pubs.acs.org/Langmuir © 2009 American Chemical Society
Reversible Electrowetting on Superhydrophobic Double-Nanotextured Surfaces Florian Lapierre and Vincent Thomy* Institut d’Electronique, de Micro electronique et de Nanotechnologie (IEMN), UMR CNRS-8520, Cit e Scientifique, Avenue Poincar e, BP 60069, 59652 Villeneuve d’Ascq, France
Yannick Coffinier, Ralf Blossey, and Rabah Boukherroub* Interdisciplinary Research Institute (IRI), Universit e de Science et Technologies de Lille (USTL), USR 3078 CNRS, Parc de la Haute Borne 50 avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq Cedex, France Received November 13, 2008. Revised Manuscript Received February 9, 2009 The paper reports on wetting, electrowetting (EW), and systematic contact angle hysteresis measurements after electrowetting of superhydrophobic silicon nanowire surfaces (NWs). The surfaces consist of C4F8-coated silicon nanowires grown on Si/SiO2 substrate. Different surfaces modulating (i) the dielectric layer thickness and (ii) the nanotexturation were investigated in this study. It was found that the superhydrophobic NWs display different EW behaviors according to their double nanotexturation with varying droplet impalement levels. Some surfaces exhibited a total reversibility to EW with no impalement (contact angle variation of 35 ( 2° at 190 VTRMS with deionized water), whereas other surfaces showed nonreversible behavior to EW with partial droplet impalement. A scenario is proposed to explain the unique properties of these surfaces.
Some biological surfaces, like Lotus leaves, exhibit the amazing property of not being wetted by water, leading to a self-cleaning effect. Their surface roughness is generally composed of microand nanostructures, creating a sort of fakir carpet on which water droplets show a quasi-spherical shape. To mimic these properties, many studies have been conducted to prepare artificial superhydrophobic surfaces: chemical modification of rough surfaces with low-energy molecules or roughening hydrophobic surfaces, different particle size deposition, and patterning materials with micro- and nanotechnology tools, leading to a well-understood wetting behavior.1-4 Such surfaces could be useful in various fields such as textile,5 microelectromechanical systems (MEMS) and lab-on-chip (LOC) or micro total analysis systems ( μTAS) inspired by the Lotus leaf properties: surfaces with high droplet mobility6,7 or self-cleaning surfaces.8 In particular for LOC applications, superhydrophobic surfaces, by reducing their contact areas with microdroplets containing bio-particles, limit the surface pollution (also called biofouling).9 Among all the techniques leading to contact angle control, electrowetting (EW) is one of the most widely used techniques for controlling the displacement of microliter to nanoliter volume water droplets. From a practical viewpoint, lab-on-chip devices, adjustable lenses, electronic displays, and a large number of microsystems are near the stage of being commercialized, proving *To whom correspondence should be addressed. Telephone: +33 (0)3 20 19 79 51. Fax: +33 (0) 20 19 78 98. E-mail:
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[email protected]. (1) Bhushan, B.; Chae, J. Y. Proceedings of the Eighth International Conference on Scanning Probe Microscopy, Sensors and Nanostructures 2007, 107, 1033–1041. (2) Bhushan, B.; Jung, Y. C. J. Phys.: Condens. Matter 2008, 20, 225010 (24 pp). (3) Patankar, N. Langmuir 2004, 20, 8209–8213. (4) Xu, W.; Liu, H.; Lu, S.; Xi, J.; Wang, Y. Langmuir 2008, 24, 10895–10900. (5) Heikenfeld, J.; Dhindsa, M. J. Adhes. Sci. Technol. 2008, 22, 319–334. (6) Burton, Z.; Bhushan, B. Nano Lett. 2005, 5, 1607–1613. (7) Fair, R. Microfluid. Nanofluid. 2007, 3, 245–281. (8) Blossey, R. Nat. Mater. 2003, 2, 301–306. (9) Luk, V. N.; Mo, G. C.; Wheeler, A. W. Langmuir 2008, 24, 6382–6389.
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how mature this technology is already.10-12 Electrowetting relies on the modification of a liquid droplet-solid surface contact angle by the application of an electrical potential between the liquid droplet and the substrate. The theoretical prediction of this phenomenon is now clearly proved by the electromechanical approach described by Jones and Mugele:13,14 under electrowetting, at a microscopic scale the contact angle is not modified while the macroscopic contact angle of the droplet decreases. On a planar hydrophobic surface, reversible electrowetting is classically observed in an air environment. On superhydrophobic (textured) surfaces, a large number of published results, using a wide variety of surfaces (micro/nanoposts,15-17 carbon nanofibers,18 or carbon nanotubes19-21), showed an irreversibility with respect to the electrowetting phenomenon. A liquid droplet deposited on the surface stays at the top of the roughness, leading to a small amount of contact with the liquid, to a high contact angle (>150°) and to a quasi-null hysteresis (rolling ball effect) (Cassie-Baxter state). Once a voltage is applied, the droplet is impaled through the surface texture leading to a contact angle lower than 150° and to a significant hysteresis (>30°) (Wenzel state). When the (10) Shamai, R.; Andelman, D.; Berge, B.; Hayes, R. Soft Matter 2008, 4, 38–45. (11) Berge, B.; Peseux, J. Eur. Phys. J. E 2000, 3, 159–163. (12) Zhao, Y.; Cho, S. K. Lab Chip 2006, 6, 137–144. (13) Jones, T. Mech. Res. Commun. 2009, 1, 2–9. (14) Mugele, F.; Buehrle, J. J. Phys.: Condens. Matter 2007, 19, 375112. (15) Krupenkin, T.; Taylor, J.; Schneider, T.; Yang, S. Langmuir 2004, 20, 3824– 3827. (16) Herbertson, D. L.; Evans, C. R.; Shirtcliffe, N. J.; Mchale, G.; Newton M. I. Sens. Actuators, A 2006, 130-131, 189–193. (17) Krupenkin, T.; Taylor, J.; Wang, E.; Kolodner, P.; Hodes, M.; Salamon, T. Langmuir 2007, 23, 9128–9133. (18) Dhindsa, M.; Smith, N.; Heikenfeld, J.; Rack, P.; Fowlkes, J.; Doktycz, M.; Melechko, A.; Simpson, M. Langmuir 2006, 22, 9030–9034. (19) Wang, Z.; Ou, Y.; Lu, T.; Koratkar, N. J. Phys. Chem. B 2007, 111, 4296– 4299. (20) Zhu, L.; Xu, J.; Xiu, Y.; Sun, Y.; Hess, D.; Wong, C. J. Phys. Chem. B 2006, 110, 15945–15950. (21) Kakade, B.; Mehta, R.; Durge, A.; Kulkarni, S.; Pillai, V. Nano Lett. 2008, 8, 2693–2696.
Published on Web 4/29/2009
DOI: 10.1021/la803756f
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voltage is turned off, the contact angle does not return to its initial value. Although the transition from the Cassie-Baxter to Wenzel state is easily obtained (by electrowetting, external pressure, evaporation, etc.), the barrier of the Wenzel to Cassie-Baxter transition is too high, in terms of free energy, to be crossed in a simple manner. Among the existing studies, a reversible transition was detected only under specific conditions, with a droplet immersed in ambient oil leading to contact angle changes of 58°22 and 40°,23 but these results do not correspond to a superhydrophobic state. Indeed, it should be noted that in an oil environment, the term “superhydrophobic surface” is irrelevant. A water droplet sitting on a planar Teflon substrate displays a contact angle of 160° in an oil environment as compared to an angle of 118° in air ambient and should or could not be classified as a superhydrophobic state. Some experiments with reversible EW have been successfully performed on ZnO nanorods in ambient air at low voltages (
