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Electrically controlled frustration of total internal reflection on transparent superhydrophobic surfaces Gou, Steven Haiping

Abstract

Superhydrophobic surfaces are those that have a very low adhesion to water due to a combination of surface chemistry and physical roughness. This low adhesion surface offers the potential for enabling low energy optical contact between a liquid and solid surface. For the purposes of this study, optical contact occurs when the distance between the liquid and solid surface is less than the evanescent wave region of light that is undergoing total internal reflection. To develop an improved understanding of superhydrophobicity, a novel visualization technique was employed using neutrally buoyant immiscible liquids, which allowed liquid drops on the order of centimeters to be studied without distortion due to gravity. This allowed the visualization of wetting behaviors of liquids on different surface geometries, revealing the importance of overhanging structures to superhydrophobicity. A nanoporous, optically transparent film of polymethyl-methacrylate (PMMA) was fabricated via oxygen plasma etching with roughness features on the order of 100 nm. The surface was made superhydrophobic by spin-coating with Teflon® AF. A new technique was developed to modulate reflectivity by changing the optical properties at an interface by moving water to and from the interface. The technique electrically controls reflectance of visible light at a transparent superhydrophobic surface by frustration of total internal reflection and was demonstrated in an experimental device. The device consisted of a water drop positioned above a dual-scale superhydrophobic surface. Application of an electrical potential difference between the water and the surface caused it to undergo electrostatic deformation. This deformation would move the water into optical contact with the transparent superhydrophobic surface, thereby changing the optical properties and enabling the modulation of the surface reflection. Removing the electrical potential caused the water to return to its original curvature, separating it from the superhydrophobic surface. Such a device has potential applications as a low-power reflective display. The results presented in this work highlight the feasibility of this technique in practical devices, and suggest that further research is warranted.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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