Binary Mixture Droplet Wetting and Evaporation Phase Change on Hydrophilic Structured Surfaces

There is a promising future for the development of structurally and chemically decorated surfaces in a variety of applications, including everyday practices as well as industrial and biomedical applications. When structured surfaces are used, their intrinsic hydrophobicity/hydrophilicity strongly affects the wettability and evaporation process of microliter droplets. In this work, pure water, pure ethanol, and their binary mixtures are used to examine their wettability and evaporation behaviour on 6 intrinsically hydrophilic micro-structured surfaces having the same height-to-diameter aspect ratio and varying the spacing between pillars. Upon deposition, the wettability of droplets on short-spacing surfaces depends highly on the liquid surface tension and spacing between pillars; however, on large-spacing surfaces, droplets behave similarly as on their smooth counterparts independently of the liquid studied. Thereafter, the full evaporation process for the same liquids is examined on the same hydrophilic structured surfaces, where three different evaporative modes are revealed: the constant contact radius (pinning), stick-slip mode and mixed mode, in the absence of the constant contact angle mode. The duration of each mode has been analysed to clearly show the dependency of the evaporation modes on the different initial wetting regimes and liquid surface tensions used. Accordingly, choosing the proper spacing of the structure combined with the proper binary mixture concentration, the initial wetting regime, the initial pinning time, and the duration of evaporation modes can be optimized according to the application and objective, making these fundamentals useful in a variety of biological, agricultural and medical fields among others.

*Author acknowledges the support received from the Omani Ministry of Higher Education, Research & Innovation and the Omani Cultural Attaché in London.