Theoretical Explanation of the Photoswitchable Superhydrophobicity

Aug 11, 2014 - Department of Biochemical Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa, Yamaga...
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Theoretical Explanation of the Photoswitchable Superhydrophobicity of Diarylethene Microcrystalline Surfaces Naoki Nishikawa,† Hiroyuki Mayama,*,‡ Yoshimune Nonomura,§ Noriko Fujinaga,† Satoshi Yokojima,⊥,∥ Shinichiro Nakamura,∥ and Kingo Uchida*,† †

Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan Research Institute for Electronic Science, Hokkaido University, N21, W10 Kita-ku, Sapporo 001-0021, Japan § Department of Biochemical Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa, Yamagata 992-8510, Japan ⊥ School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan ∥ RIKEN Research Cluster for Innovation, Nakamura Laboratory, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡

S Supporting Information *

ABSTRACT: Two types of superhydrophobic surfaces which show lotus and petal effects were induced on photochromic diarylethene microcrystalline surfaces by UV and visible light irradiation and temperature control. On the surfaces showing the lotus effect, a lowadhesion superhydrophobic property is attributed to the surface structure being covered with densely standing needle-shaped crystals of the closed-ring isomer. On surfaces showing the petal effect, a high-adhesion superhydrophobic surface consists of fine needleshaped crystals with high density together with a few rod-shaped crystals, where an invasion phenomenon occurs between these rodshaped crystals. Furthermore, the different superhydrophobic properties of the surfaces are theoretically explained using multipillar surface models.

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microstructures of rose petals (Cassie impregnation state)9 (Figure S1 in Supporting Information). Recently, we have reported the photoinduced reversible wettability changes concomitant with topographical changes on the microcrystalline surface of diarylethene 1.14−18 This is a particular case, since photochromic wettability control has usually utilized the effect of polarity changes.19−23 We reexamined the borderline distinction between lotus and petal surfaces that appear on our diarylethene microcrystalline surfaces by using photoinduced topographical changes to clarify the differences in microstructures and dynamic wetting behaviors (Figure S2 in S.I.). We also used our finding to understand the observed phenomena theoretically.

INTRODUCTION Many natural surfaces of plants and insects have been studied, and specific super-water-repellant properties have been reported.1−9 In particular, the surfaces of lotus leaves and rose petals have attracted interest because they show different types of super-water-repellency, and this difference was explained by the difference in surface structure.9 The lotus effect is attributed to low adhesive superhydrophobicity, where the contact angle (CA) and sliding angle (SA) of a water droplet are 161 and 2°, respectively. This effect implies the Cassie−Baxter state.10 Conversely, the rose petal effect shows a high-adhesion superhydrophobic surface, where water droplets pinned on the surface cannot roll off, even though the CA of a water droplet is 154°. This phenomenon is known as the petal effect, and it implies the Wenzel state.11 The wettability of a solid surface with liquids is governed by two factors, one chemical and the other geometrical.12,13 The geometrical factor, in particular, determines the dynamic behavior of wettability, and it has been explained by Wenzel11 and Cassie et al.10 The Wenzel state is a model that allows the invasion of water droplets, and the Cassie state is a model that prevents the invasion of water droplets by taking in air pockets. On a lotus leaf, water is prohibited from intruding into the air pocket of the leaf by the nanomicrostructure (Cassie state); however, water slightly penetrates into the air pocket between the © 2014 American Chemical Society

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EXPERIMENTAL SECTION: PREPARATION OF FILMS AND THEIR CHARACTERIZATION

The films were prepared by coating a chloroform solution containing diarylethene 1 (100 mg mL−1) onto the substrate, and the solvent was evaporated in vacuo. The film thickness after scratching was measured at approximately 20 μm with a laser microscope (Keyence VK-8550). Received: June 30, 2014 Revised: August 6, 2014 Published: August 11, 2014 10643

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A scanning electron microscope (Keyence VE-8800) and an optical microscope (Leica DMLP) were used to study the surface microstructures. Static CAs and SAs were measured with an optical contact angle meter (Kyowa Interface Science Co., Ltd., Drop Master 500). Photoirradiation (visible light >500 nm) was carried out using a Ushio 500 W xenon lamp with a cutoff filter (Toshiba color filter Y-50), and UV irradiation was carried out with a Spectroline hand-held UV lamp, E-series (= 254 nm, 820 mW/cm2 (distance = 15 cm)). Photoirradiation experiments were carried out at the eutectic point using an FP82HT hot stage. The crystal data were reported in a previous paper.14 For the fractal analysis, a sample was set with conductive carbon tape on a cover glass on the electron microscope stage, and an Au−Pd alloy was evaporated onto the sample surface. The sample was cracked together with the cover glass substrate and set perpendicularly on the stage for cross-section observation. The fractal dimension of the cross-section of the rough solid surfaces was calculated from the trace curves of the surfaces by the box-counting method. A two-dimensional area containing these trace curves was divided by identical boxes of side-size r. The number of boxes containing trace curve N(r) was counted, and then side-size r was changed. The number of boxes was counted again for a new side-size r, and the above process was repeated. On the basis of the box-counting method, the fractal dimension can be calculated from the following relationship

N (r ) ∝ r −D

Figure 1. SEM images (×2000) and contact angle of the diarylethene microcrystalline films stored at 30 °C [(a) 3, (b) 15, and (c) 24 h] and 50 °C [(d) 1, (e) 3, and (f) 15 h] after UV irradiation for 10 min. Scale bar: 5 μm.

(1)

where D is the fractal dimension of the cross-section, and the dimension of surface Ds is obtained approximately by Ds = D + 1.

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a low-adhesion superhydrophobic surface, appearing as a lotus effect. After 1 h of storage at 50 °C in the dark, superwater repellency and the lotus effect were observed. After 5 h, CA did not change much but SA increased to 15.5°. After 15 h, the lotus effect was lost and a water droplet was pinned on the surface. This showed a high-adhesion superhydrophobic surface, indicating the petal effect. The change from the lotus effect to the petal effect is attributed to the growth of larger rod-shaped crystals by Ostwald ripening. Fractal analysis of the surfaces, which were stored at 30 and 50 °C, was carried out by box counting for the cross-sectional trace curves of the surface’s cross-section (Figure S3). In addition, the fractal dimension (noninteger dimension) could be found in a suitable size range defined by the lower and higher cutoffs (l and L), which reflect characteristic scales such as the average width and length of the needle-shaped crystals in this study. The time profiles of these values are summarized in Figure 2. The UV-irradiated film was stored at 30 °C in the dark. After 3 h, the small needle-shaped crystals appeared on the film surface, and the fractal region covered a size ranging from 0.5 μm (lower cutoff corresponding to the average width) to 8 μm (upper cutoff corresponding to the average length), and the fractal dimension of the surface Ds was determined to be 2.21. After 15 h, needle-shaped crystals covered the film surface, and the fractal region shifted to that of 8 to 15 μm box sizes. The increase in the fractal region resulted from the growth of the needle-shaped crystals to longer lengths. Finally after 24 h, needle-shaped crystals further covered the film surface, the fractal region remained the same, and fractal dimension Ds increased from 2.33 to 2.54 (Fig. 2a). The increase in fractal dimension resulted from higher surface roughness. This fractal dimension and region indicate the lotus effect. In order to verify the lotus effect, fractal analysis was conducted. This revealed a sufficiently rough surface in the nanomicroregion to form air pockets and prevent the invasion of water droplets, such as the case of Cassie states. For this reason, water adhesion becomes low as discussed later.

RESULTS AND DISCUSSION Diarylethene 1o is a photochromic molecule that is converted to blue closed-ring isomer 1c upon irradiation with UV light, and it reverts to the original 1o by irradiation with visible light (Scheme 1).24−28 The topographical changes in the microScheme 1. Photoisomerization of Diarylethene

crystalline surface of 1o at the eutectic temperature of 1o and 1c have already been explained in a previous paper by using the phase diagram of 1o (cubic-shaped-crystals) and 1c (needleshaped-crystals) (Figure S2 in Supporting Information), indicating the lotus effect.14 Furthermore, the surface topographical changes above the eutectic temperature and the formation of a surface showing the petal effect as well as the photoswitchability of the lotus and petal effects have been reported in previous papers.16,17 We carried out new studies on the topographical changes of microcrystalline surface of 1o upon UV irradiation and monitored these changes by SEM. The top and side views of the surface are shown in Figure 1. We monitored the changes in surface topography and hydrophobicity of the sample by SEM and CA analysis (Figure 1). With photoinduced topographical changes at 30 and 50 °C after UV irradiation (10 min) in the dark, the gradual growth of the needle-shaped-crystals of 1c was observed and the CA of the water droplet increased. At 30 °C, the CA and SA after 15 h were 154° and >32°, respectively. After 24 h, these angles changed to 163° and 2°, respectively. Then, the surface was covered with needle-shaped crystals of 1c. This surface showed 10644

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Figure 3. Relation between fractal dimension and fractal region distributions on the microcrystalline surface of diarylethene 1. The scale bar for the extended image (×5000) is 2 μm (c); that for others (×2000) is 5 μm (a, b, and d−f).

Figure 2. Time profiles of the dimension and the lower and higher cutoffs (l and L) of the fractal regions by box-counting analysis at (a) 30 and (b) 50 °C. Figure 4. Alternate formation of two types of surfaces by the procedure of alternate irradiation with UV and visible light.

The UV-irradiated film was stored at 50 °C in the dark, and after 1 h, needle-shaped crystals covered the film surface, the fractal region ranged from 0.25 to 8 μm in size, and the fractal dimension of the surface Ds was determined to be 2.35 (Fig. 2b). After 3 h, needle-shaped crystals further covered the film surface and the fractal region shifted to one of 8 to 50 μm in size. The enlargement of the fractal region resulted from the growth of longer needle-shaped crystals. Finally after 15 h, rodshaped crystals appeared by Ostwald ripening, and the fractal region shifted to one of 50 to 100 μm in box size. The increase in the fractal region resulted from crystal growth due to the Ostwald ripening. This fractal dimension and region indicate the petal effect. In order to verify the petal effect, fractal analysis was conducted, which revealed sufficient surface roughness in the microregion to allow the invasion of water droplets, such as the case of Cassie impregnating states, and to enlarge the interaction with water. The relation between the order of the surface roughness and this invasion is discussed later. As a result, water adhesion becomes greater. These surfaces are classified into high-adhesion superhydrophobic surfaces and low-adhesion superhydrophobic surfaces by the wetting dynamics, such as the lotus and petal effects. Consequently, the photoswitching of surfaces showing the lotus effect and those showing the petal effect is the result of controlled water adhesion (Figure 3). With a photoswitchable diarylethene microcrystalline surface, two types of surfaces showing different dynamic wetting behaviors can be prepared reversibly by alternate UV and visible light irradiation as shown in Figure 4. (a) Lotus surface:

diarylethene microcrystalline films were stored at low temperature after UV irradiation. (b) Petal surface: visible light was used to irradiate the lotus surface, and then diarylethene microcrystalline films were stored at high temperature after UV irradiation. (c) Lotus surface: visible light was used to irradiate the petal surface, and then diarylethene microcrystalline films were stored at low temperature after UV irradiation. By such alternate irradiation with UV and visible light, two types of dynamic surface behaviors could be obtained.

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THEORETICAL DISCUSSION Now, let us discuss a possible scenario to explain why the lotus and petal effects were observed. Here, we divide this scenario into four parts. We first consider the wetting manner and fractal nature. Then, we present a general scheme to discuss the difference between the lotus and petal effects based on the potential energy of a droplet and the local pinning energy; furthermore, we propose criteria between two effects. Next, we describe the invasion of liquid into the spacing between pillars under ideal conditions because the petal effect is observed when the distance between the rod-shaped crystals is distant. This is helpful to understand the invasion on nonideal surfaces. Finally, we comprehensively discuss the relationship between the general scheme and the microcrystalline surfaces of diarylethene 1. Correlation between the Wetting Manner and Fractal Nature. Before the theoretical discussion of the lotus and petal 10645

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effects, we point out that fractal geometries are reflected in the wetting manner in the lotus and petal effects and fractal dimension as follows. Obviously, the wetting manner is different as shown in Figure 3. The minimum wetting area is a pillar head in the lotus effect (the Cassie states), which is directly connected to the lower cutoff in the size range in which the fractal dimension appears. In the sliding, the shape of the droplet is not deformed on the ideal surface, but it is deformed on the actual surfaces. This is originated from the pinning effects induced by chemical and physical defects.29 The former arises from chemical inhomogeneities, i.e., the distribution of chemically different surfaces. The contact line could be pinned at the boundary between two chemically different domains due to the difference in the equilibrium contact angle. The contact angle at the boundary changes if the droplet moves on a slope with a suitable tilting angle. On the other hand, the latter is due to surface structures. The critical factor is the change in contact angle at the tip of a physical defect. In the lotus effect, we expect that the pillars (physical defects) along the advancing front line trap the contact line, and the minimum condition in the sliding is to give a perturbation energy larger than the local pinning energy to trap the contact line at the lowest pin. The local pinning energy would be proportional to the width of a pin (the lower cutoff). Therefore, it would be possible to find the correlation between the sliding phenomena and the lower cutoff. On the other hand, the minimum wetting area is the sum of a pillar head and a pillar side in the petal effect (the Cassie impregnating states). Actually, the size (width) of a pillar head of the rod-shaped crystal is an intermediate size between the lower and upper cutoffs though the upper cutoff corresponds to the length of the rod-shaped crystals. Therefore, the width cannot be found in the box-counting analysis, but it is possible to find it in the SEM images. As a result, the petal effect would be discussed on the basis of the lower cutoff and the local pinning energy. Criterion of the Lotus and Petal Effects. Before the full discussion, we point out that the difference in SA was observed even on surfaces showing sliding behaviors, which we call the lotus effect in SA of less than 5°. This indirectly indicates that the potential energy change of a droplet ΔU and the pinning energy Epin compensate for each other. The origins of the pinning effect causing pinning energy are mainly induced by physical defects and chemical contamination on the surface. The former increases the air−liquid interfacial area of a pinned droplet on a tilting slope because of the deformation of the droplet. The latter makes a discrete change in the contact angle on the boundary between chemically different surfaces due to the difference in the equilibrium contact angle in Figure S2 (SI). Therefore, the contact line is easily pinned at the boundary. To advance the contact line, the shape of the droplet is deformed from a hemisphere to a teardrop, where the air− liquid interfacial area is also enhanced. To treat this problem, we simply mention them under the situation as shown in Figure 5a,b in which the pitch is p, the contact angle is θCB, and the sliding angle is ϕ. Here, we assume that the multipillar surface is ideal, i.e., a completely periodic structure and identical pillars with each having an edge with a perfect right angle. In this situation, we roughly assume that the droplet on a tilting slope is locally pinned by the pillar at the lowest front part, as shown in Figure 5c, by local pinning energy Elocal pin. We have previously discussed how to evaluate the pinning energy of a droplet on a cylindrical pillar defect.30 In the present situation,

Figure 5. Schematic representations of a droplet on a tilting ideal CB surface with tilting angle ϕ = 0° (a) and ϕ = SA (b) and local pinning in the base of a droplet on a multipillar surface (c).

we assume that Elocal pin is proportional to the width of the pinned front as discussed later. In addition, the potential energy of the droplet on a tilting slope is another key factor. Considering these, it is possible to discuss that the droplet slips down if the potential energy of the droplet ΔU becomes larger than Elocal pin. We define that this is the minimum condition for sliding. In other words, the critical factor is the balance between Elocal pin and ΔU. Before the discussion of Elocal pin, let us discuss the potential energy ΔU of a droplet (mass m) with a tilting angle of slope ϕ. ΔU is ΔU = mgh

(2)

As the minimum condition for sliding, ΔU must be larger than Epin when the displacement of the droplet along the slope is the pillar pitch p. Otherwise, the petal effect would be observed. The change in height of the center of mass h is p sin ϕ (3) 2 where the factor of 1/2 is applied because the change in width along the surface is twice the change in the position of the center of mass. The work of sliding the droplet, W, is equal to ΔU: h≈

mgp sin ϕ (4) 2 This is the driving force of sliding. Next, let us discuss Elocal pin. We roughly assume that Elocal pin is proportional to the length of the depinned front line, considering the full pinning energy Efull pin. This is the suppression factor. We also roughly assume that Elocal pin depends on the length, with Efull pin as w E local pin ≈ Efull pin (5) 2πR W=

where Efull pin is the pinning energy when a droplet equally penetrates along the apex of a cylindrical pillar,30 w is the diameter of a pillar (Figure 5), and R is the radius of the basement (Figure 6b).30 Efull pin is obtained from the deviation in the air−liquid interfacial energy between the deformed and equilibrium shapes as described in the following discussion. Here, we would like to briefly discuss our basic approach. Along our theoretical treatment,30 we present a geometry of the depinning condition on a physical defect in Figure S4a−d. The droplet is depinned when the contact angle at the apex reaches (θeq + 90°). Next we mention that on a chemical defect in Figure S4e−g. The droplet is depinned when the contact angle at the boundary reaches θeq2. Furthermore, we present the depinning condition on an imaginary surface from a DAE 10646

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we assume that γLΔA′L is equal to Efull pin, where γL is the interfacial energy density of the air−liquid interface (the surface tension of the liquid). To obtain Efull pin, it is necessary to calculate the air−liquid interfacial areas of the spherical droplet A′L,sp (the equilibrium state) and pancake A′L,pk, respectively. A′L,sp is30 ⎛ ⎞2/3 4 ′ = 2π ⎜ ⎟ r0 2(1 − cos θCB) AL,sp 3 ⎝ 2 − 3 cos θCB + cos θCB ⎠

(6)

where r0 is the radius of a droplet before wetting. Next, let us consider AL,pk ′ . We give the details of the theoretical treatment of the pinning energy in Supporting Information. We skip the details here, but A′L,pk is ′ = 4π[r4 2 sin θeq + (Rr4 − Ar4 2)θeq ] + πR2 AL,pk

(7)

(4/3)πr03.30

Here, r4 is determined under constant volume VL = Since the volume of the pancake is equal to VL, r4 is determined from the following relation. 4 4 π[ r4 3 + πRr4 2 + 2R2r4] − πr0 3 = 0 (8) 3 3

In the present case, A = cos(90°) = 0. Therefore, Efull pin is obtained from ′ − AL,sp ′ ) Efull pin = γL(AL,pk

(9)

This is the energy required to depin the contact line along the apex of the pillar. Now, we assume γL = 72 mN/m. Here, we define that the full pinning energy is equal to the work needed to depin the contact line along the apex. In the present experiments, the droplet is locally pinned at the contact line by the needle- and rod-shaped crystals. Therefore, we assume that Elocal pin is proportional to the width of the crystals. Elocal pin is described by eq 5, where w is also the width of a needle-shaped or rod-shaped crystal. Here, we emphasize that p in eq 4 and w in eq 5 are quite different in the lotus and petal effects. In the following discussion, we thus try to characterize the lotus and petal effects based on W and Elocal pin. Lotus Effect. The experimental and theoretical values are shown in Table 1. The orders of w and p are distributed as 0.2− 0.5 and 5−20 μm, respectively, while ϕ = 2−32° as shown in Figure 3. Accordingly, the order of Elocal pin is roughly estimated to be ∼10−11 J by eqs 5−9, while that of W is also ∼10−11 J by eq 4. As a result, we found the tendency where W is slightly larger than Elocal pin, except for the sample stored at 30 °C for 24 h. Therefore, it is possible to describe the condition of the lotus effect as

Figure 6. Schematic representations of a pinned droplet along the apex of a cylindrical pillar (a) and its deformation under an imaginary situation to evaluate the full pinning energy (b) in which the droplet is pushed down by an imaginary plate of air and the surfaces of the multipillar and the imaginary plate have the same height.

surface in Figure S5 as a conceptual model of Figure S4e. We assume that the droplet is depinned if the contact angle at the apex reaches 180° (the contact angle of water for air). On the basis of this, we mention the geometry shown in Figure 6. First, the droplet on the CB surface is spherical with contact angle θCB, and its outside is an imaginary plate. Next, we push the droplet by the imaginary plate (Figure 6b). The geometry changes from a spherical shape to a pancake. When the contact angle of the pancake reaches 180° (the contact angle of liquid to air, θair, is 180°), the contact line of the droplet begins to spread over the imaginary surface of air. In this process, the air−liquid interfacial area is increased by the deformation ΔA′L. This means that the state of the pancake is unstable. Therefore, Table 1. Theoretical Values of Parameters Based on Experiments storage temp (°C) storage time (h) volume (μl) θCB (deg) lower cutoff (μm) upper cutoff (μm) Elocal pin (J) SA (deg) w (μm) p (μm) ϕc (deg) W (J) dynamic behavior

3 2 111 0.5 8 ∼2.5 × 10−7

30 15 2 154 0.5 15 ∼3.6 × 10−11 32 0.5 10 21.6 ∼5.2 × 10−11 sliding

24 2 154 0.5 15 ∼1.6 × 10−11 2 0.5 5 18.9 ∼1.7 × 10−12 lotus effect 10647

1 2 153 0.24 9 ∼1.5 × 10−11 11.5 0.2 10 8.5 ∼2.0 × 10−11 lotus effect

50 3 2 154 0.5 50 ∼1.4 × 10−11 15.5 0.2 5 17.1 ∼1.3 × 10−11 lotus effect

15 2 156 1 100 >7.2 × 10−10 10 100 >47.4 ∼9.8 × 10−10 petal effect

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W ≥ E local pin

(10)

Furthermore, it is possible to discuss the theoretical sliding angle. We obtain the criteria of the sliding with respect to the critical tilting angle ϕc as ⎛ 2E local pin ⎞ ϕc = sin−1⎜ ⎟ ∝ sin−1 w ⎝ mgp ⎠

(11)

Equation 11 first formulates the relationship among the sliding angle, pinning energy, and mass of a droplet on the multipillar surface although the sliding angle on a tilting smooth surface was already discussed.31 The latter relation in eq 11 explains the experimental results of SA becoming larger on the rod- and needle-shaped crystal surfaces with respect to w. The theoretical values of ϕc are also shown in Table 1. Equation 11 explains the experimental tendency in SA except for the sample stored at 30 °C for 24 h. With small ϕc, we ideally obtain ϕc =

Figure 8. Schematic representations of the sliding behaviors including the lotus effect (a) and the petal effect (b) under the experimental conditions.

out the importance of the pillar’s tilting angle in the lotus effect. Further considerations will be required to understand the deviations in the near future. Petal Effect. From the SEM image of Figure 1f, we estimated that w ≈ 10 μm and p ≈ 100 μm, while ϕ > 90° as shown in Figure 3. Therefore, the order of Elocal pin is roughly estimated to be ∼10−9 J in calculation, while W is 10−9 J. In the actual samples, it is easy to imagine that the long rod-shaped crystals touch the droplet as shown in Figure 8b, in contrast to Figure 8a. This enlarges Elocal pin and ϕc. As a result, we concluded that Elocal pin > 7.2 × 10−10 J and ϕc > 47.4° We thus found that the petal effect is maintained under the following condition:

2E local pin mgp

(12)

Why do the behaviors of the sample stored at 30 °C for 24 h deviate from what we would expect in given eqs 10 and 11? There seems to be two main reasons. The first is that the actual values of w and p may be different. In the box-counting method used for this surface, data scattering below 0.5 μm is observed in Figure S3 (SI). This suggests the possibility that an effective w may be smaller than 0.5 μm. The second is the tilting angle of the needle-shaped crystals. Figure 7 shows schematic

W < E local pin

(13)

With this background, we next discuss the invasion of liquid. Invasion of the Multipillar Surface by Liquid. The Cassie impregnating states are illustrated with respect to the spacing between the rod-shaped crystals as the upper inset in Figure 3. Very recently, as an experimental approach to finding the criteria of invasion, we have been directly observing the three-dimensional geometry of the air−liquid interface in the spacing among three pillars.32 In that study, we found that the geometry is spherical. This shows that the invasion between pillars is equal to the invasion into a cylindrical hole due to having the same geometry. To clarify the invasion, we now describe the situation in Figure 9. We define the radii of the base and hollow as r2 and r5, respectively, while the radii of the droplet and the curvature of the air−liquid interface in the base are r1 and r6, respectively, as shown in Figure 9. Here, 2r5 corresponds to the spacing in the multipillar surface. The droplet presses the hollow by pressure P as

Figure 7. Schematic representations of a droplet on a tilting CB surface (a) and the contact angle in the vicinity of the lowest contact line on the tilting slopes: ideal multipillar surface (b), slightly tilting pillars (c), and tilting pillars (d). The dashed curves represent the air− liquid interface with θair for the lowest pillar head.

P=

representations of the CB states with ideal pillars (Figure 7b), slightly tilting pillars (Figure 7c), and tilting pillars (Figure 7d). A droplet on the tilting slope could be deformed by the potential energy and the increasing contact angle of the lower contact line (i.e., the advancing angle). When the contact angle for the lowest pillar head becomes larger than θair, the contact line could move to one of the next pillars (SI). In this scenario, the local depinning easily occurs in Figure 8d in which the tilting angle of the pillars is relatively large. Therefore, we point

πr52(1 − cos θCB)r1ρg πr52

(14)

where ρ is the density of the liquid and g is the acceleration of gravity. The volume of the liquid over the hollow is given approximately by πr52(1 − cos θCB)r1 for r1 > 2r5 in eq 14. The Laplace pressure ΔP is 2γ ΔP = L r6 (15) From eqs 14 and 15, we obtain 10648

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order in size is 1.5 decades in the experiments. The long needle-shaped crystals enlarge the order. (3) Fine needleshaped crystals with high density together with a few rodshaped crystals: the petal effect appears because the invasion occurs between the rods-shaped crystals and Elocal pin is enhanced. The ratio is relatively large due to the rod-shaped crystals. Its order is 2 decades in the experiments. The rodshaped crystals enlarge the order.

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CONCLUSIONS Upon alternate irradiation with UV and visible light, reversible crystal growth and melting were observed. As a result, we achieved the control for two types of rough surfaces, a lowadhesion superhydrophobic surface that shows the lotus effect and a high-adhesion superhydrophobic surface that shows the petal effect. The lotus effect is attributed to a surface structure that is covered with needle-shaped crystals, and the petal effect is attributed to surfaces covered with needle-shaped and rodtype crystals. By visible-light irradiation of both types of rough surfaces, needle-shaped and rod-shaped crystals were melted, and then the generation of cubic-shaped crystals was observed. Fractal analysis was carried out for these surfaces using the boxcounting method, and the results revealed that the lotus effect is attributed to the rough surface having nanomicrostructures, while the petal effect is attributed to the rough surface in the microregion structures. The surface roughness of the nanomicroregion prevents the invasion of water droplet by an air pocket. As a result, this region shows a low-adhesion superhydrophobic surface. The roughness of the microregion allows the invasion of water droplet and enlarges the interaction with water. As a result, this region shows a high-adhesion superhydrophobic surface. Achieving photocontrol of a crystal’s type and size led to controlling the two types of water-adhesion superhydrophobic states, and as a result, the photoswitching of the lotus and petal effects could be attained. The observed dynamic wetting behaviors are thus explained by a simple scenario considering the trade-off between the surface-structure-depending adhesion energy and the potential energy change. The rough estimation of the theoretical parameter calculated from the experimental results semiquantitatively explains the relations among surface fractal dimension, surface morphology and dynamic wetting behaviors.

Figure 9. Schematic representation of a droplet over a hollow: bird’seye view (a) and side view (b).

⎛ (1 − cos θ )r r ρg ⎞ CB 1 5 ⎟⎟ φ = sin−1⎜⎜ 2γL ⎝ ⎠

(16)

where φ is the angle of the hollow and r5 = r6 sin φ. Equation 16 explains the tendency of φ to increase when r5 increases. Roughly, invasion occurs under the following condition of the depinning angle: φ+

π ≥ θCB 2

(17)

Equations 16 and 17 show that invasion occurs easily when the distance is separated. Moreover, eqs 16 and 17 are maintained on an ideal CB surface. However, the actual diarylethene surfaces with needle- and rod-shaped crystals are quite far from the ideal multipillar surface because there are short and long needles with random orientation and tilt. In particular, the distance between the rod-shaped crystals is separated. Therefore, it is easy to understand the reason that the invasion easily occurs on the surface with rod- and needle-shaped crystals. It should be noted that the invasion makes a continuously long contact line and Elocal pin becomes quite large because the local pinning occurs over a wide area of the front line. Relationship between Wetting Dynamics and Fractal Diarylethene Surfaces. On the basis of the experimental and theoretical results, we discuss the relationship among the lotus effect, the petal effect, and diarylethene surfaces. Considering Elocal pin and the condition of penetration, we have three conditions: (1) Fine needle-shaped crystals with low density (large separation between crystals): the petal effect could be observed due to the large Elocal pin caused by the invasion. The surface fractal dimension is relatively close to 2.0 because the number of crystals is small. The ratio of the upper and lower cutoffs, L/l, is relatively small. In the experiment, its order is 1 decade. (2) Fine needle-shaped crystals with high density: the lotus effect appears, and Elocal pin is small. The surface fractal dimension is relatively higher than 2.0, and the ratio is small. Its

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of surface topographical changes by UV irradiation, SEM images and contact angles of the diarylethene microcrystalline films, fractal analysis of the microcrystalline surfaces of 1o, schematic representations of droplets in several different occasions, and geometries of a droplet on an air−DAE surface before and after deformation using an imaginary plate. The material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +81-166-68-2726. Fax: +81-166-68-2782. E-mail: [email protected]. *Tel: +81-77-543-7462. Fax: +81-77-543-7483. E-mail: [email protected]. 10649

dx.doi.org/10.1021/la502565j | Langmuir 2014, 30, 10643−10650

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Article

Present Address

Which Contact Angle of a Water Droplet Exceeds 170° by Reversible Topographical Changes on a Diarylethene Microcrystalline Surface. Langmuir 2012, 28, 17817−17824. (19) Ge, H.; Wang, G.; He, Y.; Wang, X.; Song, Y.; Jiang, L.; Zhu, D. Photoswitched Wettability on Inverse Opal Modified by a SelfAssembled Azobenzene Monolayer. ChemPhysChem 2006, 7, 575− 578. (20) Feng, X.; Zhai, J.; Jiang, L. The Fabrication and Switchable Superhydrophobicity of TiO2 Nanorod Films. Angew. Chem., Int. Ed. 2005, 44, 5115−5118. (21) Ichimura, K.; Oh, K. S.; Nakagawa, M. Light-Driven Motion of Liquids on a Photoresponsive Surface. Science 2000, 288, 1624−1626. (22) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K. Photoreversibly Switchable Superhydrophobic Surface with Erasable and Rewritable Pattern. J. Am. Chem. Soc. 2006, 128, 14458−14459. (23) Lim, H. S.; Kwak, D.; Lee, D. Y.; Lee, S. G.; Cho, K. UV-Driven Reversible Switching of a Roselike Vanadium Oxide Film between Superhydrophobicity and Superhydrophilicity. J. Am. Chem. Soc. 2007, 129, 4128−4129 4128.. (24) Duerr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 2003. (25) Feringa, B. L.; Browne, W. B. Molecular Switches; Wiley-VCH: Weinheim, Germany, 2011. (26) Irie, M. Diarylethenes for Memories and Switches. Chem. Rev. 2000, 100, 1685−1716. (27) Crano, J. C.; Guglielmetti, R. Organic Photochromic and Thermochromic Compounds; Plenum Press: New York, 1999; Vol. 1. (28) Matsuda, K.; Irie, M. Diarylethene as a Photoswitching Unit. J. Photochem. Photobiol., C 2004, 5, 169−182. (29) de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer: New York, 2003; p 69. (30) Mayama, H.; Nonomura, Y. Theoretical Consideration of Wetting on a Cylindrical Pillar Defect: Pinning Energy and Penetration Phenomena. Langmuir 2011, 27, 3550−3560. (31) Furmidge, C. G. L. Studies at phase interfaces. I. The sliding of liquid drops on solid surfaces and theory of spray retention. J. Colloid Sci. 1962, 17, 309−324. (32) Tanaka, T.; Mayama, H.; Nonomura, Y. Direct Geometric Observation of an Agar Gel Droplet on a Multipillar Surface. Chem. Lett. 2012, 41, 960−961.

(H.M.) Department of Chemistry, Asahikawa Medical University, 2-1-1-1 Midorigaoka−higashi, Asahikawa, Hokkaido 078-8510, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a grant-in-aid for scientific research on innovative area (no. 23106704) and research (C) (nos. 22540417, 23540473, and 26400424) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), and the MEXT-Supported Program for the Strategic Research Foundation at Private Universities.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on August 25, 2014. Revisions were made to Figure 1 and the Supporting Information, and the corrected version was reposted on August 26, 2014.

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dx.doi.org/10.1021/la502565j | Langmuir 2014, 30, 10643−10650