Evaporation of drops on superhydrophobic surfaces: The effect of deformation due to the gravitational field

Abstract

This paper presents an analytical/numerical study of the evaporation characteristics of sessile drops deformed by the presence of the gravitational field, focusing on the specific case of superhydrophobic substrates. The effect is systematically analyzed for contact angles between 150° and 180° and water drops with volume varying between 2.7 μl and 14.0 ml for a total of 91 cases. The evaporation characteristics are analyzed in terms of the vapor fluxes on the drop surface and evaporation rates, and the effect of drop deformation is quantified by relating the evaporation rate to the contact angle and the Bond number, Bo=ρgReq2/σ, where Req is the volume equivalent drop radius. Some peculiarities that characterize the superhydrophobic region are pointed out, like the existence of a relative large region of Bo (from 0 to 10–15), where the evaporation rate of the deformed drops is lower than that of a spherical cap of the same size, and an almost negligible effect of the contact angle on evaporation characteristics, when compared to the hydrophilic/hydrophobic substrate cases. A relatively simple correlation is defined as a correction to the well-known Picknett–Bexon correlation, valid for non-deformed drops, and it is proposed for a quick evaluation of the evaporation rates from sessile drops, deformed by the gravitational field, on superhydrophobic substrates.