Interaction of Surface Energy Components Between Solid and Liquid on Wettability, and Its Application to Textile Anti-Wetting Finish

Article Interaction of Surface Energy Components between Solid and Liquid on Wettability, and Its Application to Textile Anti-Wetting Finish

Kwanwoo Song 1, Jinwook Lee 1,2, Seong-O Choi 3,4 and Jooyoun Kim 1,5,*

1 Department of Textiles, Merchandising and Fashion Design, Seoul National University, Seoul 08826, Korea; [email protected] (K.S.); [email protected] (J.L.) 2 Department of Chemical Education, Seoul National University, Seoul 08826, Korea 3 Department of Anatomy and Physiology, Kansas State University, Manhattan, KS 66506, USA; [email protected] 4 Nanotechnology Innovation Center of Kansas State, Kansas State University, Manhattan, KS 66506, USA 5 Research Institute of Human Ecology, Seoul National University, Seoul 08826, Korea * Correspondence: [email protected]; Tel.: +82-2-880-6846

Received: 27 February 2019; Accepted: 10 March 2019; Published: 14 March 2019

Abstract: With various options of anti-wetting finish methods, this study intends to provide basic information that can be applied in selecting a relevant anti-wetting chemical to grant protection from spreading of liquids with different surface energy profiles. With such an aim, the anti-wetting effectiveness of fluorinated coating and silane coating was investigated for liquids having different surface energy components, water (WA), methylene iodide (MI) and formamide (FA). The wetting thermodynamics was experimentally investigated by analyzing dispersive and polar component surface energies of solids and liquids. The role of surface roughness in wettability was examined for fibrous nonwoven substrates that have varied surface roughness. The presence of roughness enhanced the anti-wetting performance of the anti-wetting treated surfaces. While the effectiveness of different anti-wetting treatments was varied depending on the liquid polarities, the distinction of different treatments was less apparent for the roughened fibrous surfaces than the film surfaces. This study provides experimental validation of wetting thermodynamics and the practical interpretation of anti-wetting finishing.

Keywords: surface energy; dispersive; polar; anti-wetting; roughness; nonwoven; etching; contact angle; thermodynamics

1. Introduction Surface wettability plays an important role in many functional applications [1] such as superhydrophobic self-cleaning materials [2], anti-fouling surfaces [3,4], liquid separation [2,5–7], corrosion protection [8], and fluid control [9–11]. With various industrial applicability, strategies for modifying surface wettability have been immensely studied [9,10,12–18]. One of the common design strategies for making an anti-wetting surface is to lower the surface energy and to implement the surface roughness [9,13,19–24]. While the material’s intrinsic wettability depends primarily on the surface energy, the role of surface roughness becomes critical when fabricating extremely anti-wetting surfaces [9,22,25]. Recent findings have shown that forming surface roughness in re-entrant structures can produce an inherently unwetting surface [25–28], regardless of the surface energy. Surface wettability is generally represented by the measure of contact angle (CA) and shedding angle (ShA). Commonly accepted criteria for superhydrophobic surface are CA > 150◦ and ShA < 10◦ [19,20].

Polymers 2019, 11, 498; doi:10.3390/polym11030498 www.mdpi.com/journal/polymers Polymers 2019, 11, 498 2 of 15

Traditionally Young’s equation [29] explains the solid–liquid interactions in wetting using the overall surface energies of solids and liquids, assuming a smooth and flat solid surface. The interfacial energy of solids and liquids depends on the dispersive and polar components of the surface energy of individual solid and liquid. From this relationship, the wettability is generally reduced when the overall surface energy of the solid surface is lowered. Early studies on anti-wetting surface modification have been focused on novel anti-wetting chemicals with a low surface energy or durable treatment methods [5,30–32]. Fluorinated compounds have been extensively applied in anti-wetting finish methods due to their very low surface energy [19,20,31,33,34]. Recently the environmental concern has been raised on the use of perfluorinated compounds having chain lengths of eight carbons (C8) or longer [35], and the alternatives to C8 perfluorinated chemicals, including silane with long carbon chains, are being explored [36–38]. The overall surface energy is the sum of the polar and dispersive components of surface energy [39,40]. Different anti-wetting agents with similar overall surface energy; however, could have distinct surface energy components, which would affect their interfacial interaction with liquids and thereby would show different wettability. Due to an increasing interest in anti-wetting materials, extensive studies are available for anti-wetting fabrication, most of which are focused on surface designs to maximize repellency or anti-wetting [5,6,26,27,31,41,42]. However, the interplay between the anti-wetting surface and liquids has rarely been discussed although the wetting thermodynamics would be different depending on the surface energy profiles of the anti-wetting agents and the wetting liquid. While the reduction of surface energy is the primary means of anti-wetting fabrication, it is often not sufficient to attain extreme repellency only by modifying the surface energy of solid surfaces. Thus, the implementation of surface roughness becomes significant in achieving the high level of anti-wetting property [43,44]. When an anti-wetting coating is applied to fabrics, the surface usually displays a higher level of anti-wetting than flat film surfaces, due to the presence of roughness formed by the microscale fibers and yarns [19,20,32,33,45]. With the additional roughness formed by the particle deposition [38,45] or the surface etching [36,46,47], the fabric surface can further enhance the anti-wetting property [9,22,25,26]. This study aims at providing the experimental investigation on the solid–liquid interactions, using the concept of surface energy components and the related thermodynamics. To investigate interface interactions, polymeric surfaces were treated with two different anti-wetting chemicals, fluorinated and silane compounds, and the wettability was measured with three different liquids. Anti-wetting coating was applied either by plasma enhanced chemical vapor deposition (PECVD) using C4F8 gas, or by physical vapor deposition using dodecyltrimethoxy silane (DTMS). The tested liquids were water (WA), formamide (FA), and methylene iodide (MI), which have different surface energy components. To examine the interplay between the surface energy components, dispersive and polar components of the tested surfaces and liquids were analyzed. The spreading parameter of the liquid on the solid surface was calculated using two components of surface energy [39]. The study also intends to examine the interplay of roughness and surface energy on the wetting of liquids on fibrous surfaces. Surfaces with different roughness, including flat film, spunlace (SL) nonwoven, spunbond (SB) nonwoven, and SB with alkali-etching, were examined for anti-wetting properties. The surface profiles were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Anti-wetting fabrics can be applied to a protective garment that needs protection against exposure to hazardous liquids. To achieve the anti-wetting performance to a particular liquid, understanding the interfacial interaction between the solid and liquid is significant. The emphasis of this study lies in the experimental validation of wetting thermodynamics, and its extension to the practical application of textile finish. The result of this study would be useful in choosing a proper anti-wetting finishing agent for the protection against a particular liquid with known surface energy components or polarity. Polymers 2019, 11, 498 3 of 15

2. Materials and Methods

2.1. Materials Sodium hydroxide (NaOH) was purchased from Junsei Chemical Co. (Tokyo, Japan). Octafluorocyclobutane (C4F8) gas was obtained from Union Gas (Yongin, South Korea). Chemicals, including dodecyltrimethoxysilane (DTMS), methylene iodide (MI), and formamide (FA), were purchased from Sigma Aldrich (St. Louis, MO, USA). Three different polymeric substrates, cellulose (Cel), polypropylene (PP), and polyethylene terephthalate (PET), were used for surface wetting analysis. Those polymers have different surface energy profiles and are commonly used as nonwoven materials for various applications. Cellulose and PET films were purchased from Goodfellow (Huntingdon, UK). PP films were prepared using PP resin obtained from SK Global Chemical Co., Ltd. (Seoul, South Korea). Cellulose and PET nonwovens were obtained from Korea Institute of Industrial Technology (KITECH, Cheonan, South Korea). The specifications of films and nonwovens used in this study are shown in Table1.

Table 1. Film and nonwoven substrates used.

Substrate Polymer Process Thickness (mm) Basis Weight (g/m2) Cellulose Casting 0.3 45 Film PP Casting 0.3 42 PET Casting 0.4 22 Cellulose Spunlace 2.0 26 Nonwoven PET Spunbond 1.4 20

2.2. Anti-Wetting Coating For hydrophobic coating, two different methods were used; plasma enhanced chemical vapor deposition (PECVD) and physical vapor deposition (PVD). For PECVD, a gas ﬂow of C4F8 was provided with 100 sccm for 25 min at 160 W in Covance plasma system (Femto Science, Hwaseong, South Korea) for the polymerization of a ﬂuorinated compound on the substrate. For the PVD method, 10 mL of dodecyltrimethoxysilane (DTMS) was vaporized in a vacuum oven (oven inner volume ~0.13 m3, LVO-2051P, Daehan Labtech, Namyangju, South Korea) at 120 ◦C for 3 h to coat the substrate.

2.3. Alkali Etching Nanoscale roughness was created on PET surfaces utilizing the alkali hydrolysis reaction of polyester [48]. For this treatment, a 30% (w/w) NaOH solution was prepared with distilled water, and a 10 cm × 10 cm PET nonwoven specimen was immersed in 50 mL solution at 80 ◦C for 60 min. The alkali treated specimens were rinsed with distilled water till pH reached 7, and dried at 80 ◦C for 2 min. The weight loss after alkali hydrolysis was measured to be ~50%. Alkali-etched PET spunbond nonwoven samples were coated with DTMS or C4F8 as previously described for hydrophobic modiﬁcation. The specimens used in this study are described in Table2. Polymers 2019, 11, 498 4 of 15

Table 2. Description of specimens.

Specimen Code Polymer Substrate Etching Chemical Treatment Cel None Cellulose Cel-f C4F8 plasma, f

Cel-Si DTMS deposition, Si PP PPFlat ﬁlm None

PET None None PET PET-f C4F8 plasma, f

PET-Si DTMS deposition, Si Cel(SL) None Spunlace (SL) Cellulose Cel(SL)-f nonwoven C4F8 plasma, f

Cel(SL)-Si DTMS deposition, Si PET(SB) None PET PET(SB)-f C4F8 plasma, f

PET(SB)-Si Spunbond (SB) DTMS deposition, Si PET(SB-etch) nonwoven None PET Alkali etching PET(SB-etch)-f C4F8 plasma, f

PET(SB-etch)-Si DTMS deposition, Si

2.4. Wettability For wetting analysis, static contact angle (CA) and shedding angle (ShA) of liquids on surfaces were measured using a contact angle analyzer (SmartDrop Lab, Femtobiomed Inc., Seongnam, South Korea). Contact angles of distilled water (WA), methylene iodide (MI), and formamide (FA) liquids were measured to examine the wettability of surfaces by liquids with different surface energy components. The surface energy components of different liquids used in this study are listed in Table3. The ratio of polarity to the overall surface energy of solid (PS) and liquid (PL) was calculated (PL in Table3,P S in Table4).

Table 3. Reference values of surface energy components of liquids [49].

d p Liquid γL (mN/m) γL (mN/m) γL (mN/m) PL (Polar Ratio) Water (WA) 72.8 21.8 51.0 0.70 Formamide (FA) 58.4 31.4 27.0 0.46 Methylene iodide (MI) 50.8 50.4 0.40 0.008 d p Note: γL, overall surface energy of liquid; γL, dispersive component surface energy of liquid; γL, polar component surface energy of liquid; PL, ratio of the polar component to the overall surface energy of liquid.

Table 4. Contact angle of water and methylene iodide, and the estimated surface energy.

Contact Angle (◦) Surface Energy (mN/m) Specimen Polar Ratio (PS) WA MI γ γd γp Cel 0 50 73.4 28.0 45.4 0.628 Cel-f 107 94 12.5 10.2 2.3 0.184 Cel-Si 101 60 28.6 28.2 0.42 0.015 PP 105 63 26.9 26.7 0.16 0.006 PET 96 61 28.5 27.2 1.34 0.047 PET-f 106 95 12.5 9.75 2.73 0.219 PET-Si 104 61 28.0 27.8 0.17 0.006 Polymers 2019, 11, 498 5 of 15

For CA measurement, 2.5 ± 0.5 µL of liquid drops was placed on a surface, and CA was measured within 1 s after the droplet settlement. At least ﬁve measurements were obtained from different spots of the surfaces, and the mean value was used for analysis. For ShA measurement, 12.5 µL of liquid was dropped vertically from 1 cm above the sample that was laid on a tilted stage. The lowest angle at which the drop starts to roll more than 2 cm on the surface was recorded as ShA, and the mean value of ﬁve measurements was used for analysis.

2.5. Surface Energy of Solid Surface Surface energy with its dispersive and polar components for the solid surfaces was estimated using the Owens–Wendt model [40]. With the known surface energy components of liquids and by measuring the contact angle on a flat surface, the surface energy components of solid substrates can be calculated, as shown in Equations (1)–(4) [40]. For the calculation, contact angles of WA and MI were measured on flat film surfaces. The surface energy of the nonwoven surface was regarded as the same as that of the respective film. q q d d p p γSL = γS + γL − 2 γS·γL − 2 γS·γL (1)

γS = γSL + γL cos θ (2) q q d d p p γL(1 + cosθ) = 2 γS·γL − 2 γS·γL (3) d p γS = γS + γS (4)

• θ: contact angle of liquid on solid surface

• γSL: interfacial energy between solid and liquid • γS: surface energy of solid d • γS: dispersive component surface energy of solid p • γS: polar component surface energy of solid • γL: surface energy of liquid d • γL: dispersive component surface energy of liquid p • γL: polar component surface energy of liquid

2.6. Surface Chemistry and Morphology The surface chemistry of substrates in terms of atomic concentration (%) was analyzed using X-ray photoelectron spectroscopy (XPS, Axis SupraTM, Kratos Analytical, Manchester, UK), to verify the presence of coating compounds on the surfaces. The surface morphology of the samples was observed using a ﬁeld-emission scanning electron microscope (FE-SEM, Supra 55VP, Carl Zeiss, Jena, Germany), with prior Pt coating (~10 nm) at 30 mA for 200 s using a sputter coater (EM ACE200, Leica, Wetzlar, Germany). For observation of surface roughness, an atomic force microscope (AFM, NX-10, Park Systems, Suwon, South Korea) was used with non-contact mode, and the root mean square of height (RMS) was measured as representative roughness information.

3. Results and Discussion

3.1. Wettability of Flat Surfaces

3.1.1. Surface Energy Modification with Hydrophobic Treatment Polymeric substrates of cellulose (Cel) and polyethylene terephthalate (PET) were treated with silane (DTMS) and fluorinated (C4F8) compounds for anti-wetting surface modification. Polypropylene (PP) consisting completely of hydrocarbons was compared with those surfaces. For the substrates, surface energy and its components were calculated by measuring the CA of water (WA) and methylene Polymers 2019, 11, 498 6 of 15

iodide (MI). The PS (polar ratio) of solid was calculated by the ratio of the polar component to the overall surface energy. The CA measurements and the surface energy estimates are shown in Table4 andPolymers Figure 20181, .10, x FOR PEER REVIEW 6 of 15

Figure 1. Surface energy components of solid surfaces.

ForThe bothchange Cel of and surface PET substrates, chemistry surfaceafter the coating coating with process DTMS was and examined C4F8 reduced by XPS the (Table overall 5). surface It was energy,observed down that tothe about atomic 28 concentration mN/m and 12 (%) mN/m, of O and respectively. Si in the DTMS With-treated the silane surfaces coating, was the signi dispersiveficantly componentlarger than other was significantlysurfaces. The reduced deposition for of both DTMS Cel was and more PET andapparent the Ps for of the surface Cel samples energy reachedthan for 0.006–0.015.the PET samples On the, probably contrary, due the fluorinatedto the higher coating affinity produced of silane higher compounds Ps (0.18–0.22) to –OH compared groups of to Cel the silanesubstrate. coating, The C while4F8-treated the overall surfaces surface showed energy a significant of fluorinated amount surface of F (12. The mN/m) result wasconfirms considerably that the lowerdecrease than of thatsurface of DTMSenergy coatedis attributed surface to (28the mN/m).presence of The coated surface chemicals. energy of PP, which consists of hydrocarbons, was calculated to be about 27 mN/m, most of which is contributed by the dispersive component (>99%). The contributionTable 5. XPS of atomic each polarconcentration and dispersive (%) of surfaces component. to the overall surface energy is presented in Figure1. Specimen C (%) O (%) F (%) Si (%) The change of surface chemistry after the coating process was examined by XPS (Table5). It was PP 97.8 2.2 - - observed that the atomic concentration (%) of O and Si in the DTMS-treated surfaces was significantly Cel 77.1 22.9 - - larger than other surfaces. The deposition of DTMS was more apparent for the Cel samples than for the Cel-f 52.0 30.9 17.2 PET samples, probably due to the higher affinity of silane compounds to –OH groups of Cel substrate. Cel-Si 51.2 27.3 - 21.6 The C4F8-treated surfaces showed a significant amount of F. The result confirms that the decrease of PET 91.9 2.2 - - surface energy is attributed to the presence of coated chemicals. PET-f 49.2 7.3 43.5 - PETTable-Si 5. XPS atomic67.8 concentration23.3 (%) of- surfaces. 8.8

3.1.2. Solid–LiquidSpecimen Interaction on C Wettability (%) O (%) F (%) Si (%) PP 97.8 2.2 - - Wettability isCel the result of 77.1 liquid–solid 22.9 interactions, and - cosθ can be - expressed as the relationship betweenCel-f surface energies52.0 of a liquid 30.9 and solid (E 17.2quations (5) and (6), where θ is the contact angle andCel Φ -Siis the interfacial51.2 function in 27.3the range of 0 to -1 [50]. 21.6 PET 91.9 2.2 - - PET-f 49.2훾푆퐿 = 훾푆 + 7.3훾퐿 − 2Φ√훾푆∙훾퐿 43.5 - (5) PET-Si 67.8 23.3 - 8.8 γ 푆 (6) 3.1.2. Solid–Liquid Interaction on Wettabilitycosθ = 2Φ√ − 1 γL Wettability is the result of liquid–solid interactions, and cosθ can be expressed as the relationship In Equation (6), cosθ would be proportional to Φ 훾 , as shown in Figure 2a. From Figure 2b, between surface energies of a liquid and solid (Equations√ (5)푆 and (6), where θ is the contact angle and Φwhileis the cos interfacialθ increased function with an in increase the range in ofthe 0 tooverall 1 [50 ].surface energy of solid, the relationship between cosθ and 훾푆 does not show a clear tendency. The result confirms the fact that the wetting cannot be √ accurately predicted by a single factorγSL of =solidγS +surfaceγL − 2energyΦ γS.· AsγL the wetting is the result of the solid(5)– liquid interactions, surface energies of both liquids and solids need to be counted in predicting the r γ wettability. From Equation (6), the interfacialcos θ = function2Φ S Φ− is1 proportional to cosθ, and the complete (6) wetting state (cosθ of 1) corresponds to the maximumγL Φ. However, cosθ is not perfectly correlated with Φ alone. Instead, the solid–liquid interplay in terms of their surface energy components needs to be considered when predicting the surface wetting.

Polymers 2019, 11, 498 7 of 15

√ In Equation (6), cosθ would be proportional to Φ γS, as shown in Figure2a. From Figure2b, while cosθ increased with an increase in the overall surface energy of solid, the relationship between cosθ and γS does not show a clear tendency. The result conﬁrms the fact that the wetting cannot be accurately predicted by a single factor of solid surface energy. As the wetting is the result of the solid–liquid interactions, surface energies of both liquids and solids need to be counted in predicting the wettability. From Equation (6), the interfacial function Φ is proportional to cosθ, and the complete wetting state (cosθ of 1) corresponds to the maximum Φ. However, cosθ is not perfectly correlated with Φ alone. Instead, the solid–liquid interplay in terms of their surface energy components needs to be considered when predicting the surface wetting. Polymers 2018, 10, x FOR PEER REVIEW 7 of 15

√ FigureFigure 2.2. WettabilityWettability withwith differentdifferent surfacesurface energyenergy surfaces.surfaces. ((aa)) coscosθθwith with different differentΦ Φ√γ훾S푆;(; (bb)) coscosθθ

withwith differentdifferentγ 훾S푆.. 3.1.3. Interplay of Surface Energy Components on Wetting 3.1.3. Interplay of Surface Energy Components on Wetting As the wetting thermodynamics is influenced by the dispersive and polar components of surface As the wetting thermodynamics is influenced by the dispersive and polar components of surface energy, different liquids with the same overall surface energy can exhibit different wettability on a solid energy, different liquids with the same overall surface energy can exhibit different wettability on a surface if the liquids have different surface energy components. The surface energy components of a solid surface if the liquids have different surface energy components. The surface energy components phase can be reflected in the polar ratio, and the Ps for different substrates is shown in Figure3a. For PP of a phase can be reflected in the polar ratio, and the Ps for different substrates is shown in Figure 3a. and the silane-treated surfaces, the polar component contribution to the overall surface energy was For PP and the silane-treated surfaces, the polar component contribution to the overall surface energy close to zero due to the dominant arrangement of nonpolar hydrocarbons on the surfaces. While the was close to zero due to the dominant arrangement of nonpolar hydrocarbons on the surfaces. While fluorinated surfaces had the lowest overall surface energy, its Ps was higher than that of PP and the the fluorinated surfaces had the lowest overall surface energy, its Ps was higher than that of PP and silane-treated surfaces, probably due to the polarity of fluorine. Polymersthe silane 2018-treated, 10, x FOR surfaces PEER REVIEW, probably due to the polarity of fluorine. 8 of 15 To examine the interplay between surface energy components of solids and liquids on wetting, Equations (1) and (5) were combined to produce Equations (7) and (8). By differentiating Equation (8), Equation (9) is derived.

푑 푑 푝 푝 √훾푆 ∙ 훾퐿 + √훾푆 ∙ 훾퐿 = Φ√훾푆∙훾퐿 (7)

푑 푑 푝 푝 훾푆 ∙ 훾퐿 훾푆 ∙ 훾퐿 (8) Φ = √ + √ = √(1 − PS)(1 − PL) + √PSPL 훾푆∙훾퐿 훾푆∙훾퐿

d 1 (1 − PL) 1 PL = − √ + √ (9) dPS 2 (1 − PS) 2 PS

FigureFigure 3.3. SurfaceSurface polaritypolarity inﬂuencinginfluencing wetting.wetting. (a)) PercentagePercentage ofof thethe polarpolar contributioncontribution toto thethe overalloverall γ surface energy of solids; (b) Interfacial function with % polarity. 푆 (10) surface energy of solids; cos(b) θInterfacial= 2 [√(1 function− PS)(1 with− PL %) +polarity.√PSPL] [√ − 1] γL

It isd noteworthy that the interfacial function Φ evolves differently depending on the polarity of when is zero (when PS = PL), Φ has the maximum value of 1. PL values for liquids are: 0.7 for liquidsd (PPSL). For MI, which is a nonpolar liquid, Φ increases as PS decreases. For polar liquids, such as WAwater and; 0 forFA, MI; Φ shows and 0.46 the formaximum FA. Using value those at certainPL values, polarity the interfacial of solids; maximumfunction, Φ Φ can appears be calculated at Ps of

0.7from for Eq WAuation and Ps(8 )of, resulting0.46 for FA in:. BasedWA, onΦ = th√e0 result.3(1 −, itPS )is+ inferred√0.7PS ;that MI, the Φ reduction= √(1 − P Sof) ;polar FA, ityΦ of=

solid√0.54 is(1 effective− PS) + for√0 .46antiPS-.wetting The maximum of a polar Φ liquid,of 1 would while appear the reduction when P Sof values dispersive are 0.7 surface, 0, and energy 0.46 for is effectiveWA, MI, forand anti FA-,wetting respectively. of nonpolar In Figure liquid. 3b, PsThus, of Cel the-Si effect, PET -Siof, Celanti-f,- wettingand PET for-f were a specific 1.47, 0.61,repellent 18.4, coatingand 21.9 would, respectively. be varied When for different Φ was liquid plotteds, and as athe function wettability of P orS of anti various-wettabilit solids,y against the maximum a certain liquidvalues can of Φbe appearestimated approximately using the surface at the polarity Ps values, Ps of. 0.7 for WA, 0.46 for FA, and 0 for MI. Combining Equations (6) and (8), Equation (10) is derived, which can be used in estimating CA of liquids on different surfaces. In Figure 4, the predicted (from Equation (10)) and measured CAs for FA on surfaces with various polar ratio are presented. While the discrepancy exists between the measured and predicted values (<6°), the wettability could be predicted to some extent.

Figure 4. Contact angle of formamide (FA), actual measurement and estimation from Equation (10).

Understanding that the wetting is determined by the interplay between surface energy components of solids and liquids, the thermodynamics for spreading was examined. Spreading occurs when a liquid wets a solid surface in the air. The spreading parameter, S, is defined by the work of adhesion (solid–liquid interaction) subtracted by the work of cohesion (liquid–liquid interaction), and S can be expressed as Equation (11) [39]. Combining Equations (1) and (11), Equation (12) is derived, where the spreading parameter, S, is expressed by the two surface energy components of a solid and liquid. If S ≥ 0, the liquid completely wets the surface in order to lower the surface energy; a lower S represents a lower wettability (or lower spreading) as a result of the lower interaction between the solid and liquid.

S = 훾푆 − (훾푆퐿 + 훾퐿) (11)

Polymers 2019, 11, 498 8 of 15

To examine the interplay between surface energy components of solids and liquids on wetting, Equations (1) and (5) were combined to produce Equations (7) and (8). By differentiating Equation (8), Equation (9) is derived. q q d d p p √ γS·γL + γS·γL = Φ γS·γL (7) s s d d p p q γS·γL γS·γL p Polymers 2018, 10, x FORΦ PEER= REVIEW + = (1 − PS)(1 − PL) + PSPL 8 of 15 (8) γS·γL γS·γL s s d 1 (1 − P ) 1 P = − L + L (9) dPS 2 (1 − PS) 2 PS r q p γS cos θ = 2 (1 − PS)(1 − PL) + PSPL − 1 (10) γL when dΦ is zero (when P = P ), Φ has the maximum value of 1. P values for liquids are: 0.7 for dPS S L L water; 0 for MI; and 0.46 for FA. Using those PL values, the interfacial function, Φ can be calculated p √ p from Equation (8), resulting in: WA, Φ = 0.3(1 − PS) + 0.7PS; MI, Φ = (1 − PS); FA, Φ = p √ 0.54(1 − PS) + 0.46PS. The maximum Φ of 1 would appear when PS values are 0.7, 0, and 0.46 for WA, MI, and FA, respectively. In Figure3b, Ps of Cel -Si, PET-Si, Cel-f, and PET-f were 1.47, 0.61, 18.4, and 21.9, respectively. When Φ was plotted as a function of PS of various solids, the maximum values of Φ appearFigure approximately 3. Surface polarity at theinfluencing Ps values wetting. of 0.7 (a)for Percentage WA, 0.46 of the for polar FA, contribution and 0 for MI. to the overall It is noteworthysurface energy thatof solids; the ( interfacialb) Interfacialfunction function withΦ evolves % polarity. differently depending on the polarity of liquids (P ). For MI, which is a nonpolar liquid, Φ increases as P decreases. For polar liquids, such ItL is noteworthy that the interfacial function Φ evolves differentlyS depending on the polarity of as WA and FA, Φ shows the maximum value at certain polarity of solids; maximum Φ appears at Ps liquids (PL). For MI, which is a nonpolar liquid, Φ increases as PS decreases. For polar liquids, such as of 0.7WA for and WA FA, and Φ Ps shows of 0.46 the formaximum FA. Based value on at the certain result, polarity it is inferredof solids; thatmaximum the reduction Φ appears of at polarity Ps of of solid0.7 is effectivefor WA and for Ps anti-wetting of 0.46 for FA of.a Based polar on liquid, the result while, it theis inferred reduction that ofthe dispersive reduction of surface polarity energy of is effectivesolid for is effective anti-wetting for anti of-wetting nonpolar of a liquid.polar liquid, Thus, while the the effect reduction of anti-wetting of dispersive for surface a speciﬁc energy repellent is coatingeffective would for be anti varied-wetting for of different nonpolar liquids, liquid. andThus, the the wettability effect of anti or-wetting anti-wettability for a specific against repellent a certain liquidcoating can be would estimated be varied using for the different surface liquid polarity,s, and Ps.the wettability or anti-wettability against a certain Combiningliquid can be Equations estimated using (6) and the (8), surface Equation polarity (10), Ps is. derived, which can be used in estimating CA of Combining Equations (6) and (8), Equation (10) is derived, which can be used in estimating CA liquids on different surfaces. In Figure4, the predicted (from Equation (10)) and measured CAs for FA of liquids on different surfaces. In Figure 4, the predicted (from Equation (10)) and measured CAs for on surfaces with various polar ratio are presented. While the discrepancy exists between the measured FA on surfaces with ◦ various polar ratio are presented. While the discrepancy exists between the and predictedmeasured valuesand predicted (<6 ), values the wettability (<6°), the wettability could be predicted could be predicted to some to extent. some extent.

FigureFigure 4. Contact 4. Contact angle angle of of formamide formamide (FA), (FA),actual actual measurement and and estimation estimation from from Equation Equation (10). (10).

UnderstandingUnderstanding that that the the wetting wetting is is determined determined by by the the interplay interplay between between surface surface energy energy componentscomponents of solidsof solid ands and liquids, liquids the, the thermodynamics thermodynamics for for spreading spreading w wasas examined. examined. Spreading Spreading occursoccurs when when a liquid a liquid wets wet a solids a solid surface surface in thein the air. air. The The spreading spreading parameter, parameter, S, S, is is deﬁned defined by by the thework work of adhesion (solid–liquid interaction) subtracted by the work of cohesion (liquid–liquid of adhesion (solid–liquid interaction) subtracted by the work of cohesion (liquid–liquid interaction), interaction), and S can be expressed as Equation (11) [39]. Combining Equations (1) and (11), Equation and S(12 can) is bederived expressed, where asthe Equationspreading parameter (11) [39]., CombiningS, is expressed Equations by the two surface (1) and energ (11),y Equationcomponents (12) is derived,of a wheresolid and the liquid spreading. If S ≥ 0 parameter,, the liquid S,completely is expressed wets bythe thesurface two in surface order to energy lower componentsthe surface of a solidenergy; and liquid. a lower If S S represents≥ 0, the a liquid lower completely wettability (or wets lower the spreading surface in) as order a result to lower of thethe lower surface interaction between the solid and liquid.

S = 훾푆 − (훾푆퐿 + 훾퐿) (11)

Polymers 2018, 10, x FOR PEER REVIEW 9 of 15

푑 푑 푝 푝 (12) S = 2(√훾푆 ∙ 훾퐿 + √훾푆 ∙ 훾퐿 - 훾퐿)

Polymerswhere 2019S is, the11, 498 spreading parameter. 9 of 15 The experimental investigation of the spreading parameter is presented in Figure 5. The relationship between S and cosθ is linearly correlated (Figure 5a), and the complete wetting (cosθ = energy; a lower S represents a lower wettability (or lower spreading) as a result of the lower interaction 1) occurs at S = 0. The slopes of the plots decrease when the polarity of liquid increases; WA showed between the solid and liquid. the lowest slope and MI showed the highest slope among the tested liquids. From Figure 5b, S S = γS − (γSL + γL) (11) increases when 훾 increases, showing the general tendency of favorable wetting of solids with higher 푆 q q surface energy. However, the rate of increased ind S was pnotp consistent; for nonpolar liquid, the rate of S = 2( γS·γL + γS·γL − γL) (12) increase for S declines as 훾푆 increases, while, for a polar liquid, the rate of increase rises as 훾푆 wherebecomes S is larger the spreading. For FA, parameter. of which polarity is about 46%, the rate of increase for S was consistent The experimental investigation of the spreading parameter is presented in Figure5. regardless of 훾푆, displaying the linear relationship between the S and 훾푆. θ The relationshipThe results of between this study S and can cos be usedis linearly as a selection correlated guide (Figure of anti5a),-wetting and the chemicals complete by wetting the type (cosof liquidθ = 1)s occurs that need at S = to 0. be The unwet. slopes For of the example, plots decrease either silane when or the fluorinated polarity of compound liquid increases; would WA be showedsufficient the to lowest grant the slope anti and-wetting MI showed property the against highest water; slope amonghowever, the if tested the solid liquids. needs From to be Figure protected5b, Sfrom increases wetting when againstγS increases, a nonpolar showing liquid the such general as MI, tendency then the of fluorin favorableated wettingcoating ofagent solids that with has higher lower surfacedispersive energy. component However, compared the rate of to increase silane in coating S was notwould consistent; be preferable for nonpolar for better liquid, anti the-wetting rate of increaseperformance. for S declines The results as γS soincreases, far showed while, the for experimental a polar liquid, validation the rate of of increase wetting rises thermodynamics as γS becomes, larger.employing For FA, different of which anti polarity-wetting is about treatment 46%,s the on rate flat of surfaces. increase for In SSection was consistent 3.2, the regardless role of surface of γS, displayingroughness thein the linear anti relationship-wetting property between is further the S and examined.γS.

FigureFigure 5.5. SpreadingSpreading parameterparameter forfor surfacesurface energyenergy andand wettability.wettability. ((aa)) PlotsPlots ofof coscosθθwith with spreadingspreading parameter;parameter; ( b(b)) Spreading Spreading parameter parameter with with different different surface surface energy energy of of the the solid. solid.

3.2. WettingThe results of Fibrous of this Surfaces study can be used as a selection guide of anti-wetting chemicals by the type of liquids that need to be unwet. For example, either silane or fluorinated compound would be sufficientThe to wettability grant the anti-wettingis the result property of the interplay against water;between however, surface if energy the solid and needs surface to be roughness. protected fromTherefore, wetting the againsteffect of a surf nonpolarace roughness liquid suchin addition as MI, to then solid’s the surface fluorinated energy coating was further agent examined that has lowerusing dispersivefibrous nonwoven component substrates compared. The to nonwoven silane coating types would used be in preferable this study for included better anti-wetting Cel(SL) and performance.PET(SB), which The are results commonly so far used showed in disposable the experimental applications. validation As the fibers of wetting of the nonwoven thermodynamics, surface employingwould form different limited anti-wetting surface roughness treatments onin flat microscale surfaces., In the Section PET( 3.2SB), thesamples role of surfaceunderwent roughness alkali inhydrolysis the anti-wetting to create property nanoscale is furtherroughness examined.. The morphology of the substrates with different roughness profiles was observed by SEM as shown in Figure 6. The Cel(SL) samples had unknown particles on 3.2.the Wettingsurfaces of even Fibrous after Surfaces the surface was cleaned; the compositions of the particles were found to be C and O, and the particles were thought to come from the nonwoven process aids. Due to the presence The wettability is the result of the interplay between surface energy and surface roughness. of such particles, Cel(SL) had additional roughness. The Cel(SL) had smaller fibers with denser layers Therefore, the effect of surface roughness in addition to solid’s surface energy was further examined than PET(SB), producing relatively smaller-scale roughness; the fiber sizes of Cel(SL) and PET(SB) using fibrous nonwoven substrates. The nonwoven types used in this study included Cel(SL) and were rather uniform, which were measured to be ~9.6 ± 1.2 μm and ~17.3 ± 0.5 μm, respectively for PET(SB), which are commonly used in disposable applications. As the fibers of the nonwoven surface Cel(SL) and PET(SB). The roughness between SL and SB surfaces, however, could not be quantified would form limited surface roughness in microscale, the PET(SB) samples underwent alkali hydrolysis by SEM. to create nanoscale roughness. The morphology of the substrates with different roughness profiles AFM can measure the variation of height profiles, in a value of root mean squared height (RMS). was observed by SEM as shown in Figure6. The Cel(SL) samples had unknown particles on the However, due to the limited z-scan range (≤4 μm), the RMS information was available only for surfaces even after the surface was cleaned; the compositions of the particles were found to be C and O, and the particles were thought to come from the nonwoven process aids. Due to the presence of such particles, Cel(SL) had additional roughness. The Cel(SL) had smaller fibers with denser layers than PET(SB), producing relatively smaller-scale roughness; the fiber sizes of Cel(SL) and PET(SB) were Polymers 2018, 10, x FOR PEER REVIEW 10 of 15

comparing the submicron scale roughness. The AFM scanning was carried out across the samples (0.8 μm × 0.8 μm), and the RMS values of PET film, PET SB, and alkali-etched PET SB were compared (Figure 7). The RMS value of PET(SB) was not different from that of PET film as the scanning was conducted on the small area of the topmost fiber surface. The PET(SB-etch), however, showed significantly large RMS (~13 nm), confirming the presence of nanoscale roughness in PET(SB-etch). The CA and ShA measurements for the varied substrates are shown in Table 6 and Figure 8. For all fibrous surfaces, the presence of roughness considerably increased the CAs, effectively enhancing the anti-wetting property. Particularly, the surfaces with the nanoscale roughness, Cel(SL)-f, Cel(SL)- Si, PET(SB-etch)-f, and PET(SB-etch)-Si, displayed the superhydrophobic property with WA CA ≥ 150° and WA ShA ≤ ~10°. The result clearly shows the critical role of nanoscale roughness on the anti- wetting property. For further analysis, the Cassie–Baxter state [51] was assumed for the surfaces that displayed the water CA ≥ 147° and ShA ≤ 15°, which were Cel(SL)-f, Cel(SL)-Si, PET(SB)-f, PET(SB-etch)-f, and PET(SB- etch)-Si. If it is assumed that the liquid drop contacts only the topmost surface of the roughened substrate, instead of intruding into the protrusions, the Cassie–Baxter model [51] can be simplified as Equation (13) [19]. In the equation, the solid fraction, f, can be roughly estimated from the measured

CA’s of fibrous surfaces (θCB) and those of flat film surfaces (θY). The estimated f, which is the topmost area fraction of the unit area, were 0.045, 0.23, and 0.142 for the Cel SL, PET SB, and etched PET SB surfaces, respectively. The result showed that the cellulose spunlace had the least area wet by the liquid, probably due to the irregular depth of fibers and fine particles on the surface.

cosθCB = f ∙ (cosθ푌 + 1) − 1 (13) Polymers 2019, 11, 498 10 of 15  θCB: contact angle in the Cassie–Baxter state

 θY: contact angle in Young’s model in Equation (2) (flat surface) rather f uniform,: solid fraction which in were the measuredCassie–Baxter to be model, ~9.6 ± 1.2whichµm andis the ~17.3 fraction± 0.5 ofµ m,wetted respectively surface forarea Cel(SL) to the and PET(SB).overall projected The roughness surface between area. SL and SB surfaces, however, could not be quantiﬁed by SEM.

Film Cel PET PP

Nonwoven Cel(SL) PET(SB) PET(SB-etch)

Polymers 2018, 10, x FOR PEER REVIEW 11 of 15

FigureFigure 6. 6.Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) images images of of surfaces. surfaces. Inserts Inserts are are optical optical microscope microscope imagesimages with with the the scale scale bar bar of of 1 mm.1 mm.

AFM can measure the variation of height profiles, in a value of root mean squared height (RMS). However, due to the limited z-scan range (≤4 µm), the RMS information was available only for comparing the submicron scale roughness. The AFM scanning was carried out across the samples (0.8 µm × 0.8 µm), and the RMS values of PET film, PET SB, and alkali-etched PET SB were compared (Figure7). The RMS value of PET(SB) was not different from that of PET film as the scanning was conducted on the small area of the topmost fiber surface. The PET(SB-etch), however, showed significantly large RMS (~13 nm), confirming the presence of nanoscale roughness in PET(SB-etch). The CA( anda) PET: ShA RMS measurements ~3 nm for the(b varied) PET(SB): substrates RMS ~3 arenm shown in(c Table) PET(SB6 and-etch): Figure RMS8. ~13 For nm all fibrousFigure surfaces, 7. Surface the presence roughness of roughness observed considerably by atomic force increased microscopy the CAs, (AFM) effectively. (a) Polyethylene enhancing the anti-wettingterephthalate property. (PET) Particularly, film; (b) PET thespunbond surfaces fabric; with (c) the alkali nanoscale-etched PET roughness, spunbond Cel(SL) fabric. -f, Cel(SL)-Si, ◦ PET(SB-etch)-f, and PET(SB-etch)-Si, displayed the superhydrophobic property with WA CA ≥ 150 ◦ and WAFrom ShA Table≤ ~10 6, the. The film result surfaces clearly treated shows with the DTMS critical and role C4F of8 showed nanoscale a similar roughness level on of theanti- anti-wettingwetting performance property. for water, while the surface treated with C4F8 showed the considerably enhanced anti-wetting performance for MI. The discrepancy of anti-wetting performance between the DTMS and C4F8 treatment became larger as the polarity of liquid decreased (PL is WA > FA > MI). As was discussed earlier, the results demonstrate that the anti-wetting agent needs to be properly selected based on the property of the applied liquid. The overall tendency of the anti-wetting performance of fibrous surfaces was similar to that of film surfaces; however, the fibrous surfaces did not show dramatic performance distinction between the DTMS and C4F8 treatment for different liquids compared with the films. It can be inferred that the decreased wetted area (with the concept of solid fraction, f) would lower the interaction of surface energy between the solid and liquid, thereby reducing the influence of different anti-wetting agents.

Table 6. Contact and shedding angles of surfaces.

CA (°) ShA (°) Specimen WA FA MI WA Cel 0 (±0.0) 0 (±0.0) 50 (±0.8) >50 (±0.0) Cel-f 107 (±1.9) 105 (±1.6) 94 (±3.0) >50 (±0.0) Cel-Si 101 (±1.4) 84 (±2.8) 60 (±1.3) >50 (±0.0) PP 105 (±3.7) 88 (±2.3) 63 (±2.3) >50 (±0.0) PET 96 (±4.2) 82 (±1.9) 61 (±1.7) >50 (±0.0) PET-f 106 (±1.2) 102 (±2.6) 95 (±2.7) >50 (±0.0) PET-Si 104 (±2.3) 90 (±2.8) 61 (±1.9) >50 (±0.0) Cel(SL) 0 (±0.0) 0 (±0.0) 0 (±0.0) >50 (±0.0) Cel(SL)-f 166 (±2.3) 159 (±2.5) 153 (±3.4) 12.8 (±0.8) Cel(SL)-Si 164 (±4.1) 152 (±2.3) 148 (±4.3) 8.8 (±0.8) PET(SB) 126 (±2.8) 84 (±4.5) 0 (±0.0) >50 (±0.0) PET(SB)-f 147 (±2.9) 143 (±2.7) 140 (±3.7) 15.8 (±0.8) PET(SB)-Si 145 (±3.8) 136 (±1.7) 127 (±2.5) >50 (±0.0) PET(SB-etch) 129 (±3.5) 0 (±0.0) 0 (±0.0) >50 (±0.0) PET(SB-etch)-f 156 (±2.9) 150 (±1.4) 150 (±2.4) 5.6 (±0.5) PET(SB-etch)-Si 151 (±3.3) 145 (±3.2) 140 (±3.7) 7.0 (±0.6) Note: shedding angle higher than 50° was not measured any further.

Polymers 2018, 10, x FOR PEER REVIEW 11 of 15

Figure 6. Scanning electron microscopy (SEM) images of surfaces. Inserts are optical microscope Polymers 2019, 11, 498 11 of 15 images with the scale bar of 1 mm.

(a) PET: RMS ~3 nm (b) PET(SB): RMS ~3 nm (c) PET(SB-etch): RMS ~13 nm

Figure 7. Surface roughness observed by atomic force microscopy (AFM). (a) Polyethylene Figure 7. Surface roughness observed by atomic force microscopy (AFM). (a) Polyethylene terephthalate terephthalate (PET) film; (b) PET spunbond fabric; (c) alkali-etched PET spunbond fabric. (PET) ﬁlm; (b) PET spunbond fabric; (c) alkali-etched PET spunbond fabric.

From Table 6, the film surfacesTable 6. treatedContact withand shedding DTMS andangles C of4F8 surfaces. showed a similar level of anti- wetting performance for water, while the surface treated with C4F8 showed the considerably CA (◦) ShA (◦) enhanced anti-wetting performance for MI. The discrepancy of anti-wetting performance between Specimen WA FA MI WA the DTMS and C4F8 treatment became larger as the polarity of liquid decreased (PL is WA > FA > MI). As was discussedCel earlier, 0the result (±0.0)s demonstrate 0 that (±0.0) the anti- 50wetting ( ±agent0.8) need >50s to be (±properly0.0) selected basedCel-f on the property107 (of± 1.9)the applied 105 liquid. (±1.6) The overall 94 tendency (±3.0) of >50 the anti (±-wetting0.0) Cel 101 (±1.4) 84 (±2.8) 60 (±1.3) >50 (±0.0) performance-Si of fibrous surfaces was similar to that of film surfaces; however, the fibrous surfaces did PP 105 (±3.7) 88 (±2.3) 63 (±2.3) >50 (±0.0) not showPET dramatic performance 96 (distinction±4.2) 82 between (± 1.9)the DTMS 61 and (C±41.7)F8 treatment >50 for (± different0.0) liquids comparedPET-f with the106 films. It (± can1.2) be inferred 102 that (±2.6) the decreased 95 wetted (±2.7) area >50(with the (± concept0.0) of solid fraction,PET-Si f) would104 lower( ±the2.3) interaction 90 of (± surface2.8) energy 61 between (±1.9) the >50solid and (± 0.0) liquid, ± ± ± ± thereby reducingCel(SL) the influence 0 of (different0.0) anti 0-wetting ( 0.0) agents. 0 ( 0.0) >50 ( 0.0) Cel(SL)-f 166 (±2.3) 159 (±2.5) 153 (±3.4) 12.8 (±0.8) Cel(SL)-Si 164 (±4.1) 152 (±2.3) 148 (±4.3) 8.8 (±0.8) PET(SB) 126Table 6. (± Contact2.8) and 84 shedding (±4.5) angles of 0surfaces (.± 0.0) >50 (±0.0) PET(SB) 147 (±2.9) 143 (±2.7) 140 (±3.7) 15.8 (±0.8) -f CA (°) ShA (°) PET(SB) 145 (±3.8) 136 (±1.7) 127 (±2.5) >50 (±0.0) Specimen-Si WA FA MI WA PET(SB-etch) 129 (±3.5) 0 (±0.0) 0 (±0.0) >50 (±0.0) Cel 0 (±0.0) 0 (±0.0) 50 (±0.8) >50 (±0.0) PET(SB-etch)-f 156 (±2.9) 150 (±1.4) 150 (±2.4) 5.6 (±0.5) Cel-f 107 (±1.9) 105 (±1.6) 94 (±3.0) >50 (±0.0) PET(SB-etch)-Si 151 (±3.3) 145 (±3.2) 140 (±3.7) 7.0 (±0.6) Cel-Si 101 (±1.4) 84 (±2.8) 60 (±1.3) >50 (±0.0) PP Note:105 shedding (± angle3.7) higher88 than( 50±2.3)◦ was not63 measured (±2. any3) further.>50 (±0.0) PET 96 (±4.2) 82 (±1.9) 61 (±1.7) >50 (±0.0) PET-f 106 (±1.2) 102 (±2.6) 95 (±2.7) >50 (±0.0) PET-Si 104 (±2.3) 90 (±2.8) 61 (±1.9) >50 (±0.0) ForCel(SL) further analysis, the0 Cassie–Baxter(±0.0) 0 state(± [510.0)] was0 assumed (±0.0) for the>50 surfaces (±0.0) that displayed ◦ ◦ the waterCel(SL) CA-f ≥ 147 and166 ShA ≤(±2.3)15 , which159 were(±2.5 Cel(SL)) 153-f, Cel(SL)(±3.4) -Si, PET(SB)12.8 (±-f0.8), PET(SB-etch) -f, and PET(SB-etch)Cel(SL)-Si -Si. If it is164 assumed (±4.1) that152 the liquid(±2.3) drop 148 contacts (±4. only3) the8.8 topmost(±0.8) surface of the roughenedPET(SB) substrate, instead126 of intruding(±2.8) into84 the(± protrusions,4.5) 0 the(±0.0) Cassie–Baxter >50 ( model±0.0) [51] can be PET(SB)-f 147 (±2.9) 143 (±2.7) 140 (±3.7) 15.8 (±0.8) simplified as Equation (13) [19]. In the equation, the solid fraction, f, can be roughly estimated from the PET(SB)-Si 145 (±3.8) 136 (±1.7) 127 (±2.5) >50 (±0.0) measuredPET(SB CA’s-etch) of fibrous surfaces129 (± (θ3.5)CB ) and0 those(± of0.0) flat film0 surfaces(±0.0) (θY).>50 The estimated(±0.0) f, which is the topmostPET(SB area-etch) fraction-f of156 the unit(±2.9) area, were150 0.045,(±1.4) 0.23, and150 0.142(± for2.4) the Cel5.6 SL, PET(±0.5) SB, and etched PET SBPET(SB surfaces,-etch) respectively.-Si 151 The( result±3.3) showed145 that(±3.2 the) cellulose140 (± spunlace3.7) 7.0 had the(±0.6) least area wet by the liquid, probablyNote: due shedding to the angle irregular higher depth than of50° fibers was not and measured fine particles any further. on the surface.

cos θCB = f·(cos θY + 1) − 1 (13)

• θCB: contact angle in the Cassie–Baxter state • θY: contact angle in Young’s model in Equation (2) (ﬂat surface) • f: solid fraction in the Cassie–Baxter model, which is the fraction of wetted surface area to the overall projected surface area. Polymers 2019, 11, 498 12 of 15 Polymers 2018, 10, x FOR PEER REVIEW 12 of 15

FigureFigure 8. 8.WettabilityWettability of of substrates substrates with with different different surface surface energy energy and and roughness roughness profiles. proﬁles.

TheFrom findings Table6 ,of the this film study surfaces can treated be applied with DTMS to anti and-wetting C 4F8 showed finish amethods similar level, where of anti-wetting a proper selectionperformance of anti for-wetting water, agent while need thes surfaceto be made treated for the with protection C4F8 showed from spreading the considerably of liquids enhanced. While itanti-wetting is important performance to attain a forlow MI. level The of discrepancy overall surface of anti-wetting energy in performance reducing the between wettability, the DTMS the componentand C4F8 treatment profiles of became surface largerenergy as also the play polarity a critical of liquid role decreasedin liquid spreading (PL is WA on > FAsurfaces > MI).. Textile As was materialsdiscussed with earlier, microscale the results roughness demonstrate would thatexhibit the enhanced anti-wetting anti- agentwetting needs compared to be properly to flat surfaces selected, ifbased the same on the type property of anti of the-wetting applied chemicals liquid. The are overall applied. tendency To further of the anti-wettingimprove the performance anti-wetting of performance,fibrous surfaces implementation was similar to of that submicron of film surfaces; scale roughness however, the would fibrous be surfacesbeneficial. did For not the show material dramatics thatperformance need adequate distinction protect betweenion against the the DTMS exposure and C of4F 8nontreatment-polar liquid for different (such as liquids MI), an compared anti-wetting with treatmentthe films. with It can a below inferred dispersive that the component decreased may wetted be area effective. (with the For concept the material of solids that fraction, need f) to would be protectedlower the from interaction the spreading of surface of energy a polar between liquid the, anti solid-wetting and liquid, chemical therebys with reducing a low the level influence of the of dispdifferentersive anti-wettingcomponent may agents. be effective. The findings of this study can be applied to anti-wetting finish methods, where a proper selection 4.of Conclusions anti-wetting agent needs to be made for the protection from spreading of liquids. While it is important to attain a low level of overall surface energy in reducing the wettability, the component With the increased awareness of sustainable finishing, various options of anti-wetting finish profiles of surface energy also play a critical role in liquid spreading on surfaces. Textile materials methods are being explored. This study intends to provide basic information that can be used in with microscale roughness would exhibit enhanced anti-wetting compared to flat surfaces, if the selecting an effective anti-wetting agent for protection against the spreading of liquids with particular same type of anti-wetting chemicals are applied. To further improve the anti-wetting performance, surface energy profiles. With such an aim, the anti-wetting effectiveness of fluorinated and silane implementation of submicron scale roughness would be beneficial. For the materials that need coating was investigated using the liquids having different surface energy components: water (WA), adequate protection against the exposure of non-polar liquid (such as MI), an anti-wetting treatment methylene iodide (MI), and formamide (FA). The wetting thermodynamics was experimentally with a low dispersive component may be effective. For the materials that need to be protected from investigated by analyzing two components of surface energies for solids and liquids, with the the spreading of a polar liquid, anti-wetting chemicals with a low level of the dispersive component theoretical discussion on the interplay of components on wettability. The effectiveness of different may be effective. anti-wetting treatments was varied for liquids with different polarities. 4. ConclusionsThe role of surface roughness in wettability was examined for fibrous nonwoven substrates that have various levels of roughness. The presence of nanoscale roughness resulted from nanoparticles or alkaliWith etching the increased was beneficial awareness for attaining of sustainable a high level finishing, of anti- variouswetting optionsperformance of anti-wetting. The difference finish ofmethods anti-wetting are beingperformance explored. between This study the treatments intends to was provide less apparent basic information for the roughened that can be surfaces used in thanselecting for the an flat effective film surfaces. anti-wetting The findings agent for of protection this study against can be theused spreading in selecting of liquids proper with anti- particularwetting chemicalsurfaces energy to grant profiles. protection With from such spreading an aim, theof specific anti-wetting liquids. effectiveness While it is important of fluorinated to attain and a silanelow level of overall surface energy for reducing wettability, the surface energy components also play a critical role in liquid spreading on surfaces. For example, an anti-wetting treatment with a low

Polymers 2019, 11, 498 13 of 15 coating was investigated using the liquids having different surface energy components: water (WA), methylene iodide (MI), and formamide (FA). The wetting thermodynamics was experimentally investigated by analyzing two components of surface energies for solids and liquids, with the theoretical discussion on the interplay of components on wettability. The effectiveness of different anti-wetting treatments was varied for liquids with different polarities. The role of surface roughness in wettability was examined for fibrous nonwoven substrates that have various levels of roughness. The presence of nanoscale roughness resulted from nanoparticles or alkali etching was beneficial for attaining a high level of anti-wetting performance. The difference of anti-wetting performance between the treatments was less apparent for the roughened surfaces than for the flat film surfaces. The findings of this study can be used in selecting proper anti-wetting chemicals to grant protection from spreading of specific liquids. While it is important to attain a low level of overall surface energy for reducing wettability, the surface energy components also play a critical role in liquid spreading on surfaces. For example, an anti-wetting treatment with a low dispersive component may be desirable for a material that needs protection from the exposure of non-polar liquids, such as MI. This study provides experimental validation of wetting thermodynamics and practical interpretation of anti-wetting finishing.

Author Contributions: Conceptualization, J.K.; methodology, J.K., K.S.; investigation, K.S., J.L., S.-O.C., J.K.; writing, J.K., S.-O.C., K.S., J.L.; supervision, J.K.; project administration, J.K.; funding acquisition, J.K. Funding: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A1B07040340); and the Creative-Pioneering Researchers Program through Seoul National University (SNU). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Nosonovsky, M.; Bhushan, B. Superhydrophobic surfaces and emerging applications: Non-adhesion, energy, green engineering. Curr. Opin. Colloid Interface Sci. 2009, 14, 270–280. [CrossRef] 2. Zhang, L.; Zhong, Y.; Cha, D.; Wang, P. A self-cleaning underwater superoleophobic mesh for oil-water separation. Sci. Rep. 2013, 3, 2326. [CrossRef][PubMed] 3. Kelleher, S.M.; Habimana, O.; Lawler, J.; O’Reilly, B.; Daniels, S.; Casey, E.; Cowley, A. Cicada wing surface topography: An investigation into the bactericidal properties of nanostructural features. ACS Appl. Mater. Interfaces 2016, 8, 14966–14974. [CrossRef] 4. MacCallum, N.; Howell, C.; Kim, P.; Sun, D.; Friedlander, R.; Ranisau, J.; Ahanotu, O.; Lin, J.J.; Vena, A.; Hatton, B.; et al. Liquid-infused silicone as a biofouling-free medical material. ACS Biomater. Sci. Eng. 2015, 1, 43–51. [CrossRef] 5. Du, C.; Wang, J.; Chen, Z.; Chen, D. Durable superhydrophobic and superoleophilic ﬁlter paper for oil–water separation prepared by a colloidal deposition method. Appl. Surf. Sci. 2014, 313, 304–310. [CrossRef] 6. Zhang, Y.-P.; Yang, J.-H.; Li, L.-L.; Cui, C.-X.; Li, Y.; Liu, S.-Q.; Zhou, X.-M.; Qu, L.-B. Facile fabrication of superhydrophobic copper-foam and electrospinning polystyrene ﬁber for combinational oil–water separation. Polymers 2019, 11, 97. [CrossRef] 7. Zhang, X.; Wang, L.; Levänen, E. Superhydrophobic surfaces for the reduction of bacterial adhesion. RSC Adv. 2013, 3, 12003–12020. [CrossRef] 8. Ejenstam, L.; Ovaskainen, L.; Rodriguez-Meizoso, I.; Wågberg, L.; Pan, J.; Swerin, A.; Claesson, P.M. The effect of superhydrophobic wetting state on corrosion protection-the AKD example. J Colloid Interface Sci. 2013, 412, 56–64. [CrossRef] 9. Liu, M.; Wang, S.; Jiang, L. Bioinspired multiscale surfaces with special wettability. MRS Bull. 2013, 38, 375–382. [CrossRef] 10. Wang, Z.; Elimelech, M.; Lin, S. Environmental applications of interfacial materials with special wettability. Environ. Sci. Technol. 2016, 50, 2132–2150. [CrossRef] 11. Matthew, J.H.; Melik, C.D. Anisotropic wetting on structured surfaces. MRS Bull. 2013, 38, 391–396. [CrossRef] Polymers 2019, 11, 498 14 of 15

12. Liu, K.; Yao, X.; Jiang, L. Recent developments in bio-inspired special wettability. Chem. Soc. Rev. 2010, 39, 324–3255. [CrossRef][PubMed] 13. Wong, T.-S.; Sun, T.; Feng, L.; Aizenberg, J. Interfacial materials with special wettability. MRS Bull. 2013, 38, 366–371. [CrossRef] 14. Park, S.C.; Kim, M.J.; Choi, K.; Kim, J.; Choi, S.-O. Influence of shell compositions of solution blown PVP/PCL core–shell fibers on drug release and cell growth. RSC Adv. 2018, 8, 32470–32480. [CrossRef] 15. Park, S.C.; Yuan, Y.; Choi, K.; Choi, S.-O.; Kim, J. Doxorubicin release controlled by induced phase separation and use of a co-solvent. Materials 2018, 11, 681. [CrossRef] 16. Yuan, Y.; Choi, K.; Choi, S.-O.; Kim, J. Early stage release control of an anticancer drug by drug-polymer miscibility in a hydrophobic fiber-based drug delivery system. RSC Adv. 2018, 8, 19791–19803. [CrossRef] 17. Mandolfino, C.; Lertora, E.; Gambaro, C.; Pizzorni, M. Functionalization of neutral polypropylene by using low pressure plasma treatment: Effects on surface characteristics and adhesion properties. Polymers 2019, 11, 202. [CrossRef] 18. Scarfato, P.; Schiavone, N.; Rossi, G.; Incarnato, L. An easy route to wettability changes of polyethylene terephthalate–silicon oxide substrate films for high barrier applications, surface-modified with a self-assembled monolayer of fluoroalkylsilanes. Polymers 2019, 11, 257. [CrossRef] 19. Yuan, Y.; Choi, S.-O.; Kim, J. Analysis of contact area between water and irregular fibrous surface for prediction of wettability. RSC Adv. 2016, 6, 73313–73322. [CrossRef] 20. Yuan, Y.; Hays, M.P.; Hardwidge, P.R.; Kim, J. Surface characteristics influencing bacterial adhesion to polymeric substrates. RSC Adv. 2017, 7, 14254–14261. [CrossRef] 21. Dou, X.-Q.; Zhang, D.; Feng, C.; Jiang, L. Bioinspired hierarchical surface structures with tunable wettability for regulating bacteria adhesion. ACS Nano 2015, 9, 10664–10672. [CrossRef] 22. Kota, A.K.; Li, Y.; Mabry, J.M.; Tuteja, A. Hierarchically structured superoleophobic surfaces with ultralow contact angle hysteresis. Adv. Mater. 2012, 24, 5838–5843. [CrossRef][PubMed] 23. Pan, S.; Guo, R.; Björnmalm, M.; Richardson, J.J.; Li, L.; Peng, C.; Bertleff-Zieschang, N.; Xu, W.; Jiang, J.; Caruso, F. Coatings super-repellent to ultralow surface tension liquids. Nat. Mater. 2018, 17, 1040–1047. [CrossRef] 24. Cortese, B.; Riehle, M.O.; D’Amone, S.; Gigli, G. Influence of variable substrate geometry on wettability and cellular responses. J. Colloid Interface Sci. 2013, 394, 582–589. [CrossRef][PubMed] 25. Kota, A.K.; Choi, W.; Tuteja, A. Superomniphobic surfaces: Design and durability. MRS Bull. 2013, 38, 383–390. [CrossRef] 26. Tuteja, A.; Choi, W.; Ma, M.; Mabry, J.M.; Mazzella, S.A.; Rutledge, G.C.; McKinley, G.H.; Cohen, R.E. Designing superoleophobic surfaces. Science 2007, 318, 1618–1622. [CrossRef][PubMed] 27. Anish, T.; Wonjae, C.; Joseph, M.M.; Gareth, H.M.; Robert, E.C. Robust omniphobic surfaces. Proc. Natl. Acad. Sci. USA 2008, 105, 18200–18205. [CrossRef] 28. Kota, A.K.; Mabry, J.M.; Tuteja, A. Superoleophobic surfaces: Design criteria and recent studies. Surf. Innov. 2013, 1, 71–83. [CrossRef] 29. Young, T. An essay on the cohesion of fluids. Philos. Trans. R. Soc. Lond. 1805, 95, 65–87. [CrossRef] 30. Lee, S.-M.; Kwon, T.H. Effects of intrinsic hydrophobicity on wettability of polymer replicas of a superhydrophobic lotus leaf. J. Micromech. Microeng. 2007, 17, 687–692. [CrossRef] 31. Baidya, A.; Ganayee, M.A.; Jakka Ravindran, S.; Tam, K.C.; Das, S.K.; Ras, R.H.A.; Pradeep, T. Organic solvent-free fabrication of durable and multifunctional superhydrophobic paper from waterborne fluorinated cellulose nanofiber building blocks. ACS Nano 2017, 11, 11091–11099. [CrossRef][PubMed] 32. Mates, J.E.; Schutzius, T.M.; Bayer, I.S.; Qin, J.; Waldroup, D.E.; Megaridis, C.M. Water-based superhydrophobic coatings for nonwoven and cellulosic substrates. Ind. Eng. Chem. Res. 2014, 53, 222–227. [CrossRef] 33. Shim, M.; Kim, J.; Park, C. The effects of surface energy and roughness on the hydrophobicity of woven fabrics. Text. Res. J. 2014, 84, 1268–1278. [CrossRef] 34. Gristina, R.; D’Aloia, E.; Senesi, G.S.; Milella, A.; Nardulli, M.; Sardella, E.; Favia, P.; d’Agostino, R. Increasing cell adhesion on plasma deposited fluorocarbon coatings by changing the surface topography. J. Biomed. Mater. Res. B 2009, 88, 139–149. [CrossRef] 35. Ding, G.; Peijnenburg, W.J.G.M. Physicochemical properties and aquatic toxicity of poly- and perfluorinated compounds. Crit. Res. Environ. Sci. Technol. 2013, 43, 598–678. [CrossRef] Polymers 2019, 11, 498 15 of 15

36. He, Y.; Jiang, C.; Yin, H.; Chen, J.; Yuan, W. Superhydrophobic silicon surfaces with micro–nano hierarchical structures via deep reactive ion etching and galvanic etching. J. Colloid Interface Sci. 2011, 364, 219–229. [CrossRef][PubMed] 37. Ray, S.S.; Gandhi, M.; Chen, S.S.; Chang, H.M.; Dan, C.T.N.; Le, H.Q. Anti-wetting behaviour of a superhydrophobic octadecyltrimethoxysilane blended PVDF/recycled carbon black composite membrane for enhanced desalination. Environ. Sci. Water Res. Technol. 2018, 4, 1612–1623. [CrossRef] 38. Hong, H.R.; Kim, J.; Park, C.H. Facile fabrication of multifunctional fabrics: Use of copper and silver nanoparticles for antibacterial, superhydrophobic, conductive fabrics. RSC Adv. 2018, 8, 41782–41794. [CrossRef] 39. Kalin, M.; Polajnar, M. The correlation between the surface energy, the contact angle and the spreading parameter, and their relevance for the wetting behaviour of DLC with lubricating oils. Tribol. Int. 2013, 66, 225–233. [CrossRef] 40. Owens, D.K.; Wendt, R.C. Estimation of the surface free energy of polymers. J Appl. Polym. Sci. 1961, 13, 1741–1747. [CrossRef] 41. Nishimoto, S.; Bhushan, B. Bioinspired self-cleaning surfaces with superhydrophobicity, superoleophobicity, and superhydrophilicity. RSC Adv. 2013, 3, 671–690. [CrossRef] 42. Jeevahan, J.; Chandrasekaran, M.; Britto Joseph, G.; Durairaj, R.B.; Mageshwaran, G. Superhydrophobic surfaces: A review on fundamentals, applications, and challenges. J. Coat. Tehnol. Res. 2018, 15, 231–250. [CrossRef] 43. Xue, Y.; Chu, S.; Lv, P.; Duan, H. Importance of hierarchical structures in wetting stability on submersed superhydrophobic surfaces. Langmuir 2012, 28, 9440–94450. [CrossRef][PubMed] 44. Fujii, T.; Aoki, Y.; Habazaki, H. Fabrication of super-oil-repellent dual pillar surfaces with optimized pillar intervals. Langmuir 2011, 27, 11752–11756. [CrossRef] 45. Park, Y.; Park, C.H.; Kim, J. A Quantitative Analysis on the Surface Roughness and the Level of Hydrophobicity for Superhydrophobic ZnO Nanorods Grown Textiles. Text. Res. J. 2014, 84, 1776–1788. [CrossRef] 46. Qian, B.; Shen, Z. Fabrication of superhydrophobic surfaces by dislocation-selective chemical etching on aluminum, copper, and zinc substrates. Langmuir 2005, 21, 9007–9009. [CrossRef] 47. Xue, C.-H.; Li, Y.-R.; Zhang, P.; Ma, J.-Z.; Jia, S.-T. Washable and wear-resistant superhydrophobic surfaces with self-cleaning property by chemical etching of fibers and hydrophobization. ACS Appl. Mater. Interfaces 2014, 6, 10153–10161. [CrossRef][PubMed] 48. Mazrouei-Sebdani, Z.; Khoddami, A. Alkaline hydrolysis: A facile method to manufacture superhydrophobic polyester fabric by fluorocarbon coating. Prog. Org. Coat. 2011, 72, 638–646. [CrossRef] 49. Janczuk, B.; Zdziennicka, A. A study on the components of surface free energy of quartz from contact angle measurements. J. Mater. Sci. 1994, 29, 3559–3564. [CrossRef] 50. Szymczyk, K.; Zdziennicka, A.; Krawczyk, J.; Jańczuk,B. Wettability, adhesion, adsorption and interface tension in the polymer/surfactant aqueous solution system: II. Work of adhesion and adsorption of surfactant at polymer–solution and solution–air interfaces. Colloids Surf. A Physicochem. Eng. Asp. 2012, 402, 139–145. [CrossRef] 51. Cassie, A.B.D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 1905–1971. [CrossRef]

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