Local evaporation flux of deformed liquid drops

TL;DR

This study investigates how deformation of liquid drops due to gravity and surface tension affects local evaporation flux, revealing that deformation can enhance overall evaporation and influence deposit formation, with implications for engineering applications like 3D printing.

Contribution

It introduces a heat-diffusion based model for local evaporation flux on deformed drops, contrasting with traditional vapour-diffusion models, and explores the effects of deformation on evaporation behavior.

Findings

Deformation leads to maximum evaporation flux at the contact line.

Deformation enhances total evaporation rate, especially at smaller contact angles.

Tilted drops evaporate faster and exhibit more heterogeneous flux distribution.

Abstract

Escaping of the liquid molecules from their liquid bulk into the vapour phase at the vapour-liquid interface is controlled by the vapour diffusion process, which nevertheless hardly senses the macroscopic shape of this interface. Here, deformed sessile drops due to gravity and surface tension with various interfacial profiles are realised by tilting flat substrates. The symmetry broken of the sessile drop geometry leads to a different evaporation behavior compared to a drop with a symmetric cap on a horizontal substrate. Rather than the vapour-diffusion mechanism, heat-diffusion regime is defined here to calculate the local evaporation flux along the deformed drop interface. A local heat resistance, characterised by the liquid layer thickness perpendicular to the substrate, is proposed to relate the local evaporation flux. We find that the drops with and without deformation evaporate with a minimum flux at the drop apex, while up to a maximum one with a significantly larger but finite value at the contact line. Counterintuitively, the deviation from the symmetric shape due to the deformation on a slope, surprisingly enhances the total evaporation rate; and the smaller contact angle, the more significant enhancement. Larger tilt quickens the overall evaporation process and induces a more heterogeneous distribution of evaporative flux under gravity. Interestingly, with this concept of heat flux, an intrinsic heat resistance is conceivable around the contact line, which naturally removes the singularity of the evaporation flux showing in the vapour-diffusion model. The detailed non-uniform evaporation flux suggests ways to control the self-assembly, microstructures of deposit with engineering applications particularly in three dimensional printing where drying on slopes is inevitable.