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Manipulation of Wetting Directions Using Nanostructures with Asymmetric Surface Energies Changquan Lai1, Carl V. Thompson1, 2 and W.K. Choi1, 3 1 Singapore MIT Alliance, National University of Singapore, Singapore 117576, Singapore. 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 3 Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore. ABSTRACT Ordered arrays of polymeric nanostructures with different shapes were generated using laser interference lithography and plasma etching. Surface energy anisotropy was produced in each nanostructure in the array through oblique angle deposition of a hydrophilic metal. When a water droplet was placed on such a surface, it was found to wet preferentially in the direction of the hydrophilic face. Depending on the shape of the nanostructure and the deposition direction, wetting can be made uni-, bi- or tri-directional. Insights obtained in this study contribute to the understanding of wetting on rough, chemically heterogeneous surfaces and provide new methods to engineer functional surfaces for the control of wetting directions. INTRODUCTION The spreading of a liquid on a solid surface, also known as wetting, usually takes place isotropically. However, if a surface can cause wetting to occur in certain specific directions, benefits such as anisotropic reduction of drag forces can arise. This has important implications for applications in the field of microfluidics and biosensing, and is the reason behind recent heightened interest in surfaces that can produce directional wetting. The earliest attempt at directional wetting involved the creation of a surface energy gradient by means of an uneven silanization of a flat silicon substrate [1]. Directional wetting was achieved but it was observed to be inseparable from droplet transport as migration of the droplet on the surface takes place simultaneously with, and carries on after, the wetting process. Jokinen et al subsequently showed that by creating anisotropic microstructures (e.g. microtriangles) on surfaces, bidirectional and tridirectional wetting without droplet transport can be accomplished [2]. Soon after, it was reported that unidirectional wetting can be observed on surfaces with bent nanopillars [3], [4]. Here, we demonstrate that nanostructures with anisotropic surface energies can be employed to generate all three types of directional wetting. EXPERIMENTAL DETAILS Three types of nanostructures were made: periodic nanogrooves (figure 1a), hexagonal arrays of nanofins (figure 1b) and square arrays of nanopillars (figure 1c). These were fabricated on Polystyrene (PS) substrates by means of laser interference lithography, which confers ordered arrangements to the nanostructures to facilitate theoretical analysis of wetting behaviors, and O2/CF4 plasma etching, with which the surface energy of the PS nanostructures could be manipulated by the plasma gas composition. Surface energy anisotropy of each
