Multiscale Modeling to Predict the Hydrophobicity of an Experimentally Designed Coating

Abstract

Fine-tuning surface energies of coatings through experimental trial and error can be very tedious and time-consuming. The use of a reliable theoretical model can provide useful guidelines for experimental and formulation design for surface hydrophobicity. In this work, we perform multiscale modeling with molecular dynamics (MD) simulations, theoretical models, and computational fluid dynamics (CFD) simulations to investigate the wetting and sliding behavior of a water droplet on experimentally fabricated coatings. The wetting behavior of a water droplet on untreated polydimethylsiloxane (PDMS) surface and PDMS surface functionalized with hydroxyl and fluoride groups is studied by MD simulations. MD simulation results show that water contact angle (WCA) increases with increasing length of fluorocarbon chains, and this is in good agreement with experimental measurement of PDMS surface functionalized with C8F17. A theoretical model using the results from the MD simulation and inputs from experimental measurements of surface morphology is further proposed to predict the WCA of functionalized microstructured PDMS surface. The validated theoretical model shows that, though increasing the filler concentration aids in enhancing surface hydrophobicity, the separation distance between neighboring features decreases, weakening the hydrophobicity property and indicating there is an optimal filler concentration for the most hydrophobic surface. To perform multiscale modeling to predict the hydrophobicity of coating, the CFD model is constructed to predict droplet sliding and bouncing on an inclined surface in the macroscopic scale by using the WCA from MD simulations and theoretical model as input. The sliding and bouncing behaviors in CFD simulations are well-matched with the experimental observations on different hydrophobic surfaces. This multiscale model presented here is useful in the formulation design for controlled surface hydrophobicity of PDMS surfaces, and it facilitates the applications of PDMS in various aqueous systems.

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