Wetting dynamics and evaporation of sessile droplets on nano-porous alumina surfaces

An experimental investigation of wetting and evaporation of sessile droplets is presented on nano-porous alumina substrates having different pore distribution (uniform, random and linearly arranged) morphologies and pore sizes (70-120 nm). Firstly, the behavior of a droplet as it spreads and wets a surface is captured using high speed videography and benchmarked with the correlation given by the Tanner's law. The Cassie-Wenzel state transition of a droplet on nano-porous surfaces is also scrutinized with 3D-Laser Confocal Microscopy which clearly reveals the sub-stages of the transition process. During wetting transition, entrapped air bubbles are seen coming out of the nano-porous substrate, which lead to microconvection. In this study, sessile droplet evaporation on the nano-textured surfaces is also investigated. Evaporation can be considered as a quasi-steady-state process, such that the vapor concentration distribution above the droplet satisfies the Laplace equation, but with a time-varying droplet surface. For benchmarking, the evaporation of sessile water and ethanol droplets on standard borosilicate glass and Teflon surfaces is examined, and results are compared with available diffusion models. Contact angle variation with time is recorded along with rate of evaporation in a controlled environment with specified humidity and temperature boundary condition. The coupling between wettability and eventual rate of droplet evaporation is established. The results clearly show that nano-structuring is an effective tool to control wettability as well as the diffusive evaporation process. In addition, the physical morphology and pore distribution affects wettability as well as evaporation rates. (C) 2013 Elsevier B.V. All rights reserved.