Phenomenology and kinetics of sessile droplet evaporation on convex contours

Abstract

In this article we probe experimentally and theoretically the evaporation phenomenology and kinetics of sessile droplets seated over convex hydrophilic and super-hydrophobic (SH) surfaces. To understand the role of convex contours, both cylinders (mono-curvature system) and spheres (two-curvature system) have been explored. The evolution of the droplet shape during evaporation was monitored by optical imaging. The observations reveal improved evaporation rates on convex substrates, and the same tends to increase with increasing curvature. The findings also show that the influence of a two-curvature system (sphere) on the evaporation rate is to a higher extent than that for a mono-curvature system (cylinder). These observations are attributed to the augmented liquid-vapor interfacial area and widened up vapor diffusion domain which renders the liquid-molecules more scope to diffuse in the surrounding environment. The dynamic behavior of the triple line (TL) on convex substrates shows higher receding velocity of the contact line especially on spherical substrates. To visualize the internal flow and understand the effect of substrate convexity on internal advection dynamics, particle image velocimetry (PIV) was carried out. These results show higher strength of circulation velocity inside a droplet evaporating over convex surfaces despite frequent de-pinning of contact line. This behavior can be explicated by the fact than on convex surfaces higher temperature gradient exists along the liquid-vapor interface of the droplet (due to more evaporative cooling, and mapped through infrared thermography), which induces stronger thermal-Marangoni convection, thereby scaling up the circulation velocity. The finding may hold implications towards the design and development of droplet-based thermofluidic devices.