Diffraction Patterns of a Water-Submerged Superhydrophobic Grating

Oct 7, 2009 - ... allowing not only the creation of a submerged configuration but also .... respectively, the refractive index (height) of PDMS pillar...
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Diffraction Patterns of a Water-Submerged Superhydrophobic Grating under Pressure Lei Lei,† Hao Li,†,‡ Jian Shi,† and Yong Chen*,†,‡,§ †

Ecole Normal Sup erieure, CNRS-ENS-UPMC UMR 8640, 24 rue Lhomond, 75231 Paris, France, ‡ Centre for Microfluidics and Nanotechnology, Peking University, 100871 Beijing, China, and § Institute for Integrated Cell-Material Science, Kyoto University, Kyoto 606-8507, Japan Received August 24, 2009. Revised Manuscript Received September 21, 2009

We report on a study of superhydrophobic surfaces submerged in water in a fluidic chamber. A surface-treated transmission grating was used as a superhydrophobic layer that had a well-defined diffraction pattern when a laser beam passed through the water-submerged grating sample, indicating a Cassie-Baxter state with trapped air between the water and grating interfaces. By appling pressure to the water in the fluidic chamber, the diffraction pattern can be changed because of the volume reduction of trapped air or water penetration into the grating. Depending on the maximum value of applied pressure in the fluidic chamber, the diffraction pattern change can be either reversible or irreversible after the release of the pressure. We attribute the irreversible change under high applied pressure to the switching from a Cassie-Baxter state to a Wenzel state.

Introduction The wettability of a solid surface can be modified by changing the roughness of the surface.1-3 By using a variety of nanofabrication methods such as photolithography, nanoimprint lithography, reactive ion etching, and soft lithography techniques,4-8 well-defined microstructure and nanostructure patterns can be defined, showing superhydrophobic wetting properties. Alternatively, many types of textured and water-repellent surfaces can also be obtained by advanced material processing.3 Whereas such a surface roughness has the same effect of lotus leaves on wettability,9 different surfaces can exhibit different flow resistances or drop rolling angles. More generally, on such surfaces falling water drops can roll at small tilt angles but condensed water drops “stick” despite apparently large contact angles. These phenomena can be well explained by the existence of two prominent states, called the Wenzel state and the Cassie-Baxter state.1 In the Wenzel state, the whole solid surface is wetted, whereas in the Cassie-Baxter state, water is likely suspended with trapped air inside the microstructure cavities underneath the water. Theoretical and experimental work has been done to understand their wetting properties and the transition between these two states,10-12 suggesting two energy minima separated by *Corresponding author. E-mail: [email protected].

(1) Quere, D. Ann. Rev. Mater. Res. 2008, 38, 71–99. (2) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14, 1857–1860. (3) Roach, P.; Shirtcliffe, N. J.; Newton, N. J. Soft Matter 2008, 4, 224–240. (4) Callies, M.; Chen, Y.; Marty, F.; Pepin, A.; Quere, D. Microelectron. Eng. 2005, 78-79, 100–105. (5) Reyssat, M.; Pepin, A.; Marty, F.; Chen, Y.; Quere, D. Europhys. Lett. 2006, 74, 306–312. (6) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097–2103. (7) Pozzato, A.; Zilio, S. D.; Fois, G.; Vendramin, D.; Mistura, G.; Belotti, M.; Chen, Y.; Natali, M. Microelectron. Eng. 2006, 83, 884–888. (8) Guo, S. S.; Sun, M. H.; Shi, J.; Liu, Y. J.; Huang, W. H.; Combellas, C.; Chen, Y. Microelectron. Eng. 2007, 84, 1673–1676. (9) Sun, M. H.; Luo, C. X.; Xu, L. P.; Chen, Y. Langmuir 2005, 21, 8978–8981. (10) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818–5822. (11) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999–5003. (12) Patankar, N. A. Langmuir 2004, 20, 7097–7102.

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an energy barrier. To move from one equilibrium state to another, the interface between the droplet and the material surface must overcome such an energy barrier.13 Previous investigations were mostly based on the observation of water drops placed on a surface, but much less work has been dedicated to such a study of totally water-submerged surfaces.14,15 Indeed, many applications of superhydrophobic coatings rely on the knowledge of wetting behavior in a water-submerged configuration. By using a laser beam and measuring its reflection at small angles, the surface air layers can be effectively evaluated in an open liquid container.15 In this work, we proposed to use a closed liquid chamber allowing not only the creation of a submerged configuration but also the application of external pressure on demand. Therefore, one can easily observe the dynamic change in the wetting states. In particular, we designed and fabricated the superhydrophobic surfaces in the form of transmission diffraction gratings so that the change in the wetting properties could be easily correlated to the change in the diffraction pattern as a function of applied pressure.

Materials and Methods Materials. A polydimethylsiloxane (PDMS) kit (GE RTV 615) was purchased from Eleco Produits (France). Negative photoresist SU-8 50 (Microchem) was from CTS (France). Silicon wafers were from Siltronix (France). All other chemicals were from Sigma (France). Ultrapure water (Millipore, 18.2 MΩ 3 cm at 25 °C) was used for all experiments. Grating and Device Fabrication. Soft lithography was used to fabricate PDMS transmission diffraction gratings. First, negative photoresist SU-8 50 was spin coated (2500 rpm, 60 s) onto a silicon wafer substrate and patterned with a mask of high-density micropillars. After the evaporation of trimethylchlorosilane (TMCS) on the surface of the SU 8 master, a prepolymer solution of PDMS in a 10:1 mixture ratio was poured on and cured at 80 °C (13) Zheng, Q. S.; Yu, Y.; Zhao, Z. H. Langmuir 2005, 21, 12207–12212. (14) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C.; Pyatt, F. B. Appl. Phys. Lett. 2006, 89, 104106. (15) Sakai, M.; Yanagisawa, T.; Nakajima, A.; Kameshima, Y.; Okada, K. Langmuir 2009, 25, 13–16.

Published on Web 10/07/2009

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Figure 1. (a) Microphotograph image of a PDMS transmission grating of a pseudotriangle lattice. (b) Photograph of a 3 μL water drop on the fabricated PDMS grating, showing a contact angle of about 150°. (c) Diffraction pattern observed with a green laser beam passing through the PDMS grating submerged in water in a fluidic chamber. Several diffraction orders can be clearly seen, but only two of them (0 and +1) are marked for more detailed analyses.

Results and Discussion

Figure 2. Schematic diagram of the experiment setup for the pressure-dependent observation of diffraction patterns of the water-submerged superhydrophobic grating. The fluidic chamber made of a grating substrate and a PDMS cover was fully filled with water. After blocking the outlet, hydraulic pressure can be applied through the inlet, allowing switching of the wetting configuration from the Cassie-Baxter and Wenzel states.

Figure 1a shows microphotograph (top view) of a fabricated PDMS grating of 10 μm diameter and 50 μm height pillars, forming a pseudotriangle lattice with lattice constants of 20 μm (a1), 22.4 μm (a2), and 22.4 μm (a3). Accordingly, we have a high enough surface roughness (r=4.92) for air trapping in the case of drop deposition. The photograph in Figure 1b was taken with a water drop of 3 μL placed on the surface, showing a contact angle of about 150°. In case of water submerging such a surface, air can still be trapped in the space between pillars so that they act as a regular diffraction grating. Figure 1c shows the observed diffraction pattern when water was injected and filled the fluidic chamber. Such a diffraction pattern cannot be observed when water fully penetrated the pillar gap area, considering the relatively small difference between the refractive index of water (1.33) and that of PDMS (1.41). Therefore, we can assume a CassieBaxter state in our water-submerged chamber with air trapping between the pillar areas. The diffraction grating is a well-understood optical component. For the sake of simplicity, we can consider a 1D transmission grating. Then, the diffraction pattern is determined by d sin θm ¼ mλ

for 1 h. Whereas the Young’s module of the fabricated PDMS layer depends on both the mixture ratio and the curing time, we chose the above ratio and curing conditions for easy device assembly and fluidic manipulation. After being peeled off, the PDMS slab was put in an oven at 80 °C overnight to make it more hydrophobic. To form the fluidic chamber, we first prepared two 7-mm-thick PDMS slabs by casting. A 1  2 cm2 square was cut in one of the slabs, and inlet and outlet holes were punched in another slab for fluid access. The two PDMS slabs were then thermally bonded together after surface activation in oxygen plasma. Afterwards, the fabricated PDMS grating was placed on a glass substrate and covered with the bonded PDMS slabs. Finally, mechanical clamps were used to avoid leakage in the airtight chamber. Diffraction Pattern Observation. Figure 2 shows a schematic diagram of the experimental setup for optical observation. Water was injected into the chamber with a syringe through the inlet and outlet of the system. Then, the outlet tube was blocked with a valve and the other one was connected to an air-pressure supply. The applied air pressure was regulated by using a digital barometer with 1 mbar precision. A laser beam of 1 mm spot size at a wavelength of 532 nm (Laser 2000) was aligned to pass through the fluidic chamber containing the transmission diffraction grating. The diffracted beam was imaged on a screen 5 cm above the sample. A CCD camera (Aigo digital viewer GE5, China) was used to capture diffraction images for different values of applied pressure. Langmuir 2010, 26(5), 3666–3669

ð1Þ

where d is the period of the grating, λ is the wavelength of the incident light, and θm is the diffraction angle of mth order. m can be positive or negative, resulting in diffracted orders on both sides of the zeroth-order beam.16,17 For a given wavelength, the diffraction efficiency and the distribution of energy between diffraction orders depend on the polarization of the incident light and the grating modulation parameter, η ¼ Δ=d

ð2Þ

where Δ is the optical path difference between pillars (nphp) and the water-air filling area (nwhw + naha); np(hp), nw(hw), and na(ha) are, respectively, the refractive index (height) of PDMS pillars, water, and air in the pillar gap area. Because the refractive index of water is quite close to that of PDMS, increasing the water penetration depth in the pillar gap area will lead to a decrease in the diffraction efficiency as well as a redistribution of energy between different diffraction orders. Experimentally, the water penetration depth in the PDMS pillar gaps could be controlled by the precise regulation of applied (16) Groisman, A.; Zamek, S.; Campbell, K.; Pang, L.; Levy, U.; Fainman, Y. Opt. Express. 2008, 16, 13499–13508. (17) Yu, H. B.; Zhou, G. Y.; Chau, F. S.; Lee, F. W. Opt. Lett. 2009, 34, 1753– 1755.

DOI: 10.1021/la903150h

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Figure 3. Variation of the transmission diffraction pattern when the maximum applied pressure was fixed at 50 (a), 60 (b), and 100 mbar (c). The three image sequences show clearly totally recoverable (a), partially recoverable (b), and nonrecoverable (c) water penetration, depending on the maximum value of applied pressure.

Figure 4. Initial Cassie-Baxter state of submerged water on superhydrophobic surfaces, which can be totally or partially recovered after pressure is applied and then released. When the applied pressure is high, irreversible switching between the Cassie-Baxter and the Wenzel state occurs.

pressure in the fluidic chamber. Figure 3 shows the variation of diffraction patterns of the same water-submerged PDMS grating for different values of applied pressure. Three types of wetting processes can be observed, depending on the maximum value of applied pressure. Figure 3a-c displays the change of the diffraction pattern when the applied pressure progressively increased to 50, 60 and 100 mbar, respectively, and then decreased to 0 mbar, corresponding totally recoverable (a), partially recoverable (b), and nonrecoverable (c) water-penetration processes. In other words, the switching between the Cassie-Baxter state and the Wenzel state is reversible when the maximum pressure is limited to 50-60 mbar. Otherwise, the switching is irreversible. Figure 4 shows a schematic of the reversibility of the three regimes. As shown in Figure 3a, the spot intensity of the zeroth-order diffraction increases first in the pressure range of 0-20 mbar and then decreases in the range of 30-50 mbar. When the applied pressure decreases progressively, a reversed pattern change can be observed, indicating the total recovery of the initial wetting state. Within this low-pressure limit, the diffraction pattern can be changed many times. The spot intensity of the first-order diffraction varies differently to compensate for the intensity of the 3668 DOI: 10.1021/la903150h

zeroth-order diffraction. When the added pressure increased to 60 mbar, the diffraction pattern changed in almost the same way as in Figure 3a but a total diffraction pattern recovery cannot be reached, which suggests some water remaining in the pillar gaps, equivalent to the case of a residual applied pressure of 10 mbar. The difference between forward and backward images at the same pressure increased when the maximum applied pressure increased. When the maximum pressure is 80 mbar, the remaining water after the pressure returns to zero corresponds to a residual pressure of 50 mbar. When the maximum pressure passed 100 mbar, the diffraction pattern became very weak because of the low refractive index contrast between water and PDMS. After the pressure was released, the diffraction pattern remained as weak as before, suggesting an irreversible switch from the Cassie-Baxter state to the Wenzel state, as illustrated in Figure 3c and Figure 4. The spot intensity variation of different diffraction orders can be evaluated more systematically. To simplify the analysis, we plot the spot intensity change in direction a1 defined in Figure 1a. Figure 5 displays the diffraction spot intensity versus applied pressure along the a1 direction for four values of maximum applied pressure: 50 (a), 60 (b), 80 (c), and 100 mbar (d). Clearly, when the maximum pressure is limited to 50 mbar (a), the spot intensity of both diffraction order 0 and 1 can be recovered after the pressure is released and their intensity varies in a compensated for way. For a maximum pressure of 60 mbar (b), the spot intensity recovery can still be seen, and it seems that the total intensity of the two diffraction orders remains unchanged after the applied pressure cycle, indicating a very limited quantity of water left in the pillar gaps. In the case of a larger maximum pressure, not only can the initial spot intensity not be recovered but the total diffraction intensity decreases significantly (c) or becomes negligible (d). A more detailed observation allowed us to conclude that for the grating (PDMS) and measuring system we used in this work 60 mbar is a critical value as the maximum applied pressure for a reversible change from a true CassieBaxter state to an intermediate wetting state between the CassieBaxter and Wenzel states. A numerical calculation of the oscillation of transmission diffraction efficiency for a specific diffraction order can be made in a similar manner as described in ref 17, but it Langmuir 2010, 26(5), 3666–3669

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Figure 5. Variation of the diffraction spot intensity versus applied pressure for both forward and backward pressure changes with a maximum applied pressure of 50 (a), 60 (b), 80 (c), and 100 mbar (d).

will be more pertinent to consider a true 2D grating used in this work. We should mention that the change in the wetting state under pressure is material-dependent. The gas solubility of PDMS is relatively high so that the trapped air in the PDMS pillar gap areas can be pushed into PDMS under pressure and the release of absorbed air is a slow process. Therefore, the observed irreversible switching from the Cassie-Baxter to the Wenzel state could be facilitated by PDMS. Nevertheless, the trapped air can also escape through the water-solid interface, indicating the possibility of state change independent of the nature of the material. In general, other types of materials and surface structures can also be studied in a similar way. For example, gratings made from transparent polymers can be placed in the fluidic chamber for the determination of the wetting state under pressure. The PDMS fluidic chamber can also be replaced by other materials for a larger pressure range. If necessary, the diffraction pattern can result from a reflective grating so that the transparency of the grating material is no longer necessary. Finally, the surface roughness, surface shape, and spot size may be important factors in diffraction pattern formation. Other techniques involving scattering,15 and more complex spectroscopic analyses can be used for the determination of both static and dynamic properties of the water-submerged surface. In principle, optical techniques can be extremely accurate and data can be closely correlated with the numerical results of modeling. Therefore, we have enough

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confidence in the proposed method not only for research into superhydrophobic surfaces but also for the application of a much larger application field.

Conclusions We described a method to control and monitor the switching of the two superhydrophobic states. A surface-treated PDMS transmission diffraction grating has been integrated into a fluidic chamber for water-submerged wetting observation under pressure. A laser diffraction pattern of a water-submerged PDMS grating has been analyzed in a systematic manner with different values of applied pressure. The method and the experimental setup that we proposed in this work are simple and inexpensive. The experimental results that we obtained are also intuitive and easy to analyze, thereby providing a new way of investigating the wettability of solid surfaces. Further work will be done on the numerical calculation to understand more quantitatively the diffraction efficiency change in 2D transmission gratings in different wetting regimes. Acknowledgment. This work was partially supported by the European Commission through project contract CP-FP 214566-2 (Nanoscale), the EADS foundation, the French Ministry for Foreign and European Affairs, and program PFCC no. 20994. The content of this work is the sole responsibility of the authors.

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