Apparent Contact Angle around the Periphery of a Liquid Drop on Roughened Surfaces

Xuemin Huang,Ian Gates

doi:10.1038/s41598-020-65122-w

Abstract

The wetting of roughened surfaces is complicated since not all of the surface of the irregular surface is wetted and thus, the three-phase contact line for the liquid drop is a complex, three-dimensional line that varies according to the dimensions of the roughness and its spatial heterogeneity. This can cause the contact line to not sit within a constant height horizontal plane especially when air is trapped underneath the liquid layer. Here, we explore the effect of roughness on the effective contact angle of a water droplet on a roughened hydrophobic surface. The results show that the apparent contact angle varies around the periphery of the droplet due to the roughness of the surface on first contact. Also, repeated wetting of the droplet on the surface reveals that the apparent contact angle changes due to residual liquid remaining on the roughened surface. The results also show that the Wenzel and Cassie-Baxter models tend to overestimate the apparent contact angle on the roughened surfaces.

Highlights

The wetting of roughened surfaces is complicated since not all of the surface of the irregular surface is wetted and the three-phase contact line for the liquid drop is a complex, three-dimensional line that varies according to the dimensions of the roughness and its spatial heterogeneity
The results demonstrate that an increase of the roughness leads to an increase of the contact angle variability, so that for a severe rough surface, a single contact angle measurement or even the mean contact angle no longer reflects the real and complex contact line between liquid and solid
For macroscale rough hydrophobic surfaces where roughness features are of order of tens to hundreds of microns, the results shows the same tendency as has been observed for nanoscale hydrophobic surfaces in that the increase of the roughness leads to higher apparent hydrophobicity

Summary

Introduction

The wetting of roughened surfaces is complicated since not all of the surface of the irregular surface is wetted and the three-phase contact line for the liquid drop is a complex, three-dimensional line that varies according to the dimensions of the roughness and its spatial heterogeneity. The equilibrium contact angle is a characteristic measure of the energy state at a liquid-fluid-solid contact line. The behaviour of the contact line on a roughened surface is complex because it depends on the implicit wettability of the solid (which in turn depends on the compositions of solid, liquid, and fluid reflected by the surface energies of the interfaces and solid) and the spatial properties of the roughness of the surface[4,5,6,7,8]. The difference of the scales of the liquid droplet and solid grain affects contact line placement length and configuration.

Methods

Results

Conclusion