coated surface was still above 150°, showing excellent liquid-repellent properties. evaluateexposed theto air durability for 200 ofdays. the coatedAs shown surface, in Figure the glass 11b, treated the coating with soybeanremained wax superhydrophobic was exposed to air with for To further evaluate the durability of the coated surface, the glass treated with soybean wax was 200the apparent days. As showncontactin angle Figure higher 11b, than the coating 150°. In remained the meantime, superhydrophobic a water droplet with could the apparentstill maintain contact its exposed to air for 200 days. As shown in Figure 11b, the coating remained superhydrophobic with anglespherical higher shape than on 150 the◦ .surface, In the meantime,demonstrating a water the dropletstability couldof the stillcoating maintain exposed its sphericalto air. The shape soybean on the apparent contact angle higher than 150°. In the meantime, a water droplet could still maintain its thewax surface, coating demonstrating could maintain the stability stability because of the coating of its exposedlong chain to air. The soybeancomposition wax coatingexposed could to a spherical shape on the surface, demonstrating the stability of the coating exposed to air. The soybean maintaincorrosive stabilityenvironment. because of its long chain alkane composition exposed to a corrosive environment. wax coating could maintain stability because of its long chain alkane composition exposed to a corrosive environment.

Figure 10. (a) The durability test by pressing with a finger; (b) photograph of water droplets after Figure 10. (a) The durability test by pressing with a finger; (b) photograph of water droplets after touching;touching; ((c)) one-timeone-time immersionimmersion inin yogurtyogurt andand ((d)) thethe changechange ofof apparentapparent contactcontact angleangle andand slidingsliding Figureangle ofof 10. thethe (a treatedtreated) The durability surfacesurface plottedplotted test by againstagainst pressing immersionimmersion with a finger; timestimes inin(b yogurt.)yogurt. photograph of water droplets after touching; (c) one-time immersion in yogurt and (d) the change of apparent contact angle and sliding

angle of the treated surface plotted against immersion times in yogurt.

Materials 2020, 13, 3308 8 of 11 Materials 2020, 13, x FOR PEER REVIEW 8 of 11

Figure 11.11. PhotographsPhotographs ofof yogurt yogurt and and water water droplets droplets (a) ( ona) aon coated a coated glass glass surface surface and (andb) after (b) 200after days. 200 days. 3.6. Comparison with Other Coatings 3.6. ComparisonIt is reported with that Other the edibleCoatings superhydrophobic coating was fabricated by paraffin wax, beeswax, microcrystallineIt is reported wax thatand the edible carnauba superhydrophobic wax, et al. The coating apparent was contact fabricated angle by and paraffin sliding wax, angle beeswax, of the coatingmicrocrystalline treated by wax soybean and was wax compared, et al. The with apparent other contact reported angle inand the sliding existing angle literature. of the Dicoatingfferent treated from microcrystalline by soybean wax wax was and compared paraffin with wax, other the soybean waxes reported wax, beeswax, in the carnaubaexisting literature. wax and riceDifferent bran waxfrom are microcrystalline renewable waxes. wax and As shown paraffin in wa Tablex, the1, the soybean apparent wax, contact beeswax, angles carnauba of the wax coating and withrice bran beeswax, wax are microcrystalline renewable waxes. wax, mixtureAs shown of candelillain Table 1, wax the andapparent rice bran contact wax angles were slightlyof the coating higher thanwith thatbeeswax, of soybean , used wax, in our mixture research. of candelilla The different wax superhydrophobic and were properties slightly of higher waxes arethan owed that of to soybean their diff wax,erent used chemical in our constitution research. The and different dissolution-precipitation superhydrophobic characterproperties in of solvent. waxes Theare owed soybean to waxtheir hasdifferent advantages chemical of wide constitution source and and low dissolution-precipitation price. Thus, it is promising character for application in solvent. in theThe food soybean industry, wax has to reduce advantages waste. of wide source and low price. Thus, it is promising for application in the food industry, to reduce waste. Table 1. The apparent contact angle and sliding angle of the superhydrophobic coatings with different waxes. Table 1. The apparent contact angle and sliding angle of the superhydrophobic coatings with different waxes. Waxes Apparent Contact Angle (◦) Sliding Angle (◦) Soybean Wax (This Research) 159 2 7 1 Waxes Apparent Contact± Angle (°) Sliding± Angle (°) Paraffin Wax [29] 158 3 7 1 Soybean Wax (This Research) 159 ± 2 ± 7 ± 1 Beeswax [29] 162 2 7 1 ± ± MicrocrystallineParaffin Wax [29] Wax [29] 158 161 ± 32 5 7 1± 1 ± ± BeeswaxCarnauba [29] Wax [29] 162 150 ± 22 22 7 ±2 1 ± ± MixtureMicrocrystalline of andWax Rice [29] Bran Wax [23] 161162 ± 22 1 5 1± 1 ± ± Carnauba Wax [29] 150 ± 2 22 ± 2 Mixture of Candelilla Wax and Rice Bran The coating with soybean wax was subjected to abrasion162 tests ± with2 sandpaper, water impact, 1 ± 1 finger touch and immersionWax into [23] yogurt to evaluate its robustness and durability. Zhao et al. [29] immersed the superhydrophobic coating of paraffin wax into 1M HCl (Sinopharm Chemical Reagent CO. LTD, Shanghai,The coating China), with 1M NaOH soybean (Sinopharm wax was Chemicalsubjected Reagent to abrasion CO. LTD,tests Shanghai,with sandpaper, China) andwater deionized impact, waterfinger madetouch by and water immersion purification into machineyogurt to (Ruide evaluate Chemical its robustness Instrument and CO. durability. LTD, Zaozhuang, Zhao et China),al. [29] respectively.immersed theAfter superhydrophobic 24 h of immersion, coating the of apparentparaffin wax contact into angles1M HCl of (Sinopharm the treated Chemical coatings wereReagent all CO. LTD, Shanghai, China), 1M NaOH (Sinopharm Chemical Reagent CO. LTD, Shanghai, China) higher than 150◦. Liu et al. [23] created an edible liquid-repellent coating by mixing rice bran wax and candelilladeionized waxwater on made polypropylene by water purification substrates. Themachine apparent (Ruide contact Chemical angle Instrument of the coating CO. lost LTD, its superhydrophobicZaozhuang, China), properties respectively. after After 8 cycles 24 h of during imme thersion, sandpaper the apparent abrasion contact test. angles After of 1200the treated cycles coatings were all higher than 150°. Liu et al. [23] created an edible liquid-repellent coating by mixing of 180◦ bending, the coating was still non-wettable without any change in appearance. The edible coatingrice bran with wax soybean and candelilla wax in wax our researchon polypropylene could endure substrates. 10 cycles The of apparent abrasion testcontact under angle the of same the pressure.coating lost Chen its superhydrophobic et al. [30] used the properties solution-dipping after 8 methodcycles during for sequential the sandpaper deposition abrasion of a trilayertest. After of branched1200 cycles poly(ethylenimine), of 180° bending, the ammonium coating was polyphosphate still non-wettable and fluorinated-decyl without any change polyhedral in appearance. oligomeric silsesquioxane.The edible coating The with superhydrophobic soybean wax in coating our research could could maintain endure its flame-retardant 10 cycles of abrasion and self-healing test under propertiesthe same pressure. even after Chen 1000 et cycles al. [30] of abrasion used the (under solution-dipping a pressure of method 44.8 kPa). for However,sequential the deposition non-wettable of a trilayer of branched poly(ethylenimine), ammonium polyphosphate and fluorinated-decyl coatings made of fluorocarbon materials are not suitable for the application of direct contact with food. polyhedral oligomeric silsesquioxane. The superhydrophobic coating could maintain its flame- Thus, the edible superhydrophobic coatings need to improve their heat resistance and robustness, retardant and self-healing properties even after 1000 cycles of abrasion (under a pressure of 44.8 kPa). which is also our future research direction. However, the non-wettable coatings made of fluorocarbon materials are not suitable for the application of direct contact with food. Thus, the edible superhydrophobic coatings need to improve their heat resistance and robustness, which is also our future research direction.

3.7. Application Test

Materials 2020, 13, 3308 9 of 11

3.7. Application Test Materials 2020, 13, x FOR PEER REVIEW 9 of 11 To demonstrate the practicality of the edible superhydrophobic coating of soybean wax, the inside of a paperTo demonstrate cup was coated the practicality with soybean of wax the toedible testits superhydrophobic performance. The coating cups were of filledsoybean with wax, various the viscousinside of liquid a paper food, cup including was coated milky with tea, soybean honey andwax chocolateto test its syrup.performance. As shown The in cups Video were S3, filled the liquid with wasvarious poured viscous out fromliquid the food, cup. including The high-viscous milky tea, liquid honey in and the soybeanchocolate wax syrup. treated As cupshown came in outVideo easily S3, withthe liquid a little was residue, poured demonstrating out from the cup. liquid-repellent The high-viscous properties liquid of in the the paper soybean cup. wax In contrast,treated cup without came coatingout easily treatment, with a little it was residue, hard demonstrating for the liquid to liquid-r flow outepellent and significant properties residues of the paper were cup. left stuckIn contrast, to the originalwithout cup.coating Figure treatment, 12 shows it thewas photographs hard for the ofliqu theid remaining to flow out viscous and significant liquid after residues pouring. were There left is hardlystuck to any the liquid original left withcup. theFigure coated 12 cup.shows Thus, the thephotographs edible non-wettable of the remain coatinging of viscous soybean liquid wax couldafter repelpouring. complex There non-Newtonian is hardly any liquid fluids left with with little the residue.coated cup. Thus, the edible non-wettable coating of soybean wax could repel complex non-Newtonian fluids with little residue.

Figure 12. Photographs of (left) remaining viscous liquid after pouring in a soybean wax coated cup Figure 12. Photographs of (left) remaining viscous liquid after pouring in a soybean wax coated cup and (right) original cup: (a) milky tea; (b) chocolate syrup and (c) honey. and (right) original cup: (a) milky tea; (b) chocolate syrup and (c) honey.

4. Conclusions In this work, an edible superhydrophobic coatingcoating was prepared with soybean wax by a one-step method, which could be widely applied to various packaging materials to eliminate liquid food residues and save resources. Through a variety of te tests,sts, the coating showed exce excellentllent stability to repel high-viscosity non-Newtonian liquidliquid foodfood andand eveneven aa hot aqueous solution (about 45 ◦°C).C). The coating maintained its superhydrophobicity after abrasion tests with sandpaper, water impact, fingerfinger touch and immersion into into yogurt. yogurt. The The superhydrophobic superhydrophobic coating coating prepared prepared by bysoybean soybean wax wax was wasedible edible and androbust; robust; it could it could be widely be widely used used to reduce to reduce liquid liquid food foodresidues residues due to due its tosimple, its simple, renewable renewable and low- and low-costcost characteristics. characteristics.

Supplementary Materials: The following are available onlineonline atat http:www.//www.mdpi.commdpi.com/xxx/s1./1996-1944 Video S1:/13 The/15/ 3308bounce/s1. Videoof a water S1: Thedroplet bounce on the of acoating water dropletsurface onof glass. the coating Video surfaceS2: The ofhitting glass. of Video a water S2: column The hitting on the of coating a water surface. column on the coating surface. Video S3: The remaining viscous liquid after pouring in (left) soybean wax coated cup and Video S3: The remaining viscous liquid after pouring in (left) soybean wax coated cup and (right) original cup. (right) original cup. Author Contributions:Contributions: Data curation, T.S.; formal analysis, T.S., Y.L.; resources,resources, G.X.;G.X.; writing—originalwriting—original draft preparation, S.F.;S.F.; writing—reviewwriting—review and and editing, editing, W.F. W.F. All Al authorsl authors have have read read and and agreed agree to to the the published published version version of theof the manuscript. manuscript. All authors have read and agreed to the published version of the manuscript.

Funding: This work was supported financially financially by funding fr fromom the Key Scientific Scientific and Technological Project of Henan Province (No. 202102210024, No. 152102210099), Key Scientific Research Project of Henan Provincial Henan Province (No. 202102210024, No. 152102210099), Key Scientific Research Project of Henan Provincial Education Department (No. 20A150018) and High-Level Talent Start-Up Research Project of Henan Institute of ScienceEducation and Department Technology (No. (No. 20A150018) 2017043). and High-Level Talent Start-Up Research Project of Henan Institute of Science and Technology (No. 2017043). Acknowledgments: The author is very grateful to Professor Yuping Zhang from the School of Chemistry and ChemicalAcknowledgments: Engineering, The Henan author Institute is very ofgrateful Science to and Professor Technology Yuping for Zhang technical from assistance. the School of Chemistry and ConflictsChemical ofEngineering, Interest: The Henan authors Institute declare of noScience conflict and of Technology interest. for technical assistance. Conflicts of Interest: The authors declare no conflict of interest.

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