Macroscopic Model for Sessile Droplet Evaporation on a Flat Surface

Evaporation of sessile droplets on a flat surface involves a complex interplay between phase change, diffusion, advection, and surface forces. In an attempt to significantly reduce the complexity of the problem and to make it manageable, we propose a simple model hinged on a surface free-energy-based relaxation dynamics of the droplet shape, a diffusive evaporation model, and a contact line pinning mechanism governed by a yield stress. Our model reproduces the known dynamics of droplet shape relaxation and of droplet evaporation, both in the absence and in the presence of contact line pinning. We show that shape relaxation during evaporation significantly affects the lifetime of a drop. We find that the dependence of the evaporation time on the initial contact angle is a function of the competition between the shape relaxation and evaporation and is strongly affected by any contact line pinning.