Wetting Transition from the Cassie–Baxter State to the Wenzel State

Article pubs.acs.org/Langmuir

Wetting Transition from the Cassie−Baxter State to the Wenzel State on Textured Polymer Surfaces Daiki Murakami,† Hiroshi Jinnai,†,‡ and Atsushi Takahara*,†,‡ †

Japan Science and Technology Agency (JST), ERATO, Takahara Soft Interfaces Project, CE80, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: The wetting transition from the Cassie−Baxter state to the Wenzel state on textured surfaces was investigated. Nano- to microscale hexagonal pillared lattices were prepared by nanoimprint lithography on fluorinated cycloolefin polymer substrates. The transition was clearly observed for water and some ionic liquids through contact angle measurements and optical microscopy. A simple model clearly demonstrated that the energy barrier in the wetting transition from the Cassie−Baxter state to the Wenzel state was dominated by the competition between the energy barrier and external forces, particularly the Laplace pressure in the present case.

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INTRODUCTION Energy barriers play a key role in both chemical and physical phenomena, including phase transition,1−3 superheating/cooling and nucleation in crystallization,4−7 protein folding8,9 and quantum mechanics.10 In chemical reactions, the stability of the intermediate state dominates the kinetics of the reaction.11 Although the energy barrier is important for describing the kinetics of various phenomena, its quantitative evaluation is not straightforward. In the past few decades, the static behavior of wetting on textured surfaces has drawn considerable attention from both experimental and theoretical points of view.12−20 Usually, liquids exhibit either the Cassie−Baxter wetting state or the Wenzel wetting state. In the Cassie−Baxter state, air trapped in the grooves between surface features forms a composite (solid/ air) hydrophobic surface, resulting in a larger contact angle θC−B compared to the contact angle θ with a flat surface:19 cos θC−B = f − 1 + f cos θ

One-way transition from the metastable Cassie−Baxter state to the stable Wenzel state is often observed. In the Cassie−Baxter state, liquids are pinned at the upside of textures, and the surfaces sag downward among them.24−26 Recently, Papadopoulous et al. observed two types of the transition mechanisms, referred as sagging and unpinning, on pillar pattern surfaces by laser scanning confocal microscopy.27 For the relatively low asperity, the solid/liquid/air three-phase contact line kept pinned at the top of pillars, and transition occurred when the underside of the sagging liquid surface touched the bottom substrate surface between pillars. In contrast, in textures with a high aspect ratio, the contact line was unpinned from the top surface and started to slide down with increasing the Laplace pressure inside the droplet. In order for the Cassie−Baxter state to be observed, there should be an energy barrier which slows the transition from this metastable state. External stimuli such as mechanical impact, compression, thermal perturbation, and Laplace pressure can trigger the wetting transition by overcoming the energy barrier.28−30 The role of the energy barrier in wetting transition was discussed in detail in several studies.31−34 In contrast, the difficulty of observing wetting transition has hindered progress in experimental approaches and our intuitive understanding of the transition process. In this study, we prepared various types of pillar pattern surfaces allowing us to systematically change the magnitude of the energy barrier during the transition. Contact angle measurements and optical microscopy with a high-speed camera revealed the occurrence of a wetting transition from the Cassie−Baxter state to the Wenzel state

(1)

where f represents the fraction of the solid/liquid interface in the entire composite surface beneath the liquid. In contrast, in the Wenzel state, the liquid on the surface enters the grooves, resulting in higher surface wettability due to the increase in contact area:20 cos θ W = R cos θ

(2)

where R is the ratio of the actual area of the solid/liquid interface to the normally projected area. The wetting transition from one state to the other has been studied extensively in recent years. The transition is reversible if both states are thermodynamically stabilized by changing thermodynamic variables, such as temperature and pressure, or by applying an electric field or other external forces.21−23 © 2014 American Chemical Society

Received: December 22, 2013 Revised: February 1, 2014 Published: February 5, 2014 2061

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and the interfacial tensions of water and the ionic liquids are listed in Table 1. Measurements. Liquid droplets of 2 μL were gently deposited on the sample surfaces with a micropipet, and the static contact angles were determined with a drop shape analyzer (DSA 10 Mk2; KRÜ SS GmbH) at room temperature. The advancing or receding contact angles are not discussed, because the perturbation of the advancing/ receding treatment (liquid dosing or sliding procedures) could break the stability of the metastable Cassie−Baxter state, which is the key issue of this paper. The surface tension of a liquid, γLV, was examined by shape analysis of pendant drops. The surface tension of CYTOP, γSV, was determined to be 16.1 mN m−1 with the Owens−Wendt method from the contact angles of methylene iodide and hexadecane. Then the interfacial tension between a liquid and CYTOP, γSL, was estimated based on Young’s equation. The wetting transition was examined by optical microscopy from the bottom of the polymer film. CYTOP assumes an amorphous state and is therefore transparent to visible light. A dispenser tube with a water droplet (approximately 1 μL) hanging from its tip was slowly lowered until the droplet touched the textured surface and was kept until the wetting transition occurred. Real-time images were recorded with a high-speed camera (FASTCAM MC 2.1; Photron USA, Inc.).

on different combinations of liquids and textures. A simple model calculation demonstrated that this transition occurred when the energy barrier was on the same order of magnitude as external forces, particularly the Laplace pressure in the present case.

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EXPERIMENTAL SECTION

Materials. Fluorinated cycloolefin polymer (CYTOP; Asahi Glass Co., Ltd.) was employed as the substrate material because of its high hydrophobicity, mechanical and thermal stability, and transparency. The chemical structure of CYTOP is shown in Figure 1. CYTOP

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RESULTS AND DISCUSSION Contact Angle Measurement for Water. In Table 2, the contact angles of water on the textured surfaces are summarized Table 2. Measured and Calculated Contact Angles of Water on Textured Surfaces

Figure 1. (a) SEM image of a textured CYTOP surface (x2000h1000). (b) Schematic diagram of the surface structure. (c) Chemical structure of CYTOP.

contact angle/deg

solution (CTX-809SP) was added dropwise to ASAHIKLIN AK-225 (Asahi Glass Co., Ltd.) to remove additives, and then the precipitate was dried at 50 °C under vacuum. Additive-free CYTOP was mechanically pressed at 1 MPa at 130 °C to obtain a freestanding film. Then, nano- to microscale hexagonal pillared lattices were prepared on the CYTOP surface by nanoimprint lithography.35 Six different pillar surfaces were employed for a systematic study. In one pattern, the pillars had a diameter x = 230 nm and a height h = 200 nm (x230h200), and in the others they had a diameter x = 2000 nm and a height h = 500, 1000, 2000, 3000, or 4000 nm (x2000-h500 to x2000-h4000). Figure 1 shows a scanning electron microscope (SEM) image of one of the textured surfaces. Water and various types of ionic liquids were used as probe liquids. The ionic liquids were kept in a vacuum oven at 60 °C for more than 6 h and cooled down before use to remove any water absorbed from the liquids. The contact angles on the flat CYTOP surface and the surface

x/nm

h/nm

meas

calcd W

calcd C−B

ΔE/mN m−1

2000 2000 230 2000 2000 2000

500 1000 200 2000 3000 4000

111.3 114.8−129.7 124.8−145.6 147.1 146.3 145.2

113.2 117.8 125.0 127.7 139.2 154.4

147.8 147.8 147.8 147.8 147.8 147.8

5.3 10.6 18.4 21.2 31.8 42.4

in the order of aspect ratio, together with the theoretically predicted values for the Wenzel and Cassie−Baxter states calculated by eqs 1 and 2. The contact angle on x2000-h500 was 111.3°, which was in agreement with the theoretically calculated value for the Wenzel state. In contrast, the contact angles of water on x2000-h2000 to x2000-h4000 were greater than 145°, which is close to the predicted value for the Cassie−

Table 1. Surface Properties of Water and Ionic Liquidsa water 1-ethyl-3-methylimidazolium dicyanamide 1-ethyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium tetrafluoroborate 1,3-dimethylimidazolium dimethylphosphate 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy)ethylsulfate 1-butyl-3-methylimidazolium hexafluorophosphate 1-hexyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium trifluoromethane sulfonate 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide a

CA/deg

γLV/mN m−1

γSL/mN m−1

γSL − γSV/mN m−1

108.7 101.7 99.8 90.3 90.3 90.1 88.5 80.5 74.1 73.4 72.1 68.3

72.0 59.6 52.4 42.4 44.1 44.6 42.6 37.0 34.2 33.9 34.9 31.8

39.2 28.2 25.0 16.4 16.4 16.2 15.0 10.0 6.7 6.4 5.4 4.3

23.1 12.1 8.9 0.2 0.2 0.1 −1.0 −6.1 −9.4 −9.7 −10.7 −11.8

CA: Contact angles on flat CYTOP surface. γLV, γSL, γSV: Interfacial tensions at liquid/vapor, solid/liquid, and solid/vapor interfaces, respectively. 2062

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Baxter state. For these surfaces, the experimental error was typically within ±1°. In the cases of x2000-h1000 and x230h200, however, the contact angles were scattered over a 15° range. Figure 2a shows the contact angles of water and ionic liquids on x230-h200 (cos θ′) against the contact angles on a flat

Figure 3. Bottom views of water droplets on textured surfaces. Bright and dark regions correspond to the Cassie−Baxter and Wenzel states, respectively.

behavior is characteristic of liquids in the Wenzel state on textured surfaces.37,38 Although the contact area shrank gradually due to water evaporation, the dark and deformed features never changed, indicating that the droplet remained stably in the Wenzel state. In contrast, the contact area on x2000-h2000 to x2000−4000 appeared bright and circular. Considering the results of the contact angle measurements, this appearance corresponds to the Cassie−Baxter state. The bright/dark contrast between the Wenzel and Cassie−Baxter states is attributed to the difference in refractive index of water and air in the grooves. Furthermore, coexisting Wenzel and Cassie−Baxter states were observed on x2000-h1000 and x230h200. At t = 0 s, dark regions appeared quickly for x2000-h1000 and slowly for x230-h200 (see the video data in the Supporting Information for details). These processes are considered to correspond to the wetting transition from the Cassie−Baxter state to the Wenzel state. The wetting transition on x2000-h1000 was a two-step process (Figure 4a). In the first step, a Wenzel state region appeared suddenly over a wide contact area (within 17 ms). This process involved the rapid penetration of water into the grooves in the vertical direction. After this first step, the bottom of the droplet was hexagonal, as in the Wenzel state for x2000h500, reflecting the hexagonal arrangement of pillars on the

Figure 2. Contact angles of water (open circles) and ionic liquids (filled circles) on (a) x230-h200 and (b) x2000-h1000. Solid and dashed lines indicate the values theoretically predicted from the Wenzel and Cassie−Baxter models, respectively.

surface (cos θ). The calculated contact angles in the Cassie− Baxter and Wenzel states are indicated with solid and dashed lines, respectively. The Wenzel state has a lower energy and is stable for θ < θC (θC represents the critical point at which the two theoretical lines intersect), while the Cassie−Baxter state is stable for θ > θC.36 The contact angles of the ionic liquids agreed fairly well with the values calculated from the Wenzel model. On the other hand, the contact angles of water were scattered, as mentioned above. The contact angles of water obtained in 10 measurements are plotted in Figure 2. It should be noted that the maximum and minimum values were consistent with the Cassie−Baxter and Wenzel models, respectively, and the contact angles were scattered between the two extremes. This result implies that the wetting transition from the metastable Cassie−Baxter state to the stable Wenzel states occurred for water, whereas ionic liquids were stable in the Wenzel state on x230-h200. Similar results were obtained for x2000-h1000 as well (Figure 2b), where water exhibited a strong deviation from the predicted value for the Wenzel state. Compared to x230-h200, the contact angles tended to cluster on the side of the Wenzel state rather than the Cassie−Baxter state. Optical Microscopy for Water. Microscopy of the wetting transition provided more intuitive information. Figure 3 shows snapshots of the contact area between a water droplet and textured surfaces taken from the bottom of the sample. On x2000-h500, the contact area appeared dark and hexagonally deformed immediately after the water droplet touched the surface. This deformation of the contact area indicated that the water droplet was in the Wenzel state because this anomalous

Figure 4. (a) Schematic illustration of a two-step wetting transition on a textured surface. (b) Geometry of water penetrating the surface grooves. 2063

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The wetting transition was visually examined for [emim][N(CN)2] as well (Figure 6). The contact area appeared as a

substrate surface. The following second step concerned the spreading of water in the lateral direction. The Wenzel region generated in the first step gradually expanded along the hexagonal lattice of pillars (step 2 in Figure 4a) until eventually the entire area under the droplet was in the Wenzel wetting state. Water on the x230-h200 surface also underwent the above two-step transition, although it proceeded rather slowly. Furthermore, the transition tended to start from the edge of the contact area, accompanied by the evaporation of the water droplet. Ultimately, only a small region of less than 10% of the initial contact area underwent complete transition before the droplet evaporated. Observation of Ionic Liquids. The same experiments were carried out with ionic liquids on the same textured surfaces. A similar trend as in the case of water was observed for 1-ethyl-3-methylimidazolium dicyanamide ([emim][N(CN)2]), which exhibited the highest contact angle on the flat CYTOP surface and the highest surface tension among the ionic liquids used (Table 1). The contact angles of [emim][N(CN)2] on x2000-h500, x2000-h1000, and x230-h200 were extremely close to the values calculated for the Wenzel state (Table 3), whereas

Figure 6. Images of [emim][N(CN)2] droplets on textured surfaces taken from the bottom of the samples. Bright and dark regions correspond to the Cassie−Baxter and Wenzel states, respectively.

dark hexagonal region on x2000-h500, x2000-h1000, and x230h200, and as a bright circular region on x2000-h3000 and x2000-h4000. These results indicate that droplets were in the Wenzel state in the former three structures and in the Cassie− Baxter state in the latter two, which is consistent with the results of the contact angle measurements. Wetting transition from the Cassie−Baxter state to the Wenzel state was clearly observed on x2000-h2000. The transition proceeded extremely rapidly (within 17 ms), as in the experiment with water on x2000-h1000 (see the video data in the Supporting Information for details). Unique behavior was also observed for 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) (Table 4).

Table 3. Measured and Calculated Contact Angles of [emim][N(CN)2] on Textured Surfaces contact angle/deg x/nm

h/nm

meas

calcd W

calcd C−B

ΔE/mN m−1

2000 2000 230 2000 2000 2000

500 1000 200 2000 3000 4000

109.2 104.3 109.9 111.8−139.6 141.3 141.5

104.4 107.1 111.3 112.7 118.6 124.8

145.0 145.0 145.0 145.0 145.0 145.0

2.7 5.5 9.5 11.0 16.4 21.9

Table 4. Measured and Calculated Contact Angles of [emim][BF4] on Textured Surfaces contact angle/deg

the contact angles on x2000-h3000 and x2000-h4000 were notably high, as predicted for the Cassie−Baxter state. Therefore, contact angle measurements confirmed that the Wenzel and Cassie−Baxter wetting regimes occurred at low and high asperities, respectively. On x2000-h2000, the values were scattered widely between the two wetting states, as in the case of water on x230-h200 and x2000-h1000. Figure 5 shows cos θ′−cos θ plots for x2000-h2000. In this case, the contact angle of water is consistent with the value predicted for the Cassie−Baxter state. Only the results for [emim][N(CN)2] were distributed between the two states, while other ionic liquids showed a strong preference for the Wenzel state.

x/nm

h/nm

meas

calcd W

calcd C−B

ΔE/mN m−1

2000 2000 230 2000 2000 2000

500 1000 200 2000 3000 4000

102.6 107.1 100.6 108.9 139.8 138.5

102.1 104.3 107.7 108.9 113.7 118.6

144.3 144.3 144.3 144.3 144.3 144.3

2.0 4.1 7.0 8.1 12.2 16.2

Patterns with scattered contact angles did not appear in this case. However, it was evident from the contact angle values that wetting transition from the Wenzel state to the Cassie−Baxter state would occur for some structure with an asperity between those of x2000-h2000 and x2000-h3000. This was also confirmed by direct imaging of the contact area through a microscope. The steady Wenzel and Cassie−Baxter states appeared at low and high asperities, respectively, and time evolution was not observed. In contrast, other ionic liquids in Table 1 showed only the stable Wenzel state on any structures. Results of contact angle measurements and microscopy for all liquids are summarized in the Supporting Information. Good reproducibility in the static contact angle measurement was confirmed for the low aspect ratio surface; x2000-h500 to -h2000 and x230-h200. For x2000-h3000 and x2000-h4000, on the other hand, the probe liquids besides water, [emin][N(CN)2], [emim][BF4] showed the large hysteresis, because of the high asperity on the surfaces. In those cases, the liquids were confirmed to be in the Wenzel state by the microscopy.

Figure 5. Contact angles of water (triangle) and ionic liquids (open circles, [emim][N(CN)2]; filled circles, others) on x2000-h2000. Solid and dashed lines represent the theoretical predictions for the Wenzel and Cassie−Baxter models, respectively. 2064

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Consideration of the Energy Barrier. We demonstrated that wetting transition occurred for different surface patterns (aspect ratios) depending on the liquid (on x2000-h1000 and x230-h200 for water; on x2000-h2000 for [emim][N(CN)2], and on a structure between x2000-h2000 and x2000-h3000 for [emim][BF4]). Considering the factors determining these differences in the wetting transition, one possible explanation may be the change in interfacial energy during the wetting transition in the vertical direction. Papadopoulous et al. observed a sagging transition for pillar surfaces with low aspect ratios and an unpinning process for ones with high aspect ratios.27 Because the asperities of the pillar surfaces were sufficiently high, we assumed that the transition proceeded with the unpinning mechanism in our systems. In addition, flatness of the liquid surface over the grooves was assumed for simplicity. Then, the total energy beneath the contact area is the sum of the interfacial energies at solid/liquid (γSL), liquid/ vapor (γLV), and solid/vapor (γSV) interfaces in the Cassie− Baxter state. For the pillar pattern, the energy per unit contact area is expressed as ⎛ πh ⎞ EC−B = fγSL + (1 − f )γLV + ⎜1 − f + ⎟γ ⎝ 2 3 x ⎠ SV

reasoning accounts for the observation that these liquids never underwent a wetting transition, and Wenzel wetting was always observed. These liquids had a small, if any, energy barrier against the wetting transition. The energy barriers for the textured surfaces were calculated for three liquids by eq 6, and the values are shown in Tables 2−4. It is noteworthy that the wetting transition took place around similar energy values (ΔE = 10 mN m−1) even for different liquids. Considering the physical meaning of the energy barrier, we assume that sagging liquid surfaces with a radius R are pinned at the top of the pillars in the metastable Cassie−Baxter state25 (Figure 7). The perimeter of a pillar is πx, and the number

(3) Figure 7. Schematic illustration of the sagging liquid droplet surface in the Cassie−Baxter state.

As the liquid advances into the grooves, the energy changes to ⎛ πy ⎞ Ey = ⎜f + ⎟γ + (1 − f )γLV ⎝ 2 3 x ⎠ SL ⎡ π (h − y ) ⎤ + ⎢1 − f + ⎥γ ⎣ 2 3 x ⎦ SV

density of pillars is 1/2√3x2. Then, the energy barrier ΔE corresponds to a pressure ΔPEB = πΔE/2√3x, which acts upward to prevent the penetration of liquid into the space between the pillars. ΔPEB was calculated as 4.5 × 103 Pa for x = 2 μm and 3.9 × 104 Pa for x = 230 nm when ΔE = 10 mN m−1. These values were on the same order of magnitude as the downward Laplace pressure inside the sagging surface, ΔPlap ∼ γ/R ∼ γ/x, because R = −x/2cos θad (θad represents the advancing contact angle) if the surface is assumed to be in the shape of a spherical cap. The self-weight of the liquid droplet causes the pressure of only a few to several tens of pascals. Therefore, the threshold value of the energy barrier is determined by the competition with the Laplace pressure, which is the main contribution in terms of external forces in the present case. We notice that both ΔPEB and ΔPlap are proportional to 1/x and therefore the competition between them is irrelevant to the dimension of size x. So the reason why slow wetting transition was observed even on x230-h200 (ΔE = 18.4 mN m−1) only in the case of water should be attributed to other contribution. The Laplace pressure of water should be greater than that of the ionic liquids due to its larger γ and θad. Furthermore, the evaporation of water afforded additional energy though movement of the contact line. These effects might generate a slow transition by slightly overcoming the energy barrier only near the contact line of the water droplet. Consequently, it was clearly demonstrated in systematic experiments that the wetting transition from the Cassie−Baxter state to the Wenzel state was dominated by competition between the energy barrier and external forces, particularly the Laplace pressure in the present case.

(4)

where y denotes the penetration depth of the liquid measured from the top of the pillars (Figure 4b). When the liquid reaches the bottom of the textured pattern (i.e., when Wenzel wetting is complete), the liquid/vapor and solid/vapor interfaces disappear; instead, there are solid/liquid interfaces and the energy is ⎛ πh ⎞ E W = ⎜1 + ⎟γ = RγSL ⎝ 2 3 x ⎠ SL

(5)

Vertical penetration of liquid into the grooves may occur instantaneously to obtain an energy gain when EW is smaller than EC−B. However, if γSL > γSV, the energy increases before attaining the low-energy Wenzel state due to the replacement of the low-energy solid/vapor interface with a high-energy solid/liquid interface. This increase in surface energy can be regarded as the above-mentioned energy barrier for the wetting transition from the Cassie−Baxter state to the Wenzel state. The magnitude of the energy barrier ΔE is easily obtained from eqs 3 and 4 as ΔE =

πh (γ − γSV ) 2 3 x SL

(6)

We note here that the energy barrier is a function of the aspect ratio (h/x), and the interface replacement energy of the surface (γSL − γSV). The replacement energies for water and ionic liquids are listed in Table 1. Water, [emim][N(CN)2], and [emim][BF4], exhibiting wetting transition on the textured surfaces, were characterized by high interface replacement energies and accordingly large energy barriers, whereas the values were only slightly positive or negative for other liquids. This

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CONCLUSION The wetting behavior of several liquids on CYTOP textured by nanoimprint lithography was investigated. Contact angle measurement revealed that, for water, [emim][N(CN)2], and [emim][BF4], the liquid was in the Wenzel state for pillar pattern surfaces with a low aspect ratio and in the Cassie− 2065

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(11) Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic Chemistry; Oxford: New York, 2001. (12) Gao, L.; McCarthy, T. J. An Attempt to Correct the Faulty Intuition Perpetuated by the Wenzel and Cassie “Laws”. Langmuir 2009, 25, 7249−7255. (13) Young, X.; Zhang, L. T. Nanoscale Wetting on GroovePatterned Surfaces. Langmuir 2009, 25, 5045−5053. (14) Marmur, A.; Bittoun, E. When Wenzel and Cassie Are Right: Reconciling Local and Global Considerations. Langmuir 2009, 25, 1277−1281. (15) Extrand, C. W. Designing for Optimum Liquid Repellency. Langmuir 2006, 22, 1711−1714. (16) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takahara, A. Super-Liquid-Repellent Surfaces Prepared by Colloidal Silica Nanoparticles Covered with Fluoroalkyl Groups. Langmuir 2005, 21, 7299−7302. (17) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. MacroscopicWetting Anisotropy on the Line-Patterned Surface of Fluoroalkylsilane Monolayers. Langmuir 2005, 21, 911−918. (18) Blossey, R. Self-Cleaning SurfacesVirtual Realities. Nat. Mater. 2003, 2, 301−306. (19) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (20) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988−994. (21) Manukyan, G.; Oh, J. M.; van den Ende, D.; Lammertink, R. G. H.; Mugele, F. Electrical Switching of Wetting States on Superhydrophobic Surfaces: A Route Towards Reversible Cassie-to-Wenzel Transitions. Phys. Rev. Lett. 2011, 106, 14501-1−14501-4. (22) Liu, G.; Fu, L.; Rode, A. V.; Craig, V. S. J. Water Droplet Motion Control on Superhydrophobic Surfaces: Exploiting the Wenzel-to-Cassie Transition. Langmuir 2011, 27, 2595−2600. (23) Vrancken, R. J.; Kusumaatmaja, H.; Hermans, K.; Prenen, A. M.; Pierre-Louis, O.; Bastiaansen, C. W. M.; Broer, D. J. Fully Reversible Transition from Wenzel to Cassie-Baxter States on Corrugated Superhydrophobic Surfaces. Langmuir 2010, 26, 3335−3341. (24) Extrand, C. W. Model for Contact Angles and Hysteresis on Rough and Ultraphobic Surfaces. Langmuir 2002, 18, 7991−7999. (25) Extrand, C. W. Criteria for Ultralyophobic Surfaces. Langmuir 2004, 20, 5013−5018. (26) Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. M.; Cohen, R. E. Robust Omniphobic Surfaces. Proc. Natl. Acad. Sci. U.S.A. 2018, 105, 18200−18205. (27) Papadopoulos, P.; Mammen, L.; Deng, X.; Vollmer, D.; Butt, H. J. How Superhydrophobicity Breaks Down. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 3254−3258. (28) Kwon, H. M.; Paxson, A. T.; Varanasi, K. K.; Patankar, N. A. Rapid Deceleration-Driven Wetting Transition during Pendant Drop Deposition on Superhydrophobic Surfaces. Phys. Rev. Lett. 2011, 106, 36102−1−36102−4. (29) Sbragaglia, M.; Christophe, A. M.; Pirat, C.; Borkent, B. M.; Lammertink, R. G. H.; Wessling, M.; Lohse, D. Spontaneous Breakdown of Superhydrophobicity. Phys. Rev. Lett. 2007, 99, 156001−1−156001−4. (30) Lafuma, A.; Quéré, D. Superhydrophobic States. Nat. Mater. 2003, 2, 457−460. (31) Giacomello, A.; Meloni, S.; Chinappi, M.; Casciola, C. M. Cassie−Baxter and Wenzel States on a Nanostructured Surface: Phase Diagram, Metastabilities, and Transition Mechanism by Atomistic Free Energy Calculations. Langmuir 2012, 28, 10764−10772. (32) Bormashenko, E.; Musin, A.; Whyman, G.; Zinigrad, M. Wetting Transitions and Depinning of the Triple Line. Langmuir 2012, 28, 3460−3464. (33) Patankar, N. A. Consolidation of Hydrophobic Transition Criteria by Using an Approximate Energy Minimization Approach. Langmuir 2010, 26, 8941−8945. (34) Ishino, C.; Okumura, K.; Quéré, D. Wetting Transitions on Rough Surfaces. Europhys. Lett. 2004, 68, 419−425.

Baxter state for ones with a high aspect ratio. Furthermore, highly scattered contact angle values were observed between the Cassie−Baxter and Wenzel states. Microscopy from the bottom of the samples showed that this behavior corresponded to a wetting transition between the two states. A simple model was employed to estimate the energy barrier ΔE in the wetting transition due to the replacement of the solid/vapor interface at the pillar wall by a solid/liquid interface. The transition occurred at ΔE of around 10 mN m−1 irrespective of the liquid. This threshold value corresponded to the Laplace pressure inside the sagging surface of liquid droplets in the Cassie− Baxter state. This clearly demonstrated that the wetting transition from the Cassie−Baxter state to the Wenzel state was dominated by the competition between the energy barrier and external forces, particularly the Laplace pressure in the present case.

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ASSOCIATED CONTENT

S Supporting Information *

Videos 1−3: Video data of the wetting transition process of water on the x2000-h1000 (Video 1) and x2000-h2000 (Video 2; plays in 5 times speed), and [emim][N(CN)2] on x2000h2000 (Video 3). Table S1; Results of the contact angle measurement and the microscopy for all probe liquids at all aspect ratios. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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