Water and Ethanol Droplet Wetting Transition during Evaporation on Omniphobic Surfaces.

Xuemei Chen,Suresh V Garimella,Justin A Weibel

doi:10.1038/srep17110

Abstract

Omniphobic surfaces with reentrant microstructures have been investigated for a range of applications, but the evaporation of high- and low-surface-tension liquid droplets placed on such surfaces has not been rigorously studied. In this work, we develop a technique to fabricate omniphobic surfaces on copper substrates to allow for a systematic examination of the effects of surface topography on the evaporation dynamics of water and ethanol droplets. Compared to a water droplet, the ethanol droplet not only evaporates faster, but also inhibits Cassie-to-Wenzel wetting transitions on surfaces with certain geometries. We use an interfacial energy-based description of the system, including the transition energy barrier and triple line energy, to explain the underlying transition mechanism and behaviour observed. Suppression of the wetting transition during evaporation of droplets provides an important metric for evaluating the robustness of omniphobic surfaces requiring such functionality.

Highlights

Superhydrophobic surfaces encountered in nature have inspired numerous theoretical and experimental investigations of wetting behaviour on rough surfaces, leading to a variety of engineered surfaces for self-cleaning[1], drug reduction[2], water harvesting[3], anti-icing[4], and condensation heat transfer enhancement[5,6,7,8]
We develop an approach to fabricate omniphobic surfaces with reentrant mushroom structures on copper substrates, and systematically investigate the effects of surface topography on water and ethanol droplet evaporation
We demonstrate that by controlling the surface topography, the ethanol droplets can be preserved in a Cassie state throughout their evaporation lifetime

Summary

Introduction

Superhydrophobic surfaces encountered in nature have inspired numerous theoretical and experimental investigations of wetting behaviour on rough surfaces, leading to a variety of engineered surfaces for self-cleaning[1], drug reduction[2], water harvesting[3], anti-icing[4], and condensation heat transfer enhancement[5,6,7,8]. Most micro/nano-structured surfaces designed to yield superhydrophobicity[9,10,11,12,13] are not suitable to support non-wetting states for low surface tension liquids, such as oils and alcohols To overcome this limitation, researchers have engineered surfaces with topographic features having specialized reentrant geometries, such as inverse trapezoidal[14], serif-T15,16, mushroom[17,18,19], micro-hoodoo[20,21], and micro-nail[22] structures. An interfacial energy-based analysis was employed to predict the Cassie-to-Wenzel wetting transition observed in the experiments

Methods

Results

Conclusion