Contribution of solid–liquid–vapor interface to droplet evaporation

Abstract

A solid–liquid–vapor (slv) interface is formed by a capillary and disjoining pressure gradient near the macroscopic contact line of a droplet. At a micro/nanostructured surface, the scale of the slv interface increases, such that it can significantly contribute toward droplet evaporation. However, the wettability of the micro/nanostructured surface and droplet volume may affect this contribution and warrant further clarification. In this study, the evaporation rate of a droplet at a macroscopic liquid–vapor (lv) interface was derived theoretically and that at fabricated surfaces was measured experimentally. Considering droplet mass conservation, the evaporation rate at the slv interface was estimated based on the difference between the experimental results and theoretical evaporation rate at the lv interface. The corresponding scale of the slv interface was estimated and further validated with experiments. It was found that the theoretical evaporation rate agreed with the experimental results, when the scale of the slv interface was negligible. For microstructured surfaces, the scale of the slv interface was estimated to be 253–940 µm for a 4 μL water droplet, significantly contributing toward the evaporation rate, in addition to the evaporation at the lv interface. The evaporation rate at the slv interface accounted for 16–48 % of the total droplet evaporation rate. The scale of the slv interface and evaporation rate increased with a decrease in the initial contact angle or an increase in the droplet volume. The results of this study are expected to contribute toward improvements in practical engineering and medical applications using micro/nanostructured surfaces for droplet evaporation.