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Wetting transition and optimal design for microstructured surfaces with hydrophobic and hydrophilic materials
Journal of Colloid and Interface Science 336 (2009) 298–303
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Wetting transition and optimal design for microstructured surfaces with hydrophobic and hydrophilic materials Chan Ick Park a, Hoon Eui Jeong a, Sung Hoon Lee a, Hye Sung Cho a, Kahp Y. Suh a,b,* a b
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Korea Institute of Advanced Machinery and Design, Seoul National University, Seoul 151-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 January 2009 Accepted 1 April 2009 Available online 15 April 2009 Keywords: Wetting transition Microstructure Soft lithography Replica molding Capillary force Polymer
a b s t r a c t We present wetting transition of a water droplet on microstructured polymer surfaces using materials with different hydrophilicity or hydrophobicity: hydrophobic polydimethyl siloxane (PDMS) (hwater 110°) and hydrophilic Norland Optical Adhesive (NOA) (hwater 70°). The microstructures were fabricated by replica molding and self-replication with varying pillar geometry [diameter: 5 lm, spacingto-diameter ratio (s/d): 1–10 (equal interval), height-to-diameter ratio (h/d): 1–5] over an area of 100 mm2 (10 mm 10 mm). Measurements of contact angle (CA) and contact angle hysteresis (CAH) demonstrated that wetting state was either in the homogeneous Cassie regime or in the mixed regime of Cassie and Wenzel states depending on the values of s/d and h/d. These two ratios need to be adjusted to maintain stable superhydrophobic properties in the Cassie regime; s/d should be smaller than 7 (PDMS) and 6 (NOA) with h/d being larger than 2 to avoid wetting transition by collapse of a water droplet into the microstructure. Based on our observations, optimal design parameters were derived to achieve robust hydrophobicity of a microstructured surface with hydrophobic and hydrophilic materials. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Wettability is one of the most important surface properties in a variety of fields such in film coating [1], tribology [2,3], MEMS/ BioMEMS [4–6], and micro/nanofluidics [7–11]. Accordingly, wettability has been studied for many decades after Wenzel [12] and Cassie–Baxter [13] suggested a model to explain wetting behavior on a structured surface. As a consequence, it is now well understood that surface geometry and chemical distribution of materials govern the wettability of a solid surface as described by Cassie– Baxter and Wenzel equations [12,13]. In particular, since the observation of excellent superhydrophobic properties of a lotus leaf in 1997 [14], extensive efforts have been made to fabricate nature-inspired superhydrophobic surfaces based on various self-assembly and lithographic methods [15–19] or to examine the wetting kinetics and transition on structured surfaces [19–33]. In this work, we present wetting transition on microstructured polymer surfaces using materials of different hydrophobicity and optimal design parameters for robust hydrophobicity. To fabricate a microstructured surface, a simple replica molding method aided by self-replication was used with UV-curable materials. Compared to previous self-assembly or self-organization methods, this soft lithographic method provides a simple route to modifying the * Corresponding author. Address: School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Korea. Fax: +82 2 883 1597. E-mail address: [email protected] (K.Y. Suh). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.04.022
topography of a surface with designing capability in a geometry controllable manner. Furthermore, self-replication is an important characteristic of the current method, which is particularly useful for generating positive or negative features without the use of an original silicon master [34]. Using the self-replication characteristic, one can duplicate various microstructured surfaces of hydrophobic and hydrophilic materials with identical geometry. By exploiting the above-mentioned advantages, we created geometry-tunable, microstructured surfaces using hydrophobic PDMS (hwater 110°) and hydrophilic NOA materials (hwater 70°) and examined their wetting properties. Although there have been a large number of studies dealing with wetting behavior on a microstructured surface, the role of hydrophilicity or hydrophobicity of a material is not well understood in designing geometrical parameters for tailored wetting properties. Aiming at this purpose, we tested various pillar geometries by changing the space to diameter (s/d) (from 1 to 10, equal interval) and height-to-diameter (from 1 to 5) ratios for a given pillar diameter (5 lm), yielding a total number of combinations as high as 100 (50 each for each material).
2. Materials and methods 2.1. Fabrication of microstructured polymer surfaces The negative silicon masters (denoted as ‘’) were prepared by conventional photolithography, which contained an array of 5 lm circular holes with different density over an area of 100 mm2
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(10 mm 10 mm). Each wafer had 10 different spacing ratios (space/diameter) and five different height ratios (height/diameter). Detailed geometrical parameters are summarized in Table 1. To prepare microstructured polymer surfaces from the above silicon masters, replica molding and self-replication were used as shown in Fig. 1. First, positive (denoted as ‘+’) PDMS (Sylgard, 184, Dow Corning) replicas were obtained by casting PDMS precursor against a silicon master with 30% curing agent to prevent ‘self-matting problem’ [35] in high aspect ratio (AR > 3) structures. Owing to a higher content of the curing agent, the PDMS replica became more rigid and hydrophobic (hwater 110°). No cracks or wrinkles were observed in the highly cured PDMS replica. To utilize self-replication characteristic, a polyurethane acrylate (PUA, 311RM, Minuta Tech.) liquid precursor was drop-dispensed onto the as-prepared PDMS replica and then a 50 lm transparent polyethylene terephthalate (PET) film was gently placed on the liquid followed by UV (k = 250–400 nm) exposure for 40 s. To expel trapped air, a roller was mildly rounded several times on top of the flat PET film. After the UV curing, the PUA replica was peeled off from the PDMS replica using a sharp tweezer [34], with the same geometry as the original silicon master. Finally, a replica of Norland Optical Adhesive (NOA) 73 (Norland Products Inc.) was prepared from the PUA replica after completely destroying reactive double bonds of the PUA via an additional curing for 12 h. The NOA replica was prepared by the replica molding after curing for 90 s and peeling-off. 2.2. Measurements of contact angle (CA) and contact angle hysteresis (CAH) The static contact angles (CAs) of water were measured with a contact angle analyzer (Drop Shape Analysis System DSA100, Kruss, Germany) by gently placing a water droplet (6 ll) on a microstructured surface over the time span of 1.5 min for each pattern. The presented values were averaged over at least six different locations in a sample. 2.3. Scanning Electron Microscopy (SEM) SEM images were taken using SEM (S-4800, Hitachi, Japan) at an acceleration voltage of 15.0 kV and an average working distance of 11.4 mm. Samples were coated with a 30 nm Pt layer prior to analysis. 3. Results and discussion 3.1. Fabrication of microstructured polymer surfaces Fig. 2 shows representative SEM images of microstructured PDMS and NOA surfaces fabricated by replica molding and self-
Fig. 1. Schematic diagram of the fabrication procedure with two different polymer materials: (left side) simple replication of microstructures with hydrophobic PDMS, (right side) sequential replication of the same microstructures with hydrophilic NOA 73 by using PUA material as an intermediate self-replica.
replication. As shown in the figure, the structures exhibited high structural fidelity with well-defined vertical edge profiles. No structural differences between the PDMS and NOA replicas were observed. For generating uniform microstructured surfaces, care should be taken during the 2nd replication step so as to preserve an original height of the silicon master. In the case of fabricating a PDMS replica (1st replication step), air can easily permeate out of the cured PDMS during thermal crosslinking, so that an exact replica can be readily obtained without air trap. In the case of fabricating a PUA replica, however, air may be trapped in the spaces between microstructures, resulting in a reduced height or a nonuniform distribution of height in the microstructure. To alleviate this problem, a roller was mildly rounded several times on top of a flat PET film to expel trapped air prior to UV exposure. Also, the curing time needs to be controlled for maintaining the PUA surface inactive to the NOA material, recognizing that the same double bonds are present in the backbone of both materials. It
Table 1 Geometrical parameters of pillar pattern used in the experiments (d: diameter, s: spacing, h: height). The pillar diameter is fixed at 5 lm for all patterns (unit: lm). h
s 5
10
15
20
25
30
35
40
45
50
5
h/d = 1 s/d = 1
h/d = 1 s/d = 2
h/d = 1 s/d = 3
h/d = 1 s/d = 4
h/d = 1 s/d = 5
h/d = 1 s/d = 6
h/d = 1 s/d = 7
h/d = 1 s/d = 8
h/d = 1 s/d = 9
h/d = 1 s/d = 10
10
h/d = 2 s/d = 1
h/d = 2 s/d = 2
h/d = 2 s/d = 3
h/d = 2 s/d = 4
h/d = 2 s/d = 5
h/d = 2 s/d = 6
h/d = 2 s/d = 7
h/d = 2 s/d = 8
h/d = 2 s/d = 9
h/d = 2 s/d = 10
15
h/d = 3 s/d = 1
h/d = 3 s/d = 2
h/d = 3 s/d = 3
h/d = 3 s/d = 4
h/d = 3 s/d = 5
h/d = 3 s/d = 6
h/d = 3 s/d = 7
h/d = 3 s/d = 8
h/d = 3 s/d = 9
h/d = 3 s/d = 10
20
h/d = 4 s/d = 1
h/d = 4 s/d = 2
h/d = 4 s/d = 3
h/d = 4 s/d = 4
h/d = 4 s/d = 5
h/d = 4 s/d = 6
h/d = 4 s/d = 7
h/d = 4 s/d = 8
h/d = 4 s/d = 9
h/d = 4 s/d = 10
25
h/d = 5 s/d = 1
h/d = 5 s/d = 2
h/d = 5 s/d = 3
h/d = 5 s/d = 4
h/d = 5 s/d = 5
h/d = 5 s/d = 6
h/d = 5 s/d = 7
h/d = 5 s/d = 8
h/d = 5 s/d = 9
h/d = 5 s/d = 10
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Fig. 2. SEM images of replicated PDMS microstructures from negative patterned silicon masters (a–e) and replicated NOA 73 microstructures (k–o) from self-replicated PUA masters (f–j). The geometrical parameters are: (a, f, and k) 5 lm height pillars with a spacing of 50 lm, (b, g, and l) 15 lm height pillars with a spacing of 50 lm, (c, h, and m) 25 lm height pillars with a spacing of 25 lm, (d, i, and n) 25 lm height pillars with a spacing of 25 lm and (e, j, and o) 25 lm height pillars with a spacing of 5 lm. For each structure, the diameter is 5 lm. Note that no differences in the replicated structures were observed between PDMS and NOA materials. Scale bars indicate 100 lm.
turned out that an overnight curing step (>12 h) was a prerequisite to ensure a sufficiently inert, non-reactive PUA surface with clean release of a NOA replica. 3.2. Wetting transition and design criteria with hydrophobic and hydrophilic materials A water droplet ‘reacts’ to a physical change of the surface in the presence of microstructure, either in the form of homogenous or heterogeneous wetting. According to Wenzel model [12], the apparent CA of a water droplet varies proportionally with the roughness factor, which gives
cos hw ¼ r cos hwater
ð1Þ
where hw is the apparent CA on a rough surface determined by Wenzel state, hwater is the equilibrium CA on a flat surface and r is the roughness factor that is defined as the ratio of the actual area of a rough surface to that of the projected area. Since the roughness factor is always larger than unity, surface properties are exaggerated when a water droplet contacts the surface in Wenzel state. On the other hand, Cassie–Baxter proposed the heterogeneous contact of a water droplet on a composite surface that is composed of solid and air, which gives [13]
cos hc ¼ f1 cos h1 þ f2 cos h2 þ
ð2Þ
where hc is the apparent CA on a composite surface determined by Cassie–Baxter state, hn is the equilibrium CA on each flat surface, and f is the area fraction of each material. In a binary surface consisting of solid and air, Eq. (2) is reduced to
cos hc ¼ f cos hwater þ ð1 f Þ cos hair ¼ f ðcos hwater þ 1Þ 1
ð3Þ
Fig. 3. Comparison of the measured CAs on microstructured PDMS and NOA surfaces with the theoretical predictions based on Cassie–Baxter and Wenzel models. Inset pictures show the images of water droplets before and after transition.
where hwater is the equilibrium CA on a flat surface and f is the solid surface (1 f: area fraction). Therefore, a water droplet is supported by hydrophobic air cushion that is present in the spaces between microstructures in Cassie–Baxter state and thus the static CA increases with increasing spacing s/d.
f ¼
Fig. 3 shows theoretical and experimental CAs of water on microstructured surfaces made of (a) hydrophobic (PDMS) and (b) hydrophilic (NOA 73) materials. Theoretical values were calculated by Wenzel and Cassie–Baxter equations using the following parameters:
where d is the diameter, s is the spacing and h is the height of pillars, respectively. Experimental results were obtained from averaged CAs measured over at least six different locations per each pattern with an average deviation of ±3°.
r ¼1þ
pdh ðd þ sÞ2
pd2 2
ðd þ sÞ
¼
¼1þ
p ðh=dÞ ð1 þ s=dÞ2
p ð1 þ s=dÞ2
ð4Þ ð5Þ
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Several notable findings are derived from Fig. 3. First, water drops prefer to exist in the Cassie–Baxter regime for a high aspect ratio, dense pattern because the surface free energy is known to be lower than that in the Wenzel regime [24]. When the s/d ratio increased, the water droplet started to collapse into the cavity, representing the transition of wetting state from Cassie to Wenzel. This transition occurs easily for a small height ratio (h/d) pattern, suggesting that the transition barrier gets lowered for a lower height. From a thermodynamic standpoint, the water droplet prefers wetting state with a lower free energy. Thus, the stability of Cassie state can be described by the threshold contact angle (h*), which is given by [36]
cos h ¼
f 1 rf
ð6Þ
If the CA of a flat surface h is larger than h*, the water droplet prefer to stay in the Cassie regime. For h > h*, on the other hand, the water droplet prefers to exist in the Wenzel regime. Second, the trend of wetting transition was similar for PDMS and NOA materials except for different critical values of s/d that are required for the collapse of a droplet. For the hydrophobic PDMS material, the Cassie state was maintained over a wide range of s/d values up to 7 for a given h/d of 5 (See Fig. 3a). For the hydrophilic NOA material, the droplet started to fall when s/d was larger than 6 for the same h/d. Thus, the design criteria for stable superhydrophobic properties in the Cassie regime need to be adjusted for materials with different hydrophobicity. For the hydrophobic PDMS, a spacing ratio of 6–7 is desirable for a large height of microstructure (h/d = 5), which would give the maximum contact angle as well as the smallest hysteresis for the conditions used in our study. Experimentally, such a microstructured surface exhibited a static CA as high as 154.5° and a hysteresis as low as 4.2°. For the hydrophilic NOA, a spacing ratio of 5–6 seems appropriate to maintain superior wetting properties even with a hydrophilic material. Such a microstructured surface showed a static CA as high as 155.2° and a hysteresis as low as 10.2°, suggesting that even a hydrophilic material could provide a stable Cassie state under a limited range of s/d. Fig. 4 shows snap-shot images of measuring hysteresis on these two different surfaces. Considering difficulty in preparing a high aspect ratio silicon master, a microstructured PDMS surface with an s/d ratio of 4–5 and an h/d ratio of 2–3 could be optimal to achieve robust hydrophobic properties. It is noted that for all the geometrical parameters tested for both materials an h/d ratio of 1 appears too small to attain a stable heterogeneous wetting state as seen from Fig. 3. It seems that the local meniscus of a droplet directly touch the bottom of the substrate, resulting in a rapid decrease of static CA with increasing s/d. 3.3. Theoretical description of wetting transition
Fig. 4. Advancing and receding CAs (dynamic CA) for the proposed optimal design parameter for each material. (a) Static CA on PDMS surface is as high as 154.5° with hysteresis as low as 4.2°. (b) Static CA on NOA 73 surface is as high as 155.2° with hysteresis as low as 10.2°.
where c is the liquid–air surface tension (72 mJ/m2), and A and L represent the cross-sectional area and perimeter of pillar, respectively. For the conditions used in our experiment, the calculated critical pressure is about 5 102 Pa. On the other hand, the internal pressure that is originated from gravity can be calculated by
Pi ¼
V qg V qg ¼ Acontact pðRðhÞÞ2 "
RðhÞ ¼
ð8Þ 6V
pð1 cos hÞð3 sin2 h þ ð1 cos hÞ2 Þ
#13 ð9Þ
where V is the volume of a droplet, q is the density of water, g is the gravitational constant, h is the apparent CA of a droplet and R(h) is the radius of a droplet in contact with the surface, respectively. The calculated internal pressure is 6.4 102 Pa. For a dense pattern with a smaller s/d, the critical pressure is larger than the internal pressure, which means that the wetting state can be maintained in the Cassie regime. As s/d increases, however, the critical pressure decreases with a comparable value to the internal pressure. For instance, the critical pressure is 6.7 102 Pa for an s/d of 6 and
Although the trend of wetting transition is similar for both materials, the underlying mechanism would be different. Here, we explain the role of two geometrical parameters (i.e., s/d and h/d) in the wetting transition in terms of Laplace and hydrostatic pressures. For the hydrophobic PDMS material, the shape of a local meniscus is convex, such that the Laplace pressure competes with the internal pressure by gravity. Fig. 5a shows the formation of a convex meniscus and two competing pressures. According to a previous study [37], the critical pressure, a maximum sustainable pressure of the Cassie state, can be given by
Pc ¼
cf cos hwater ð1 f ÞðA=LÞ
ð7Þ
Fig. 5. Schematics showing the shape of a local meniscus for (a) hydrophobic PDMS and (b) hydrophilic NOA 73 materials and the interplay between internal and Laplace pressures.
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5.1 102 Pa for an s/d of 7, suggesting that the collapse of a droplet might start to proceed for an s/d of about 7. This is in excellent agreement with our experimental results shown in Fig. 3a. Variations with different h/d values are not significant provided that s/ d is larger than 2. Next, h/d needs to be larger than 2 to avoid direct contact of a local meniscus to the bottom of the substrate. Since the curvature of the droplet is constant, the droplet between two neighboring pillars can be drooped where the maximum drooping depth (d) between neighboring pillars is given by [21,38]
d
pffiffiffi ð 2ðs þ dÞ dÞ2 R
ð10Þ
where R is the radius of a droplet. Thus, if d is larger than the height of pillars, the Cassie to Wenzel transition would take place. According to Eq. (10), when the s/d ratio is 8, d is about 3.5 lm, suggesting that the local meniscus forms far below the edge of pillars. By considering the fact that the fabricated pillars have a slightly rounded corner, the meniscus can further penetrate into the pillars, which in turn increases the probabilities of touching the bottom surface [39]. Moreover, d can be increased further by external perturbations such as vibration or mechanical loading. Therefore, the contribution of Wenzel state can increase significantly with increasing s/d, resulting in a rapid decrease of CA as shown in Fig. 3. For h/d > 2, the local meniscus forms near the edge of pillars and the trend of wetting transition becomes similar with varying s/d values. For the hydrophilic NOA material, the shape of a local meniscus is concave as shown in Fig. 5b. In this case, the direction of the internal and Laplace pressures is the same (acting downwards), so that a droplet is expected to penetrate into the pillars immediately after contact. Interestingly, the droplet still can float on such a structured surface presumably due to the three-dimensional balance of the resultant Laplace pressure, which must be uniform under static conditions all along the interior of the droplet. More specifically, the pillars are creating variable saddle-points that provide both negative and positive curvature contours, which would give rise to the levitation of a droplet. As with hydrophobic materials, the only way of seeing the bottom surface is the direct contact of a local meniscus for a relatively large value of s/d, which indeed occurs for s/d > 5 (h/d > 2) based on our experimental observations. Although not shown, the droplet quickly penetrates into the pillars made of silicon (hwater 20°) after contact for the entire pillar structures tested, revealing that the initial CA plays an important role in the wetting transition, in particular for a hydrophilic material. A further study would be required to address this issue in more detail.
after transition. To elaborate on this wetting transition, the area fraction of Wenzel state f (%) was calculated for each pattern and displayed in Fig. 6. The upper red (f = 0.9) and lower blue lines (f = 0.1) represent the boundary between the Wenzel and the mixed wetting states and the Cassie and the mixed wetting states, respectively. As can be seen from the plots, there seem three regimes for wetting states that can be classified into three groups: group 1 (Wenzel), 2 (Mixed), and 3 (Cassie). In group I, the Wenzel fraction is larger than 90%, which means the droplet nearly wets the surface with little trapped air. In group 2, a droplet exists in a mixed wetting state of Cassie and Wenzel. The Wenzel fraction was different for different materials used in the experiment. For the hydrophobic PDMS, the fraction was populated in a narrow window, ranging from 12.97% to 19.92%, without a notable change with varying s/d ratio. It is hypothesized that the hydrophobic nature of PDMS inhibits the complete filling of the droplet into the cavities even after the impregnation. Only high aspect ratio, dense patterns can ensure the Cassie state (group 3) as seen from Fig. 5a. For the hydrophilic NOA, on the other hand, there is essentially no intermediate, mixed regime for wetting (group 2), such that most data were populated in the Wenzel (group 1) and Cassie (group 3) regimes. It appears that the droplet can spontaneously fill the cavities once the impregnation initiates because of the hydrophilic nature of NOA. To visualize the mixed wetting state, optical images of a droplet were examined under wetting transition as shown in Fig. 7. Once the transition occurs, the portion of the bright center becomes larger with a thinning of the dark, edge region, which can be used as an indicator of the transition. Also, some trapped air was observed for the mixed wetting state, suggesting that the surface might be comprised of binary regions of Wenzel and Cassie states (Fig. 7c). In conjunction with these findings, several researchers have recently suggested a ‘co-existing’ state as a combination of Cassie and Wenzel states. Dorrer et al. presented evidence of the possibil-
3.4. Analysis of a mixed wetting state It is known that a water droplet prefers to exist in the Wenzel state on most artificially fabricated surfaces to lower surface free energy. Only for high aspect ratio, dense patterns, the Cassie state becomes thermodynamically more stable [24]. To date, researchers have attempted to explain the wetting transition from Cassie to Wenzel mathematically based on free energy calculations [24,40,41]. It has been found that there is an energy barrier between Cassie–Baxter and Wenzel regimes, so that a water droplet could exist in a mixed wetting state that does not belong to the Cassie or to the Wenzel regimes. Once an additional stimulus is applied, a complete transition can take place. Our experimental observations generally agree with the aforementioned wetting transition. The measured CAs were higher than those in the Wenzel state by < 20° for the hydrophobic PDMS material, and higher by < 35° for the hydrophilic NOA material
Fig. 6. Plots of the Wenzel fraction f (%) as a function of the spacing ratio for two materials. As can be seen from the figure, the data can be categorized into three groups, i.e., group 1: Wenzel, group 2: Mixed, and group 3: Cassie.
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In summary, our study demonstrated that the wetting transition is highly related to structural parameters of a microstructured surface along with materials properties such as hydrophobicity. The present work can offer a guideline to attain superhydrophobicity or tailored wetting properties when a hydrophilic or a hydrophobic material needs to be used as a structural component. Acknowledgments This work was supported by a Grant-in-Aid for Strategy Technology Development Programs from the Korea Ministry of Knowledge Economy (No. 10030046). This work was also supported in part by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (Grant KRF-J03003), the Micro Thermal System Research Center, and the Engineering Research Institute. References
Fig. 7. Optical micrograph images of a droplet on the micropillars, showing three different wetting states: (a) Cassie, (b) Wenzel, and (c) Mixed states. Once the wetting transition occurs, the center portion (bright region) becomes larger with a thinning of the dark, edge region as shown in (b). For the mixed wetting state, there is partially trapped air (Cassie state) as shown in (c). Arrows indicate the trapped air.
ity of ‘co-existing’ state in a droplet by condensation [42]. Also, Zheng et al. suggested a ‘mixed’ state based on their sink depth model [37] and the experimental data from Yoshimitsu et al. [22]. Although a further study would be required to elucidate the underlying mechanism of wetting transition, our results demonstrate that the transition is highly associated with structural parameters of a surface as well as material properties such as hydrophobicity. 4. Conclusions In this work, we have presented the wetting transition of a water droplet on microfabricated polymer surfaces with different hydrophobicity by employing replica molding and self-replication methods. It was found that the trend of wetting transition was similar for hydrophobic PDMS (hwater 110°) and hydrophilic NOA (hwater 70°) materials except for a different spacing ratio for the collapse of a droplet. For the PDMS material, the Cassie state was maintained over a wide range of s/d up to 7 for a given h/d of 5. For the NOA material, the droplet impregnated when s/d was larger than 6 for the same h/d. Based on the measurements of static CA and CAH, we have derived optimal design parameters for both hydrophobic and hydrophilic materials to achieve robust hydrophobicity with a larger static CA and a lower CAH. In particular, specifically, the design criteria for stable superhydrophobic properties in the Cassie regime need to be adjusted for materials with different hydrophobicity. For example, a spacing ratio of 6–7 was desirable for a large height of microstructure (h/d = 5) for the PDMS, for which a microstructured surface exhibited a static CA as high as 154.5° and a hysteresis as low as 4.2°. For the hydrophilic NOA, a spacing ratio of 5–6 was appropriate for apparent robust hydrophobicity, for which a microstructured surface showed a static CA as high as 155.2° and a hysteresis as low as 10.2°. These findings suggest that even a hydrophilic material could provide a stable Cassie state under a limited range of s/d.
[1] I. Veretennikov, A. Agarwal, A. Indeikina, H.C. Chang, J. Colloid Interface Sci. 215 (2) (1999) 425–440. [2] R.A. Singh, H.J. Kim, J. Kim, S. Yang, H.E. Jeong, K.Y. Suh, E.S. Yoon, J. Mech. Sci. Technol. 21 (4) (2007) 624–629. [3] Z. Burton, B. Bhushan, Nano Lett. 5 (8) (2005) 1607–1613. [4] S.K. Cho, H.J. Moon, C.J. Kim, J. Microelectromech. Syst. 12 (1) (2003) 70– 80. [5] J. Lee, H. Moon, J. Fowler, T. Schoellhammer, C.J. Kim, Sens. Actuators A 95 (2– 3) (2002) 259–268. [6] H. Ren, R.B. Fair, M.G. Pollack, Sens. Actuators B 98 (2–3) (2004) 319–327. [7] A.A. Darhuber, S.M. Troian, W.W. Reisner, Phys. Rev. E 6403 (3) (2001). art. no. 031603. [8] L. Ionov, N. Houbenov, A. Sidorenko, M. Stamm, S. Minko, Adv. Funct. Mater. 16 (9) (2006) 1153–1160. [9] H.E. Jeong, P. Kim, M.K. Kwak, C.H. Seo, K.Y. Suh, Small 3 (5) (2007) 778–782. [10] P. Kim, S. Baik, K.Y. Suh, Small 4 (1) (2008) 92–95. [11] C.P. Ody, C.N. Baroud, E. de Langre, J. Colloid Interface Sci. 308 (1) (2007) 231– 238. [12] R.N. Wenzel, Ind. Eng. Chem. 28 (8) (1936) 988–994. [13] A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 546–551. [14] W. Barthlott, C. Neinhuis, Planta 202 (1) (1997) 1–8. [15] L. Zhai, M.C. Berg, F.C. Cebeci, Y. Kim, J.M. Milwid, M.F. Rubner, R.E. Cohen, Nano Lett. 6 (6) (2006) 1213–1217. [16] Z.G. Guo, W.M. Liu, Appl. Phys. Lett. 90 (22) (2007) 193108. [17] P. Roach, N.J. Shirtcliffe, M.I. Newton, Soft Matter 4 (2) (2008) 224–240. [18] D. Kim, J. Kim, H.C. Park, K.H. Lee, W. Hwang, J. Micromech. Microeng. 18 (1) (2008) 015019. [19] X.J. Feng, L. Jiang, Adv. Mater. 18 (23) (2006) 3063–3078. [20] C.W. Extrand, Langmuir 19 (3) (2003) 646–649. [21] Y.C. Jung, B. Bhushan, Scripta Mater. 57 (12) (2007) 1057–1060. [22] Z. Yoshimitsu, A. Nakajima, T. Watanabe, K. Hashimoto, Langmuir 18 (15) (2002) 5818–5822. [23] A. Marmur, Langmuir 19 (20) (2003) 8343–8348. [24] N.A. Patankar, Langmuir 19 (4) (2003) 1249–1253. [25] D. Quere, Nat. Mater. 1 (1) (2002) 14–15. [26] E. Bormashenko, R. Pogreb, G. Whyman, M. Erlich, Langmuir 23 (12) (2007) 6501–6503. [27] A. Lafuma, D. Quere, Nat. Mater. 2 (7) (2003) 457–460. [28] M. Nosonovsky, Langmuir 23 (6) (2007) 3157–3161. [29] M.A. Rodriguez-Valverde, J. Colloid Interface Sci. 327 (2) (2008) 477–479. [30] E. Bormashenko, Y. Bormashenko, T. Stein, G. Whyman, E. Bormashenko, J. Colloid Interface Sci. 311 (1) (2007) 212–216. [31] K.M. Hay, M.I. Dragila, J. Liburdy, J. Colloid Interface Sci. 325 (2) (2008) 472– 477. [32] M. Iwamatsu, J. Colloid Interface Sci. 297 (2) (2006) 772–777. [33] K. Yamamoto, S. Ogata, J. Colloid Interface Sci. 326 (2) (2008) 471–477. [34] S.J. Choi, P.J. Yoo, S.J. Baek, T.W. Kim, H.H. Lee, J. Am. Chem. Soc. 126 (25) (2004) 7744–7745. [35] B. Aksak, M.P. Murphy, M. Sitti, Langmuir 23 (6) (2007) 3322–3332. [36] J. Bico, U. Thiele, D. Quere, Colloids Surf. A Physicochem. Eng. Aspects 206 (1– 3) (2002) 41–46. [37] Q.S. Zheng, Y. Yu, Z.H. Zhao, Langmuir 21 (26) (2005) 12207–12212. [38] M. Reyssat, J.M. Yeomans, D. Quere, Europhys. Lett. 81 (2) (2008) 26006. [39] Y. Xiu, L. Zhu, D.W. Hess, C.P. Wong, Nano Lett. 7 (11) (2007) 3388–3393. [40] C.W. Extrand, Langmuir 18 (21) (2002) 7991–7999. [41] A. Marmur, Langmuir 20 (9) (2004) 3517–3519. [42] C. Dorrer, J. Ruhe, Langmuir 23 (7) (2007) 3820–3824.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Wetting transition and optimal design for microstructured surfaces with hydrophobic and hydrophilic materials Chan Ick Park a, Hoon Eui Jeong a, Sung Hoon Lee a, Hye Sung Cho a, Kahp Y. Suh a,b,* a b
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Korea Institute of Advanced Machinery and Design, Seoul National University, Seoul 151-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 January 2009 Accepted 1 April 2009 Available online 15 April 2009 Keywords: Wetting transition Microstructure Soft lithography Replica molding Capillary force Polymer
a b s t r a c t We present wetting transition of a water droplet on microstructured polymer surfaces using materials with different hydrophilicity or hydrophobicity: hydrophobic polydimethyl siloxane (PDMS) (hwater 110°) and hydrophilic Norland Optical Adhesive (NOA) (hwater 70°). The microstructures were fabricated by replica molding and self-replication with varying pillar geometry [diameter: 5 lm, spacingto-diameter ratio (s/d): 1–10 (equal interval), height-to-diameter ratio (h/d): 1–5] over an area of 100 mm2 (10 mm 10 mm). Measurements of contact angle (CA) and contact angle hysteresis (CAH) demonstrated that wetting state was either in the homogeneous Cassie regime or in the mixed regime of Cassie and Wenzel states depending on the values of s/d and h/d. These two ratios need to be adjusted to maintain stable superhydrophobic properties in the Cassie regime; s/d should be smaller than 7 (PDMS) and 6 (NOA) with h/d being larger than 2 to avoid wetting transition by collapse of a water droplet into the microstructure. Based on our observations, optimal design parameters were derived to achieve robust hydrophobicity of a microstructured surface with hydrophobic and hydrophilic materials. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Wettability is one of the most important surface properties in a variety of fields such in film coating [1], tribology [2,3], MEMS/ BioMEMS [4–6], and micro/nanofluidics [7–11]. Accordingly, wettability has been studied for many decades after Wenzel [12] and Cassie–Baxter [13] suggested a model to explain wetting behavior on a structured surface. As a consequence, it is now well understood that surface geometry and chemical distribution of materials govern the wettability of a solid surface as described by Cassie– Baxter and Wenzel equations [12,13]. In particular, since the observation of excellent superhydrophobic properties of a lotus leaf in 1997 [14], extensive efforts have been made to fabricate nature-inspired superhydrophobic surfaces based on various self-assembly and lithographic methods [15–19] or to examine the wetting kinetics and transition on structured surfaces [19–33]. In this work, we present wetting transition on microstructured polymer surfaces using materials of different hydrophobicity and optimal design parameters for robust hydrophobicity. To fabricate a microstructured surface, a simple replica molding method aided by self-replication was used with UV-curable materials. Compared to previous self-assembly or self-organization methods, this soft lithographic method provides a simple route to modifying the * Corresponding author. Address: School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Korea. Fax: +82 2 883 1597. E-mail address: [email protected] (K.Y. Suh). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.04.022
topography of a surface with designing capability in a geometry controllable manner. Furthermore, self-replication is an important characteristic of the current method, which is particularly useful for generating positive or negative features without the use of an original silicon master [34]. Using the self-replication characteristic, one can duplicate various microstructured surfaces of hydrophobic and hydrophilic materials with identical geometry. By exploiting the above-mentioned advantages, we created geometry-tunable, microstructured surfaces using hydrophobic PDMS (hwater 110°) and hydrophilic NOA materials (hwater 70°) and examined their wetting properties. Although there have been a large number of studies dealing with wetting behavior on a microstructured surface, the role of hydrophilicity or hydrophobicity of a material is not well understood in designing geometrical parameters for tailored wetting properties. Aiming at this purpose, we tested various pillar geometries by changing the space to diameter (s/d) (from 1 to 10, equal interval) and height-to-diameter (from 1 to 5) ratios for a given pillar diameter (5 lm), yielding a total number of combinations as high as 100 (50 each for each material).
2. Materials and methods 2.1. Fabrication of microstructured polymer surfaces The negative silicon masters (denoted as ‘’) were prepared by conventional photolithography, which contained an array of 5 lm circular holes with different density over an area of 100 mm2
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(10 mm 10 mm). Each wafer had 10 different spacing ratios (space/diameter) and five different height ratios (height/diameter). Detailed geometrical parameters are summarized in Table 1. To prepare microstructured polymer surfaces from the above silicon masters, replica molding and self-replication were used as shown in Fig. 1. First, positive (denoted as ‘+’) PDMS (Sylgard, 184, Dow Corning) replicas were obtained by casting PDMS precursor against a silicon master with 30% curing agent to prevent ‘self-matting problem’ [35] in high aspect ratio (AR > 3) structures. Owing to a higher content of the curing agent, the PDMS replica became more rigid and hydrophobic (hwater 110°). No cracks or wrinkles were observed in the highly cured PDMS replica. To utilize self-replication characteristic, a polyurethane acrylate (PUA, 311RM, Minuta Tech.) liquid precursor was drop-dispensed onto the as-prepared PDMS replica and then a 50 lm transparent polyethylene terephthalate (PET) film was gently placed on the liquid followed by UV (k = 250–400 nm) exposure for 40 s. To expel trapped air, a roller was mildly rounded several times on top of the flat PET film. After the UV curing, the PUA replica was peeled off from the PDMS replica using a sharp tweezer [34], with the same geometry as the original silicon master. Finally, a replica of Norland Optical Adhesive (NOA) 73 (Norland Products Inc.) was prepared from the PUA replica after completely destroying reactive double bonds of the PUA via an additional curing for 12 h. The NOA replica was prepared by the replica molding after curing for 90 s and peeling-off. 2.2. Measurements of contact angle (CA) and contact angle hysteresis (CAH) The static contact angles (CAs) of water were measured with a contact angle analyzer (Drop Shape Analysis System DSA100, Kruss, Germany) by gently placing a water droplet (6 ll) on a microstructured surface over the time span of 1.5 min for each pattern. The presented values were averaged over at least six different locations in a sample. 2.3. Scanning Electron Microscopy (SEM) SEM images were taken using SEM (S-4800, Hitachi, Japan) at an acceleration voltage of 15.0 kV and an average working distance of 11.4 mm. Samples were coated with a 30 nm Pt layer prior to analysis. 3. Results and discussion 3.1. Fabrication of microstructured polymer surfaces Fig. 2 shows representative SEM images of microstructured PDMS and NOA surfaces fabricated by replica molding and self-
Fig. 1. Schematic diagram of the fabrication procedure with two different polymer materials: (left side) simple replication of microstructures with hydrophobic PDMS, (right side) sequential replication of the same microstructures with hydrophilic NOA 73 by using PUA material as an intermediate self-replica.
replication. As shown in the figure, the structures exhibited high structural fidelity with well-defined vertical edge profiles. No structural differences between the PDMS and NOA replicas were observed. For generating uniform microstructured surfaces, care should be taken during the 2nd replication step so as to preserve an original height of the silicon master. In the case of fabricating a PDMS replica (1st replication step), air can easily permeate out of the cured PDMS during thermal crosslinking, so that an exact replica can be readily obtained without air trap. In the case of fabricating a PUA replica, however, air may be trapped in the spaces between microstructures, resulting in a reduced height or a nonuniform distribution of height in the microstructure. To alleviate this problem, a roller was mildly rounded several times on top of a flat PET film to expel trapped air prior to UV exposure. Also, the curing time needs to be controlled for maintaining the PUA surface inactive to the NOA material, recognizing that the same double bonds are present in the backbone of both materials. It
Table 1 Geometrical parameters of pillar pattern used in the experiments (d: diameter, s: spacing, h: height). The pillar diameter is fixed at 5 lm for all patterns (unit: lm). h
s 5
10
15
20
25
30
35
40
45
50
5
h/d = 1 s/d = 1
h/d = 1 s/d = 2
h/d = 1 s/d = 3
h/d = 1 s/d = 4
h/d = 1 s/d = 5
h/d = 1 s/d = 6
h/d = 1 s/d = 7
h/d = 1 s/d = 8
h/d = 1 s/d = 9
h/d = 1 s/d = 10
10
h/d = 2 s/d = 1
h/d = 2 s/d = 2
h/d = 2 s/d = 3
h/d = 2 s/d = 4
h/d = 2 s/d = 5
h/d = 2 s/d = 6
h/d = 2 s/d = 7
h/d = 2 s/d = 8
h/d = 2 s/d = 9
h/d = 2 s/d = 10
15
h/d = 3 s/d = 1
h/d = 3 s/d = 2
h/d = 3 s/d = 3
h/d = 3 s/d = 4
h/d = 3 s/d = 5
h/d = 3 s/d = 6
h/d = 3 s/d = 7
h/d = 3 s/d = 8
h/d = 3 s/d = 9
h/d = 3 s/d = 10
20
h/d = 4 s/d = 1
h/d = 4 s/d = 2
h/d = 4 s/d = 3
h/d = 4 s/d = 4
h/d = 4 s/d = 5
h/d = 4 s/d = 6
h/d = 4 s/d = 7
h/d = 4 s/d = 8
h/d = 4 s/d = 9
h/d = 4 s/d = 10
25
h/d = 5 s/d = 1
h/d = 5 s/d = 2
h/d = 5 s/d = 3
h/d = 5 s/d = 4
h/d = 5 s/d = 5
h/d = 5 s/d = 6
h/d = 5 s/d = 7
h/d = 5 s/d = 8
h/d = 5 s/d = 9
h/d = 5 s/d = 10
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Fig. 2. SEM images of replicated PDMS microstructures from negative patterned silicon masters (a–e) and replicated NOA 73 microstructures (k–o) from self-replicated PUA masters (f–j). The geometrical parameters are: (a, f, and k) 5 lm height pillars with a spacing of 50 lm, (b, g, and l) 15 lm height pillars with a spacing of 50 lm, (c, h, and m) 25 lm height pillars with a spacing of 25 lm, (d, i, and n) 25 lm height pillars with a spacing of 25 lm and (e, j, and o) 25 lm height pillars with a spacing of 5 lm. For each structure, the diameter is 5 lm. Note that no differences in the replicated structures were observed between PDMS and NOA materials. Scale bars indicate 100 lm.
turned out that an overnight curing step (>12 h) was a prerequisite to ensure a sufficiently inert, non-reactive PUA surface with clean release of a NOA replica. 3.2. Wetting transition and design criteria with hydrophobic and hydrophilic materials A water droplet ‘reacts’ to a physical change of the surface in the presence of microstructure, either in the form of homogenous or heterogeneous wetting. According to Wenzel model [12], the apparent CA of a water droplet varies proportionally with the roughness factor, which gives
cos hw ¼ r cos hwater
ð1Þ
where hw is the apparent CA on a rough surface determined by Wenzel state, hwater is the equilibrium CA on a flat surface and r is the roughness factor that is defined as the ratio of the actual area of a rough surface to that of the projected area. Since the roughness factor is always larger than unity, surface properties are exaggerated when a water droplet contacts the surface in Wenzel state. On the other hand, Cassie–Baxter proposed the heterogeneous contact of a water droplet on a composite surface that is composed of solid and air, which gives [13]
cos hc ¼ f1 cos h1 þ f2 cos h2 þ
ð2Þ
where hc is the apparent CA on a composite surface determined by Cassie–Baxter state, hn is the equilibrium CA on each flat surface, and f is the area fraction of each material. In a binary surface consisting of solid and air, Eq. (2) is reduced to
cos hc ¼ f cos hwater þ ð1 f Þ cos hair ¼ f ðcos hwater þ 1Þ 1
ð3Þ
Fig. 3. Comparison of the measured CAs on microstructured PDMS and NOA surfaces with the theoretical predictions based on Cassie–Baxter and Wenzel models. Inset pictures show the images of water droplets before and after transition.
where hwater is the equilibrium CA on a flat surface and f is the solid surface (1 f: area fraction). Therefore, a water droplet is supported by hydrophobic air cushion that is present in the spaces between microstructures in Cassie–Baxter state and thus the static CA increases with increasing spacing s/d.
f ¼
Fig. 3 shows theoretical and experimental CAs of water on microstructured surfaces made of (a) hydrophobic (PDMS) and (b) hydrophilic (NOA 73) materials. Theoretical values were calculated by Wenzel and Cassie–Baxter equations using the following parameters:
where d is the diameter, s is the spacing and h is the height of pillars, respectively. Experimental results were obtained from averaged CAs measured over at least six different locations per each pattern with an average deviation of ±3°.
r ¼1þ
pdh ðd þ sÞ2
pd2 2
ðd þ sÞ
¼
¼1þ
p ðh=dÞ ð1 þ s=dÞ2
p ð1 þ s=dÞ2
ð4Þ ð5Þ
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Several notable findings are derived from Fig. 3. First, water drops prefer to exist in the Cassie–Baxter regime for a high aspect ratio, dense pattern because the surface free energy is known to be lower than that in the Wenzel regime [24]. When the s/d ratio increased, the water droplet started to collapse into the cavity, representing the transition of wetting state from Cassie to Wenzel. This transition occurs easily for a small height ratio (h/d) pattern, suggesting that the transition barrier gets lowered for a lower height. From a thermodynamic standpoint, the water droplet prefers wetting state with a lower free energy. Thus, the stability of Cassie state can be described by the threshold contact angle (h*), which is given by [36]
cos h ¼
f 1 rf
ð6Þ
If the CA of a flat surface h is larger than h*, the water droplet prefer to stay in the Cassie regime. For h > h*, on the other hand, the water droplet prefers to exist in the Wenzel regime. Second, the trend of wetting transition was similar for PDMS and NOA materials except for different critical values of s/d that are required for the collapse of a droplet. For the hydrophobic PDMS material, the Cassie state was maintained over a wide range of s/d values up to 7 for a given h/d of 5 (See Fig. 3a). For the hydrophilic NOA material, the droplet started to fall when s/d was larger than 6 for the same h/d. Thus, the design criteria for stable superhydrophobic properties in the Cassie regime need to be adjusted for materials with different hydrophobicity. For the hydrophobic PDMS, a spacing ratio of 6–7 is desirable for a large height of microstructure (h/d = 5), which would give the maximum contact angle as well as the smallest hysteresis for the conditions used in our study. Experimentally, such a microstructured surface exhibited a static CA as high as 154.5° and a hysteresis as low as 4.2°. For the hydrophilic NOA, a spacing ratio of 5–6 seems appropriate to maintain superior wetting properties even with a hydrophilic material. Such a microstructured surface showed a static CA as high as 155.2° and a hysteresis as low as 10.2°, suggesting that even a hydrophilic material could provide a stable Cassie state under a limited range of s/d. Fig. 4 shows snap-shot images of measuring hysteresis on these two different surfaces. Considering difficulty in preparing a high aspect ratio silicon master, a microstructured PDMS surface with an s/d ratio of 4–5 and an h/d ratio of 2–3 could be optimal to achieve robust hydrophobic properties. It is noted that for all the geometrical parameters tested for both materials an h/d ratio of 1 appears too small to attain a stable heterogeneous wetting state as seen from Fig. 3. It seems that the local meniscus of a droplet directly touch the bottom of the substrate, resulting in a rapid decrease of static CA with increasing s/d. 3.3. Theoretical description of wetting transition
Fig. 4. Advancing and receding CAs (dynamic CA) for the proposed optimal design parameter for each material. (a) Static CA on PDMS surface is as high as 154.5° with hysteresis as low as 4.2°. (b) Static CA on NOA 73 surface is as high as 155.2° with hysteresis as low as 10.2°.
where c is the liquid–air surface tension (72 mJ/m2), and A and L represent the cross-sectional area and perimeter of pillar, respectively. For the conditions used in our experiment, the calculated critical pressure is about 5 102 Pa. On the other hand, the internal pressure that is originated from gravity can be calculated by
Pi ¼
V qg V qg ¼ Acontact pðRðhÞÞ2 "
RðhÞ ¼
ð8Þ 6V
pð1 cos hÞð3 sin2 h þ ð1 cos hÞ2 Þ
#13 ð9Þ
where V is the volume of a droplet, q is the density of water, g is the gravitational constant, h is the apparent CA of a droplet and R(h) is the radius of a droplet in contact with the surface, respectively. The calculated internal pressure is 6.4 102 Pa. For a dense pattern with a smaller s/d, the critical pressure is larger than the internal pressure, which means that the wetting state can be maintained in the Cassie regime. As s/d increases, however, the critical pressure decreases with a comparable value to the internal pressure. For instance, the critical pressure is 6.7 102 Pa for an s/d of 6 and
Although the trend of wetting transition is similar for both materials, the underlying mechanism would be different. Here, we explain the role of two geometrical parameters (i.e., s/d and h/d) in the wetting transition in terms of Laplace and hydrostatic pressures. For the hydrophobic PDMS material, the shape of a local meniscus is convex, such that the Laplace pressure competes with the internal pressure by gravity. Fig. 5a shows the formation of a convex meniscus and two competing pressures. According to a previous study [37], the critical pressure, a maximum sustainable pressure of the Cassie state, can be given by
Pc ¼
cf cos hwater ð1 f ÞðA=LÞ
ð7Þ
Fig. 5. Schematics showing the shape of a local meniscus for (a) hydrophobic PDMS and (b) hydrophilic NOA 73 materials and the interplay between internal and Laplace pressures.
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5.1 102 Pa for an s/d of 7, suggesting that the collapse of a droplet might start to proceed for an s/d of about 7. This is in excellent agreement with our experimental results shown in Fig. 3a. Variations with different h/d values are not significant provided that s/ d is larger than 2. Next, h/d needs to be larger than 2 to avoid direct contact of a local meniscus to the bottom of the substrate. Since the curvature of the droplet is constant, the droplet between two neighboring pillars can be drooped where the maximum drooping depth (d) between neighboring pillars is given by [21,38]
d
pffiffiffi ð 2ðs þ dÞ dÞ2 R
ð10Þ
where R is the radius of a droplet. Thus, if d is larger than the height of pillars, the Cassie to Wenzel transition would take place. According to Eq. (10), when the s/d ratio is 8, d is about 3.5 lm, suggesting that the local meniscus forms far below the edge of pillars. By considering the fact that the fabricated pillars have a slightly rounded corner, the meniscus can further penetrate into the pillars, which in turn increases the probabilities of touching the bottom surface [39]. Moreover, d can be increased further by external perturbations such as vibration or mechanical loading. Therefore, the contribution of Wenzel state can increase significantly with increasing s/d, resulting in a rapid decrease of CA as shown in Fig. 3. For h/d > 2, the local meniscus forms near the edge of pillars and the trend of wetting transition becomes similar with varying s/d values. For the hydrophilic NOA material, the shape of a local meniscus is concave as shown in Fig. 5b. In this case, the direction of the internal and Laplace pressures is the same (acting downwards), so that a droplet is expected to penetrate into the pillars immediately after contact. Interestingly, the droplet still can float on such a structured surface presumably due to the three-dimensional balance of the resultant Laplace pressure, which must be uniform under static conditions all along the interior of the droplet. More specifically, the pillars are creating variable saddle-points that provide both negative and positive curvature contours, which would give rise to the levitation of a droplet. As with hydrophobic materials, the only way of seeing the bottom surface is the direct contact of a local meniscus for a relatively large value of s/d, which indeed occurs for s/d > 5 (h/d > 2) based on our experimental observations. Although not shown, the droplet quickly penetrates into the pillars made of silicon (hwater 20°) after contact for the entire pillar structures tested, revealing that the initial CA plays an important role in the wetting transition, in particular for a hydrophilic material. A further study would be required to address this issue in more detail.
after transition. To elaborate on this wetting transition, the area fraction of Wenzel state f (%) was calculated for each pattern and displayed in Fig. 6. The upper red (f = 0.9) and lower blue lines (f = 0.1) represent the boundary between the Wenzel and the mixed wetting states and the Cassie and the mixed wetting states, respectively. As can be seen from the plots, there seem three regimes for wetting states that can be classified into three groups: group 1 (Wenzel), 2 (Mixed), and 3 (Cassie). In group I, the Wenzel fraction is larger than 90%, which means the droplet nearly wets the surface with little trapped air. In group 2, a droplet exists in a mixed wetting state of Cassie and Wenzel. The Wenzel fraction was different for different materials used in the experiment. For the hydrophobic PDMS, the fraction was populated in a narrow window, ranging from 12.97% to 19.92%, without a notable change with varying s/d ratio. It is hypothesized that the hydrophobic nature of PDMS inhibits the complete filling of the droplet into the cavities even after the impregnation. Only high aspect ratio, dense patterns can ensure the Cassie state (group 3) as seen from Fig. 5a. For the hydrophilic NOA, on the other hand, there is essentially no intermediate, mixed regime for wetting (group 2), such that most data were populated in the Wenzel (group 1) and Cassie (group 3) regimes. It appears that the droplet can spontaneously fill the cavities once the impregnation initiates because of the hydrophilic nature of NOA. To visualize the mixed wetting state, optical images of a droplet were examined under wetting transition as shown in Fig. 7. Once the transition occurs, the portion of the bright center becomes larger with a thinning of the dark, edge region, which can be used as an indicator of the transition. Also, some trapped air was observed for the mixed wetting state, suggesting that the surface might be comprised of binary regions of Wenzel and Cassie states (Fig. 7c). In conjunction with these findings, several researchers have recently suggested a ‘co-existing’ state as a combination of Cassie and Wenzel states. Dorrer et al. presented evidence of the possibil-
3.4. Analysis of a mixed wetting state It is known that a water droplet prefers to exist in the Wenzel state on most artificially fabricated surfaces to lower surface free energy. Only for high aspect ratio, dense patterns, the Cassie state becomes thermodynamically more stable [24]. To date, researchers have attempted to explain the wetting transition from Cassie to Wenzel mathematically based on free energy calculations [24,40,41]. It has been found that there is an energy barrier between Cassie–Baxter and Wenzel regimes, so that a water droplet could exist in a mixed wetting state that does not belong to the Cassie or to the Wenzel regimes. Once an additional stimulus is applied, a complete transition can take place. Our experimental observations generally agree with the aforementioned wetting transition. The measured CAs were higher than those in the Wenzel state by < 20° for the hydrophobic PDMS material, and higher by < 35° for the hydrophilic NOA material
Fig. 6. Plots of the Wenzel fraction f (%) as a function of the spacing ratio for two materials. As can be seen from the figure, the data can be categorized into three groups, i.e., group 1: Wenzel, group 2: Mixed, and group 3: Cassie.
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In summary, our study demonstrated that the wetting transition is highly related to structural parameters of a microstructured surface along with materials properties such as hydrophobicity. The present work can offer a guideline to attain superhydrophobicity or tailored wetting properties when a hydrophilic or a hydrophobic material needs to be used as a structural component. Acknowledgments This work was supported by a Grant-in-Aid for Strategy Technology Development Programs from the Korea Ministry of Knowledge Economy (No. 10030046). This work was also supported in part by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (Grant KRF-J03003), the Micro Thermal System Research Center, and the Engineering Research Institute. References
Fig. 7. Optical micrograph images of a droplet on the micropillars, showing three different wetting states: (a) Cassie, (b) Wenzel, and (c) Mixed states. Once the wetting transition occurs, the center portion (bright region) becomes larger with a thinning of the dark, edge region as shown in (b). For the mixed wetting state, there is partially trapped air (Cassie state) as shown in (c). Arrows indicate the trapped air.
ity of ‘co-existing’ state in a droplet by condensation [42]. Also, Zheng et al. suggested a ‘mixed’ state based on their sink depth model [37] and the experimental data from Yoshimitsu et al. [22]. Although a further study would be required to elucidate the underlying mechanism of wetting transition, our results demonstrate that the transition is highly associated with structural parameters of a surface as well as material properties such as hydrophobicity. 4. Conclusions In this work, we have presented the wetting transition of a water droplet on microfabricated polymer surfaces with different hydrophobicity by employing replica molding and self-replication methods. It was found that the trend of wetting transition was similar for hydrophobic PDMS (hwater 110°) and hydrophilic NOA (hwater 70°) materials except for a different spacing ratio for the collapse of a droplet. For the PDMS material, the Cassie state was maintained over a wide range of s/d up to 7 for a given h/d of 5. For the NOA material, the droplet impregnated when s/d was larger than 6 for the same h/d. Based on the measurements of static CA and CAH, we have derived optimal design parameters for both hydrophobic and hydrophilic materials to achieve robust hydrophobicity with a larger static CA and a lower CAH. In particular, specifically, the design criteria for stable superhydrophobic properties in the Cassie regime need to be adjusted for materials with different hydrophobicity. For example, a spacing ratio of 6–7 was desirable for a large height of microstructure (h/d = 5) for the PDMS, for which a microstructured surface exhibited a static CA as high as 154.5° and a hysteresis as low as 4.2°. For the hydrophilic NOA, a spacing ratio of 5–6 was appropriate for apparent robust hydrophobicity, for which a microstructured surface showed a static CA as high as 155.2° and a hysteresis as low as 10.2°. These findings suggest that even a hydrophilic material could provide a stable Cassie state under a limited range of s/d.
[1] I. Veretennikov, A. Agarwal, A. Indeikina, H.C. Chang, J. Colloid Interface Sci. 215 (2) (1999) 425–440. [2] R.A. Singh, H.J. Kim, J. Kim, S. Yang, H.E. Jeong, K.Y. Suh, E.S. Yoon, J. Mech. Sci. Technol. 21 (4) (2007) 624–629. [3] Z. Burton, B. Bhushan, Nano Lett. 5 (8) (2005) 1607–1613. [4] S.K. Cho, H.J. Moon, C.J. Kim, J. Microelectromech. Syst. 12 (1) (2003) 70– 80. [5] J. Lee, H. Moon, J. Fowler, T. Schoellhammer, C.J. Kim, Sens. Actuators A 95 (2– 3) (2002) 259–268. [6] H. Ren, R.B. Fair, M.G. Pollack, Sens. Actuators B 98 (2–3) (2004) 319–327. [7] A.A. Darhuber, S.M. Troian, W.W. Reisner, Phys. Rev. E 6403 (3) (2001). art. no. 031603. [8] L. Ionov, N. Houbenov, A. Sidorenko, M. Stamm, S. Minko, Adv. Funct. Mater. 16 (9) (2006) 1153–1160. [9] H.E. Jeong, P. Kim, M.K. Kwak, C.H. Seo, K.Y. Suh, Small 3 (5) (2007) 778–782. [10] P. Kim, S. Baik, K.Y. Suh, Small 4 (1) (2008) 92–95. [11] C.P. Ody, C.N. Baroud, E. de Langre, J. Colloid Interface Sci. 308 (1) (2007) 231– 238. [12] R.N. Wenzel, Ind. Eng. Chem. 28 (8) (1936) 988–994. [13] A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 546–551. [14] W. Barthlott, C. Neinhuis, Planta 202 (1) (1997) 1–8. [15] L. Zhai, M.C. Berg, F.C. Cebeci, Y. Kim, J.M. Milwid, M.F. Rubner, R.E. Cohen, Nano Lett. 6 (6) (2006) 1213–1217. [16] Z.G. Guo, W.M. Liu, Appl. Phys. Lett. 90 (22) (2007) 193108. [17] P. Roach, N.J. Shirtcliffe, M.I. Newton, Soft Matter 4 (2) (2008) 224–240. [18] D. Kim, J. Kim, H.C. Park, K.H. Lee, W. Hwang, J. Micromech. Microeng. 18 (1) (2008) 015019. [19] X.J. Feng, L. Jiang, Adv. Mater. 18 (23) (2006) 3063–3078. [20] C.W. Extrand, Langmuir 19 (3) (2003) 646–649. [21] Y.C. Jung, B. Bhushan, Scripta Mater. 57 (12) (2007) 1057–1060. [22] Z. Yoshimitsu, A. Nakajima, T. Watanabe, K. Hashimoto, Langmuir 18 (15) (2002) 5818–5822. [23] A. Marmur, Langmuir 19 (20) (2003) 8343–8348. [24] N.A. Patankar, Langmuir 19 (4) (2003) 1249–1253. [25] D. Quere, Nat. Mater. 1 (1) (2002) 14–15. [26] E. Bormashenko, R. Pogreb, G. Whyman, M. Erlich, Langmuir 23 (12) (2007) 6501–6503. [27] A. Lafuma, D. Quere, Nat. Mater. 2 (7) (2003) 457–460. [28] M. Nosonovsky, Langmuir 23 (6) (2007) 3157–3161. [29] M.A. Rodriguez-Valverde, J. Colloid Interface Sci. 327 (2) (2008) 477–479. [30] E. Bormashenko, Y. Bormashenko, T. Stein, G. Whyman, E. Bormashenko, J. Colloid Interface Sci. 311 (1) (2007) 212–216. [31] K.M. Hay, M.I. Dragila, J. Liburdy, J. Colloid Interface Sci. 325 (2) (2008) 472– 477. [32] M. Iwamatsu, J. Colloid Interface Sci. 297 (2) (2006) 772–777. [33] K. Yamamoto, S. Ogata, J. Colloid Interface Sci. 326 (2) (2008) 471–477. [34] S.J. Choi, P.J. Yoo, S.J. Baek, T.W. Kim, H.H. Lee, J. Am. Chem. Soc. 126 (25) (2004) 7744–7745. [35] B. Aksak, M.P. Murphy, M. Sitti, Langmuir 23 (6) (2007) 3322–3332. [36] J. Bico, U. Thiele, D. Quere, Colloids Surf. A Physicochem. Eng. Aspects 206 (1– 3) (2002) 41–46. [37] Q.S. Zheng, Y. Yu, Z.H. Zhao, Langmuir 21 (26) (2005) 12207–12212. [38] M. Reyssat, J.M. Yeomans, D. Quere, Europhys. Lett. 81 (2) (2008) 26006. [39] Y. Xiu, L. Zhu, D.W. Hess, C.P. Wong, Nano Lett. 7 (11) (2007) 3388–3393. [40] C.W. Extrand, Langmuir 18 (21) (2002) 7991–7999. [41] A. Marmur, Langmuir 20 (9) (2004) 3517–3519. [42] C. Dorrer, J. Ruhe, Langmuir 23 (7) (2007) 3820–3824.