Fundamentals of dropwise wetting and evaporation phase-change of binary mixture droplets on micro-decorated surfaces

Abstract

Almost every aspect of our daily lives involves liquid-surface interactions and is intimately related to the physicochemical properties of the substrate as well as those of the liquid. Understanding the mechanisms taking place at the initial state of the evaporation process i.e., wettability, and during the evaporation of droplets is of great interest to many industrial, biological, and medical applications. This research investigates experimentally the initial wettability states and the evaporation modes for droplets of pure water, pure ethanol and their binary mixture, accessing a wide range of surface tensions, on hydrophobic and hydrophilic micro-pillared surfaces with fixed height and diameter whilst varying the spacing between the pillars.

On one hand, the initial wetting states of pure fluids and their binary mixtures on intrinsically hydrophobic micro-decorated surfaces are first studied and a wetting regime map is proposed. This regime map predicts the droplets’ symmetrical and asymmetrical shapes and wetting dependence on the fluid surface tension and the surface structure on the hydrophobic microstructured surfaces, which in turn govern the subsequent evolution of the droplet contact angle and contact radius. Four different evaporation modes have been observed which are consistent with the literature and further two evaporation modes have been revealed here for the first time, namely, increasing contact angle mixed-sick- slip mode and decreasing contact angle mixed-stick-slip mode.

On the other hand, on intrinsically hydrophilic surfaces, the same systematic experimental study is applied using the same fluids and the same microstructured surfaces. It is remarked that the wettability and evaporation on hydrophilic structured surfaces can be affected by ambient exposure after subjecting the surfaces to air plasma cleaning, which eventually removes any deposition of hydrocarbons ever present in the ambient. Unlike the hydrophobic surfaces, the hemi-wicking and spreading regimes are further observed on these surfaces which, consequently, affect the evaporation process. The same six evaporation modes have been observed on these surfaces with different durations though. Investigating the initial wetting and the evaporation modes can lead to a better understanding of choosing the proper structure and wettability (and/or ambient exposure) combined with the correct binary mixture concentration to be specifically tailored to different applications.