A method to Generate Biomimetic Superhydrophobic Engineering Surfaces
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1008-T08-16
A Method to Generate Biomimetic Superhydrophobic Engineering Surfaces Yilei Zhang, and Sriram Sundararajan Mechanical Engineering, Iowa State University, Ames, IA, 50011 ABSTRACT A versatile hybrid processing method that combines electrostatic deposition of microparticles and subsequent anisotropic plasma etching is described that can generate superhydrophobic engineering surfaces with tunable bimodal roughness and a thin hydrophobic fluorocarbon film. These surfaces exhibit contact angles with water of more than 160∫. INTRODUCTION Superhydrophobic (super-water repellant) surfaces are found in nature on the leaf surfaces of many plants such as the lotus and colrabi1. These surfaces also exhibit self-cleaning capability by which rolling water droplets remove dirt and debris from their surfaces. Therefore, efforts to fabricate artificial superhydrophobic surfaces (exhibiting water contact angle greater than 150∫) have received much attention, motivated by both fundamental research and their potential use in practical applications. The wetting behavior of superhydrophobic surfaces is governed by both their chemical composition and geometric microstructure. A careful check of the lotus leaf surface reveals a binary roughness structure (at the 10 - 20 µm and nano meter scale respectively) and epicuticular wax layer1. Both micro and nano scale roughness are helpful in increasing contact angle 2. Several methods have been employed to generate engineering surfaces that can mimic the structure and chemistry of natural superhydrophobic surfaces 3. Polymer coatings or layer-bylayer deposited particles with both low surface energy and micro structures can be attached to the bulk to achieve superhydrophobic properties4. Microfabrication techniques are widely used to generate predetermined micro roughness while aligned carbon nanotubes have been utilized for nano scale roughness 5. Since many engineering materials are hydrophilic, a further step is needed to coat the roughened substrates with a hydrophobic layer self assembled monolayers 6, polymer films7 or diamond-like carbon films8. It is of interest to develop processing methods that allow a high degree of control over the resulting surface structure and that can simultaneously impart hydrophobicity to render a material superhydrophobic. EXPERIMENT The details of the process (Fig. 1) are described as follows using a silicon substrate. First, a clean Si(100) surface with a negatively charged native oxide layer is achieved using a Piranha etch (3:1 solution of H2O2 and H2SO4). Next, a poly (diallyldimethyl ammonium) chloride (PDDA) monolayer was deposited onto the Si (100) surface via dip-coating to form a polycationic layer. Silica spheres with an average diameter of 20 µm were mixed with MilliQ water and sonicated for 20 minutes to form a colloidal solution into which the silicon substrate
was immersed. This particle size corresponds to the micro scale feature size seen on lotus leaves1. The negatively charged silica particles are randomly attracted to the posi
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