Two-Fluid Wetting Behavior of a Hydrophobic Silicon Nanowire

Article pubs.acs.org/Langmuir

Two-Fluid Wetting Behavior of a Hydrophobic Silicon Nanowire Array Yongkwan Kim,† Yunsie Chung,† Ye Tian,†,‡ Carlo Carraro,† and Roya Maboudian*,† †

Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, United States School of Materials Science and Engineering, Harbin Institute of Technology Harbin, Heilongjiang 150001, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The two-fluid wetting behavior of surfaces textured by an array of silicon nanowires is investigated systematically. The Si nanowire array is produced by a combination of colloidal patterning and metal-catalyzed etching, with control over its roughness depending upon the wire length. The nanowires are made hydrophobic and oleophobic by treatment with hydrocarbon and fluorinated self-assembled monolayers, respectively. Static, advancing, and receding contact angles are measured with water, hexadecane, and perfluorotripentylamine in both single-fluid (droplet on solid in an air environment) and two-fluid (droplet on solid in a liquid environment) configurations. The single-fluid measurements show wetting behavior similar to that expected by the Wenzel and Cassie−Baxter models, where the wetting or non-wetting behaviors are amplified with increasing roughness. The two-fluid systems on the rough surface exhibit more complex configurations because either the droplet or the environment fluid can penetrate the asperities depending upon the wettability of each fluid. It is observed that, when the Young contact angles are significantly increased or reduced from single-liquid to two-liquid systems, the effect of roughness is relatively minimal. However, when the Young contact angles are similar, roughness has almost identical influence on apparent contact angles in single- and two-liquid systems. The Wenzel and Cassie−Baxter models are modified to describe various two-fluid wetting states. In cases where metastable behavior is observed for the droplet, advancing and receding measurements are performed to suggest the equilibrium state of the droplet.

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wetting devices.9,10 Surface-coating applications where the environment is exposed to both water and oil can also benefit from understanding how rough surfaces interact with one liquid in the presence of another liquid. In this work, we examine the two-liquid wetting behavior on hydrophobic silicon nanowires of varying lengths to determine how the presence of a liquid environment in combination with roughness affects the two-liquid contact angle. In particular, the surfaces of the silicon nanowires are treated with hydrocarbonand fluorocarbon-terminated self-assembled monolayers to create hydrophobic and oleophobic surfaces. The single- and two-fluid static contact angle measurements are performed with water, hexadecane (HD), and perfluorotripentylamine (FTPA) to examine the interaction of polar, hydrocarbon, and fluorinated liquids with the hydrophobic rough nanowire surfaces. While we focus primarily on experimental observation of the wetting behavior in two-liquid systems and explanation based on qualitative analysis, we also aim to incorporate theoretical approaches, such as the Young, Wenzel, and Cassie−Baxter (CB) models, to provide further insights.

INTRODUCTION Wetting behavior of liquids on solid surfaces is a topic of great interest because it provides fundamental insights on interfacial phenomena in numerous fields, such as printing, lubrication, and coatings.1 In particular, many studies have focused on fabrication and characterization of superhydrophobic (or superoleophobic) surfaces on which water (or oil) contact angles exceed 150°, to take advantage of their self-cleaning and liquid-repellant properties.2−4 It is now well-known that hydrophobicity can be enhanced by chemically modifying the solid substrates and changing the surface topography.2,3,5 To date, most of the contact angle analyses have focused on single-liquid systems, in which the contact angle is measured at the three-phase interface of a solid, liquid, and vapor. In comparison, a limited number of studies have investigated twoliquid systems, where the vapor phase is replaced by another immiscible liquid.6−8 These studies have been mostly on contact angles of water or other polar liquids on a flat surface in an oil environment. Two-liquid studies on rough surfaces have focused mostly on demonstration of wetting reversibility for electrowetting applications.9,10 A better understanding of twofluid wetting behavior, in particular, on surfaces with roughness, can aid in many useful applications, for example, in reducing the wetting hysteresis and, thus, improving reversibility of electro© 2014 American Chemical Society

Received: August 23, 2014 Revised: October 19, 2014 Published: October 30, 2014 13330

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Chemical Modification of Surfaces. After the silicon nanowire array is fabricated, the surface is chemically modified by either hydrocarbon [by treatment with octadecyltrichlorosilane (ODTS)] or fluorinated [by treatment with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS)] monolayers.12 Before coating with the self-assembled monolayer, the silicon samples are successively cleaned by sonication in acetone (5 min), isopropanol (5 min), and UV ozone (10 min). The ODTS-treated surface is prepared by submerging the nanowire sample in toluene solution containing 0.1% ODTS (Alfa Aesar) by volume and leaving for 24 h in a relative humidity of 150°) even without apparent roughness. Equation 4 also imposes a criterion that the sum of two angles in inversed systems (θLLb and θLbL) must be 180°. However, because static contact angles tend to follow advancing angles more closely,24 the actual sum of two systems is found in the range of 180−200°. Panels a and b of Figure 5 show the effect of the nanowire length on the two-liquid contact angles formed by various 13333

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57.2° 53.7° 143.9°± 6.7°

predicted experimental

44.9° ± 7.1° 48.5° ± 10.9° 121.5° 125.0°

combinations of water, HD, and FTPA, denoted as droplet/ bulk liquids (e.g., H2O/HD = water droplet in a HD environment). On ODTS-coated surfaces (Figures 5a), θH2O/HD is high (170°) without any roughness and hardly changes with the nanowire length. While the maximum water contact angles of around 160° are achieved in single-liquid systems with nanowires longer than ∼0.5 μm (Figures 2a), almost no additional roughness is required to reach maximum angles in the two-liquid system. On the other hand, the inverted system of HD/H2O exhibits a greatly reduced contact angle of 6.5° on a flat ODTS surface. Because the angle is already close to 0°, roughness plays an insignificant role just as in the H2O/HD system. It can be noted that, for highly hydrophobic and oleophilic surfaces, such as an ODTS-coated substrate, having a water droplet in oily liquid or vice versa enhances both hydrophobic and oleophilic characteristics and the roughness parameter has a minimal effect. The FTPA/HD contact angles display a similar non-wetting trend as H2O/HD on the ODTS surface. This result is not immediately obvious from the single-liquid measurements because FTPA has slightly lower contact angles than HD in the single-liquid case (Figures 2a). However, eq 4 points out that not only the single liquid contact angles but also the

predicted

162.4° 50.9°

experimental

149.2° ± 13.1° 48.3° ± 15.6°

predicted

17.6° 129.1° 31.8° 46.9°

experimental

Figure 5. (a) Two-liquid contact angles of H2O/HD, FTPA/HD, FTPA/H2O, HD/FTPA, and HD/H2O on an ODTS-treated silicon nanowire array surface with increasing nanowire length. (b) Twoliquid contact angles of H2O/HD, H2O/FTPA, FTPA/HD, FTPA/ H2O, and HD/H2O on a PFTS-treated silicon nanowire array surface with increasing nanowire length.

148.2° 133.1°

6.5° ± 5.6° 47.4° ± 7.4°

predicted experimental

168.0° ± 2.6° 152.6°± 4.1°

experimental

predicted

36.6° ± 3.5°

experimental

predicted

Article

ODTS PFTS

FTPA/HD HD/FTPA HD/H2O H2O/HD droplet/bulk

Table 1. Experimental and Predicted Contact Angles of Two-Liquid Systems on Flat ODTS- and PFTS-Treated Si Surfaces

H2O/FTPA

FTPA/H2O

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surface tensions of the liquids involved (which essentially indicate the interfacial tension values of the respective liquids with the solid) determine whether wetting will be favorable or not in a two-liquid system. The contact angle greater than 90° can be anticipated from the lower energy of the HD/ODTS interface compared to that of the FTPA/ODTS interface. An opposite wetting trend is observed with an inversed system of HD/FTPA. On the flat ODTS surface, the contact angles of FTPA/HD and HD/FTPA are 149° and 37°, respectively, and the sum of two angles is close to 180°, as expected by eq 4. These two systems have nearly a symmetric relationship, but as the nanowire length increases, the FTPA/ HD system reaches its maximum angle first (h = 0.25 μm), while the HD/FTPA continues to decrease. It can be noted that a HD droplet in FTPA behaves almost the same way as in air (Figure 2a). The flat surface contact angles of HD in air and in FTPA are very close (HD/air = 37.4°, and HD/FTPA = 36.6°), and the low θHD/FTPA indicates that the wetting of the ODTS surface is favorable with the droplet likely in the Wenzel state. According to the Wenzel equation (eq 1), the apparent contact angle only depends upon roughness and the angle on a flat surface. Because these two parameters are very similar for HD/air and HD/FTPA systems, the single- and two-liquid systems exhibit the trends that almost coincide. The results reveal that the effect of roughness in single- and two-liquid systems is almost identical when their Young contact angles are similar. For example, the FTPA/H2O contact angle on the flat ODTS surface (44.9°) does not differ much from the HD/air and HD/FTPA angles, and it also displays a similar tendency against the roughness factor. Figure 5b shows that the H2O/HD contact angle behavior is similar on the PFTS-treated surface to that on the ODTStreated surface in that little roughness is required to reach the maximum angle of 170°. This behavior is also observed on the H2O/FTPA system. Enhanced hydrophobicity on the PFTStreated surface in the presence of hydrocarbon oil or fluorinated liquid indicates preferential wetting of those fluids over water, as confirmed by the single liquid measurements (Figure 2b). Figure 5b also shows that the contact angles of FTPA/HD, FTPA/H2O, and HD/H2O on the flat PFTS-coated surface are very close. This is consistent with the predicted angles listed in Table 1, which all fall within the error range of the experimental values and are coincidently similar as a result of three different combinations of surface tensions and single-liquid contact angles. The above two-fluid combinations also exhibit very similar characteristics with respect to nanowire lengths. To further explain the experimentally observed trends of the contact angle on rough surfaces in two-liquid systems, some of the data, in particular, HD/FTPA and FTPA/HD data for the ODTS surface and HD/H2O for the PFTS surface, have been replotted in panels a and b of Figure 6 to facilitate comparison to theoretical models. For eqs 1 and 2 to be valid in a twoliquid system simply by substituting the vapor phase with the bulk liquid phase, the bulk liquid must be in the Wenzel configuration, as shown in panels a and b of Figure 7. For the ODTS surface, wetting by FTPA and HD is favorable, and the Wenzel and CB models are used to describe the two-liquid systems HD/FTPA and FTPA/HD (Figure 6a). The theoretical prediction by the CB equation is in good agreement with the experimental data, in contrast to the single-liquid predictions that underestimate the final contact angles (panels a and b of Figure 2). On the other hand, the Wenzel model again underestimates the contact angle as observed in the single-

Figure 6. (a) FTPA/HD and HD/FTPA contact angle data along with two-liquid CB and Wenzel model predictions. (b) Static, advancing, and receding angles of the HD/H2O system with CB/CB and Wenzel/CB two-liquid model predictions.

Figure 7. (a) Two-fluid Wenzel state where both the droplet and environment liquid wet the asperities. (b) Two-fluid CB state where the droplet sits on the fluid−solid composite. (c) Wenzel/CB state in a two-fluid system in which only the droplet wets the asperities. (d) CB/CB state in a two-fluid system in which both liquids sit on the air− solid composite.

liquid systems, possibly because of the discrepancy in the estimated geometry as explained for the single-fluid measurements. In cases where the environment liquid cannot wet the surface, more complicated configurations can result in twoliquid systems. For example, in the HD/water system on a PFTS surface, water cannot wet a rough PFTS surface and the system may be found as either a Wenzel/CB or CB/CB state (panels c and d of Figure 7). The equation describing each state 13335

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Table 2. Summary of Two-Fluid Configurations with Examples and Their Descriptions droplet configuration (a) two-liquid Wenzel (b) two-liquid CB (c) Wenzel/CB (d) CB/CB

example

description

HD/FTPA on ODTS H2O/HD on ODTS or PFTS HD/H2O on PFTS (receding) HD/H2O on PFTS (advancing)

low contact angle of the droplet fluid that decreases with increasing roughness high contact angle even at low roughness, with enhanced hydrophobicity of water in the oily environment near 0 contact angle even at low roughness approximating the receding angle of the droplet in a non-wetting environment intermediate angle closer to the advancing angle

liquid that exhibits favorable wetting of the solids. Moreover, single-liquid contact angles and surface and interfacial tensions provide a reasonable prediction of whether wetting of surfaces in two-liquid systems is favorable or not. We observe that, when the Young contact angles are significantly increased or reduced from single-liquid to two-liquid systems, the effect of roughness is relatively minimal. However, when the Young contact angles are similar, roughness has almost identical influence on apparent contact angles in single- and two-liquid systems. The hysteresis of HD/air and HD/water has been measured to analyze the metastability of a HD droplet. The result from the single-liquid system has revealed that the receding and advancing angles are consistent with the Wenzel and CB predictions, respectively, and the static contact angles follow the advancing angles more closely. Two equations are derived to model more complicated Wenzel/CB and CB/CB two-fluid states. We have observed that the receding and advancing contact angles of HD/water are in good agreement with the Wenzel/CB and CB/CB models, respectively. Finally, the receding angles of HD/water achieve complete wetting at all roughness, showing that a system in the Wenzel/CB state has contact angles near 0°. The results provide new insights into fluid wetting behavior when the system is made more complex by the presence of both roughness and an environmental liquid and should prove useful as practical applications of various twofluid systems are developed, for example, in microfluidic and electrowetting devices.

can be derived on the basis of the same approach used to derive the Wenzel and CB equations.34 Before they are introduced, one more geometric parameter, α, is defined α=

pillar lateral area 2πdh = projected surface area 3 a2

(7)

where the variables are as defined in Figure 4. The derived equations are as follows (see the Supporting Information for derivation): Wenzel/CB * = Φ cos θLL + (1 − Φ + α)cos θLV cos θLL b b +

γLV γLL

b

(1 − Φ)γL V b

γLL

(8)

b

CB/CB * = Φ cos θLL + cos θLL b b

(1 − Φ)(γL V − γLV ) b

γLL

b

(9)

where θ*LLb is the apparent contact angle and the rest of the variables are as defined previously. To examine the case of HD/water on the PFTS surface more closely, including the possible metastable behavior of the HD droplet, Figure 6b replots the two-fluid contact angles along with additional measurements of the contact angle hysteresis of HD/water on the PFTS surface in the two-liquid system. For comparison, the two theoretical models are also shown. The Wenzel/CB equation predicts that the contact angle of HD/ H2O drops to 0° for all nanowire lengths and the CB/CB model displays a constant angle of 31°. The Wenzel/CB model correctly predicts the receding angle data, which show complete wetting for all roughness values. At small roughness (h < 0.42 μm), the CB/CB model is a good representation of the advancing angle, despite the slight underestimation. In comparison to the single-liquid system (Figure 3c), the hysteresis in the two-liquid system is initially higher and is greatly reduced as the surface becomes rougher. Furthermore, the receding angle reveals that complete wetting of the PFTS surface by HD can be achieved by simply having water as a bulk liquid even without any apparent roughness. The two-liquid contact behavior of various configurations shown in Figure 7 as observed throughout this study is summarized in Table 2.

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ASSOCIATED CONTENT

S Supporting Information *

Derivation of models describing Wenzel/CB and CB/CB configurations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation Grants EEC-0832819 (through the Center of Integrated Nanomechanical Systems) and DMR-1207053. The authors also thank the support of the China Scholarship Council.

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CONCLUSION This study investigates the contact angles of water, HD, and FTPA on periodically structured hydrophobic silicon surfaces in both single- and two-liquid systems. Our experimental results show that the hydrophobicity of ODTS and PFTS surfaces is greatly enhanced by replacing the vapor phase with a bulk

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