Water Droplet Motion Control on Superhydrophobic Surfaces

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ARTICLE pubs.acs.org/Langmuir

Water Droplet Motion Control on Superhydrophobic Surfaces: Exploiting the Wenzel-to-Cassie Transition Guangming Liu,† Lan Fu,‡ Andrei V. Rode,§ and Vincent S. J. Craig†,* †

Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia ‡ Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia § Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia

bS Supporting Information ABSTRACT: Water droplets on rough hydrophobic surfaces are known to exist in two states; one in which the droplet is impaled on the surface asperities (Wenzel state) and the other, a superhydrophobic state in which air remains trapped beneath the droplet (Cassie state). Here, we demonstrate that water droplets can transit from the Wenzel-to-Cassie state even though the former is energetically favored. We find that two distinct superhydrophobic states are produced. One is a true Cassie state, whereas the other exhibits superhydrophobicity in the absence of a vapor phase being trapped in the surface roughness. Furthermore, we can selectively drive the motion of water droplets on tilted structured hydrophobic surfaces by exploiting Wenzel-to-Cassie transitions. This can be achieved by heating the substrate or by directly heating the droplet using a laser.

’ INTRODUCTION When a water droplet is placed on a tilted structured hydrophobic surface, it can either remain in position or roll down the surface like a liquid ball.1 The outcome is determined by the relative magnitude of the slope and the contact angle hysteresis—when the former is greater, rolling is favored. If the water droplet sits atop the surface structures it adopts a superhydrophobic or Cassie state,2 whereas if the water droplet is impaled on the surface structures it is in a hydrophobic or Wenzel state.3 On many surfaces droplet history determines what state the droplet is in. In the Wenzel state, the water droplet is strongly pinned on the surface, has a large contact angle hysteresis and the droplet remains stationary, whereas Cassie droplets have low contact angle hysteresis and this gives rise to droplet mobility and the self-cleaning properties associated with superhydrophobicity which are highly desirable for some practical applications.4 Although the Cassie-to-Wenzel transition has been well studied and can easily be achieved in a number of ways including, depositing the droplet from some height,5 applying pressure or force on the droplet,6 applying a voltage7 or vibrating the substrate,8 the Wenzel-to-Cassie transition has only recently been observed by the coalescence of Wenzel and Cassie droplets,4 applying a short pulse of electrical current through a conductive substrate,9 or through vibrations10 and only when the Cassie droplet was the thermodynamically favored state and the Wenzel droplet was in a metastable state. The Wenzel-to-Cassie transition is generally assumed to be irreversible and has never been observed for the case where the Wenzel droplet is at the global minimum-energy state.10,11 In the r 2011 American Chemical Society

present report, we will show that water droplets can transit from the Wenzel-to-Cassie state even though the former is the energetically more favorable state for the surfaces employed. Additionally, we use this transition to selectively control the motion of water droplets on tilted superhydrophobic surfaces. This approach provides a platform for complex automated microfluidic control on simple surfaces.

’ MATERIALS AND METHODS Patterned Hydrophobic Substrates. Micropatterned hydrophobic substrates were produced using a gallium arsenide (GaAs) wafer coated with a silicon dioxide (SiO2) film that was chemically treated to render it hydrophobic. The single crystal GaAs (001) wafer with a thickness of approximately 380 μm was supplied by American Xtal Technology (AXT) and used as received. Posts of height ∼22 μm were produced on a 100 μm  100 μm square grid on the GaAs substrate by conventional photolithography. The substrate was wet chemically etched in H3PO4:H2O2:H2O = 1:1:3 (v:v) solution at 18 °C for ∼14 min to the targeted depth of ∼22 μm and then the photoresist was removed by rinsing with acetone. A ∼180 nm SiO2 film was deposited on the miropatterned GaAs substrate by plasma enhanced chemical vapor deposition (PECVD) at 300 °C under SiH4 (5% in N2/N2O flow) for the following chemical modification. The three-dimensional morphology of the substrate was obtained by surface profilometry (Wyko NT 9000) and the two-dimensional morphology of the substrate was Received: November 23, 2010 Revised: January 13, 2011 Published: February 15, 2011 2595

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Figure 1. Characterization of the micropatterned substrate used in this investigation. (A) Image obtained from surface profile measurements, depicting the three-dimensional morphology of the substrate. (B) Cross section of the substrate also obtained from surface profile measurements. (C) Top view of the substrate obtained by optical microscopy. obtained by optical microscopy (LeiCA DM4000M). Note that the thermal conductivity of GaAs (∼0.5 W cm-1 K-1) is approximately three times lower than that of silicon,12 which is advantageous for the local-heating experiments. Hexadecane (99%) and chloroform (99%) were purchased from Sigma-Aldrich and dried by molecular sieve before use. 1H,1H,2H,2H-perfluorooctyltrichlorosilane (97%) was purchased from Alfa Aesar and used as received. The ethanol was purified by distillation prior to use. All water used was filtered through a coarse wool filter, charcoal filter, and reverse osmosis membrane before a final filtration through a Millipore Gradient system (Memtec) giving a resistivity of 18.2 MΩ cm-1. A hydrophobic monolayer of 1H,1H,2H,2H-perfluorooctyltrichlorosilane was chemically bonded to the SiO2 film to render the surface hydrophobic. This treatment was chosen as the hydrophobic monolayer has excellent thermal stability and is essentially inert in air until heated to a temperature of ∼500 °C.13 The substrate was washed with ethanol and dried under a stream of pure N2 before a RF plasma treatment at a power level of 30 W for 60 s. Then 50 μL of 1H,1H,2H,2H-perfluorooctyltrichlorosilane was added into a solvent mixture of hexadecane (5 mL) and chloroform (1 mL). The cleaned substrate was placed into the solution for ∼10 min. After surface reaction the substrate was removed from the solution, rinsed with ethanol, dried by N2, and baked at 120 °C in an oven for approximately 30 min to complete the formation of Si-O bonds. The contact angles formed by water droplets on the solid substrate were measured using a KSV (Helsinki, Finland) CAM 200 contact angle goniometer at different temperatures. On a flat SiO2 surface covered by the same monolayer the advancing contact angle θA for water is ∼121° and the receding contact angle θR is ∼90° at room temperature. The static contact angle θS, corresponds to the true thermodynamic nature of the interfaces and was found to be ∼110°, though we acknowledge that it can only be truly determined on an ideal perfectly smooth, perfectly homogeneous surface without hysteresis. Heating by Conduction. Droplets were deposited onto the substrate and then the substrate was placed on a preheated small, locally built, temperature controlled heating stage. The temperature of the heating stage was measured using a thermocouple. Experiments (not reported here) were also performed where droplets were allowed to fall onto a preheated substrate, these experiments resulted in similar behavior except in the Leidenfrost regime where bouncing readily occurred. In the process of local heating, a temperature controlled soldering iron was used to heat the underside of the substrate locally to selectively drive the motion of water droplets. Laser Heating of Droplets. We used a custom-built high-repetition-rate laser system delivering up to 50 W of laser power. The laser beam has a spot size of 2.03 mm  1.92 mm FWHM and the droplet was located at the focal spot. The amount of laser energy absorbed in a 4 μL

droplet of water using 35 W of laser power in the presented experiments was ∼2.0  10-7 J per pulse, heating the droplet by ∼7.3  10-5 K during each pulse. The absorbed energy was estimated taking into account the reflectivity at the air-GaAs, GaAs-SiO2, SiO2-H2O, and H2O-air interfaces (nGaAs = 3.478;14 nSiO2 = 1.450;14 nH2O = 1.32115 at 1.064 μm); kH2O = 1.51  10-6. At this wavelength the substrate is completely transparent (the imaginary part of the complex refractive index kGaAs 160° before the Leidenfrost regime is obtained. (B) Water droplet lifetime as a function of temperature. With increasing substrate temperature the droplet lifetime on the micropatterned hydrophobised surface initially exhibits a rapid decrease as the temperature rises from 65 to 120 °C and remains low up to 190 °C. Above 210 °C, droplets become highly mobile, which is characteristic of the Leidenfrost effect. This droplet mobility precludes accurate determination of the droplet lifetime as the droplet does not remain in the field of view for long but it is known that the lifetime of a Leidenfrost droplet is extended due to poor heat conduction following the formation of a vapor cushion between the surface and the droplet. 2597

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Figure 4. Heating induced wetting transitions of a water droplet on the micropatterned hydrophobised surface. (A) The droplet exhibits a hydrophobicto-superhydrophobic transition induced by heating at the temperature of ∼190 °C; however, this transition is not the true Wenzel-to-Cassie transition as the droplet is pinned and light cannot be seen between the droplet and the surface in the superhydrophobic state, indicating that some water is still trapped between pillars. Upon cooling the contact angle is maintained and the droplet remains pinned. (B) The water droplet exhibits a true Wenzel-toCassie transition induced by heating at the temperature of ∼210 °C. The light now can be seen between the droplet and the surface at the superhydrophobic state and the droplet is mobile on the surface. Upon cooling, the mobility and contact angle are maintained.

dramatically. At lower temperatures the droplet remains stationary and at higher temperatures it is very mobile. Inspection reveals that light cannot be seen between the droplet and the surface after the hydrophobic-to-superhydrophobic transition at the temperature of ∼190 °C (see Figure 4A), which indicates that the transition from hydrophobic to superhydrophobic state that has occurred in regime II is not the true Wenzel-to-Cassie transition, as water remains between the pillars in this superhydrophobic state. The presence of the water ensures that the contact angle hysteresis remains high20 and the droplet remains stationary on the surface in regime II. This state is therefore a distinct state that is a hybrid between the Cassie and Wenzel states. The contact angle is Cassie like, whereas the adhesion is Wenzel like. We note that if the droplet is now cooled the superhydrophobic contact angle and pinning of the droplet remain. This also supports the conclusion that the high contact angle exhibited on the rough hydrophobic surface is a result of the condition at the three phase contact line rather than the nature of the surface under the droplet.21 When the temperature is increased to ∼210 °C, a second superhydrophobic state is seen (see Figure 4B). Now, light can be seen between the droplet and the surface after the water droplet transits from the hydrophobic state to the superhydrophobic state, indicating that a true Wenzel-to-Cassie transition has occurred. Here, a vapor layer is formed between the droplet and the surface, which leads to the Wenzel-to-Cassie transition. It is interesting to consider the nature of the transition from the hydrophobic to the superhydrophobic state. At the higher temperatures where the droplet enters the Leidenfrost regime this can be attributed to a dewetting transition brought about by the nucleation of a vapor phase between the droplet and the surface. However, it is unclear whether the same mechanism is responsible for the transition to the pinned superhydrophobic state. In this case, the droplet remains in contact with the substrate and fluid fills the gaps between the columns on the micropatterned hydrophobic surface. One can posit that a

Leidenfrost effect is occurring only in the region of the three phase line but this seems unlikely, when it is not occurring under the center of the droplet where the cooling due to evaporation is less, although preferential nucleation at the three phase line is possible. As molecules evaporate from the droplet, they exert a recoil pressure on the surface of the droplet. We have calculated the magnitude of this pressure from the rate of evaporation and the surface area of the droplet and found it to be orders of magnitude less than the Laplace pressure and therefore unimportant in any consideration of the contact angle or surface energy. Clearly the contact angle measurements are not made under equilibrium conditions and therefore applying an equilibrium thermodynamic description to the contact angle, such as Young’s equation is inappropriate. However it is possible that the increase in contact angle arises due to a change in the relative surface energies with temperature. The droplet is cooled by evaporation and remains at 100 °C even when the substrate temperature is much higher,18 thus the liquid-vapor surface energy remains unchanged at substrate temperatures above 100 °C. Similarly one might expect that the solid-liquid interfacial energy will change only a little as the temperature at the interface will change little. Therefore, it is most likely that if the transition is driven by changes in interfacial energies the solidvapor interface is likely responsible and this may occur through a progressive reduction in the adsorption of water vapor to the solid surface as the temperature is increased.22 Elbaum et al. have shown that for a volatile fluid, undersaturation of the fluid in the vapor phase results in an increase in contact angle.23 An example of the importance of vapor adsorption on the contact angle is the observation that the contact angle of pentane on mica is finite when in equilibrium with the vapor phase but completely wetting when the vapor is significantly undersaturated.24 If interfacial energies are driving the transition then it can be seen as a “Sullivan” transition19 that is driven by a change in the thermodynamics of wetting at high temperatures that operates on the perimeter of the droplet and results in a very high contact angle 2598

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Langmuir for the droplet but is unable to overcome the large energy barrier required to expel the water trapped in the structured surface beneath the droplet. That is, at the elevated temperatures, the transition state may correspond to the thermodynamically most favorable state. What is significant is that upon cooling the droplet remains in the superhydrophobic state which is not the thermodynamic minimum. We note here that we have previously shown that the contact angle also increases when droplets on flat hydrophobic surfaces are heated.25

’ MOTION CONTROL OF DROPLETS Applying this technology to selectively drive the motion of water droplets on superhydrophobic surfaces is an interesting challenge that could provide a platform for a range of applications from microfluidics to self-cleaning surfaces. What needs to be demonstrated is that for a surface with multiple droplets, a single droplet or a selection of droplets can be moved with control. A simple approach for exercising such control is to exploit the difference in adhesion and therefore mobility exhibited by droplets in the Cassie and Wenzel states, along with selective transitioning of droplets between these states. Our approach is to place two or more water droplets on a tilted superhydrophobic surface in the Wenzel state, and then selectively transit one droplet from the Wenzel state to Cassie state by the local application of heat. The unheated droplets remain in the original state whereas the heated droplets undergo a transition to the Cassie state and become mobile. If the superhydrophobic surface has a contact angle hysteresis that is less than the surface slope, the transitioned droplet will roll down the slope due to gravity. We tested this idea using a slope of ∼12°, as for our surface the contact angle hysteresis in the Cassie state is ∼11° (θA ∼ 166°/ θR ∼ 155°), whereas the contact angle hysteresis in the Wenzel state is ∼63° (θA ∼ 130°/θR ∼ 67°). Therefore, a slope of greater than 11° and less than 67° is required if gravity is to be used as the driving force for selective droplet motion. We found that a selected water droplet can readily be induced to roll down the tilted substrate by inducing the Wenzel-to-Cassie transition by applying heat locally to the underside of the substrate. (Videos of these experiments are available as Supporting Information). The droplet continues to roll until it reaches the border between the flat surface and the micropatterned surface (the flat surface has a greater contact angle hysteresis of ∼31°, θA ∼ 121°/θR ∼ 90°). We can selectively drive one of two water droplets to roll down the tilted substrate or one of four droplets by applying the tip of a soldering iron (∼450 °C) to the back of the substrate (videos 1-3, Supporting Information). Also, we can selectively move one droplet and then a second from a group of three droplets on the tilted superhydrophobic surface using a similar method (video 4, Supporting Information). We have also investigated the Wenzel-to-Cassie transition and the subsequent motion of water droplets on another micropatterned substrate that has different surface features including the pillar height, the pillar top surface area and the distance between the pillars to show that the phenomena observed in our experiments is a general effect and not dependent on the precise geometry of the surface employed. Posts of height ∼13 μm were produced on a 50 μm  50 μm square grid on the GaAs substrate for the second substrate (see Supporting Information). A convenient means of heating the droplets is to use a pulsed laser26 to selectively heat one droplet and induce it to move down the slope (Nd:YVO4; 1.06 μm; 15 ps; 35 W; 1.5 MHz) (videos 5

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and 6, Supporting Information). In this way the liquid in the droplet is heated directly, rather than via the substrate. This provides the most effective and well-controlled way of inducing the hydrophobic-to-superhydrophobic transition on the surface. At this wavelength the substrate is completely transparent (the imaginary part of the complex refractive index kGaAs