Substrate concavity influenced evaporation mechanisms of sessile droplets

Abstract

In this article, we probe the morphing of the evaporation kinetics of sessile droplets on curved hydrophilic and superhydrophobic (SH) surfaces. Concave grooves of different radii have been employed, and optical diagnostics of the droplet profile has been carried out to monitor evolution of the evaporation progress. Our observations reveal curtailed evaporation rates on curved hydrophilic surfaces due to the droplet confinement phenomenon, whereas the rates improve for curved SH surfaces. We study the modulation of triple line transients and contact angle dynamics under the influence of substrate curvature. We show that the effective evaporation rate is determined by the interplay of substrate curvature and a proposed confinement ratio. Furthermore, the internal flow field within the evaporating droplet is studied using particle image velocimetry. Our findings show that minor changes in internal velocity occur due to hydrophilic substrate curvature, whereas for a curved SH surface, the circulation velocity is augmented. A mathematical analysis based on diffusion driven evaporation is proposed to predict the transient variation of evaporation for curved hydrophilic substrates. We explain the enhanced evaporation rate on curved SH on the basis of enhanced circulation velocity and increase in liquid-vapor interfacial shear. The model predictions confirm well to the experimental observations.