Soluto-thermo-hydrodynamics influenced evaporation of sessile droplets

TL;DR

This study investigates how solvated salts influence the evaporation dynamics of sessile droplets on different surfaces, revealing complex behaviors driven by internal advection and interfacial flows that cannot be explained by simple diffusion models.

Contribution

The paper introduces a comprehensive analysis combining experiments and theory to explain salt-modulated evaporation rates, highlighting the role of internal advection and interfacial shear in evaporation dynamics.

Findings

Salt concentration affects evaporation rates differently on hydrophilic and superhydrophobic surfaces.

Internal advection and interfacial shear significantly influence evaporation behavior.

Diffusion models alone cannot predict the observed evaporation modulation.

Abstract

The present article experimentally and theoretically probes the evaporation kinetics of sessile saline droplets. Observations reveal that presence of solvated ions leads to modulated evaporation kinetics, which is further a function of surface wettability. On hydrophilic surfaces, increasing salt concentration leads to enhanced evaporation rates, whereas on superhydrophobic surfaces, it first enhances and reduces with concentration. Also, the nature and extents of the evaporation regimes constant contact angle or constant contact radius are dependent on the salt concentration. The reduced evaporation on superhydrophobic surfaces has been explained based on observed via microscopy crystal nucleation behaviour within the droplet. Purely diffusion driven evaporation models are noted to be unable to predict the modulated evaporation rates. Further, the changes in the surface tension and static contact angles due to solvated salts also cannot explain the improved evaporation behaviour. Internal advection is observed using PIV to be generated within the droplet and is dependent on the salt concentration. The advection dynamics has been used to explain and quantify the improved evaporation behaviour by appealing to the concept of interfacial shear modified Stefan flows around the evaporating droplet. The analysis leads to accurate predictions of the evaporation rates. Further, another scaling analysis has been proposed to show that the thermal and solutal Marangoni advection within the system leads to the advection behaviour. The analysis also shows that the dominant mode is the solutal advection and the theory predicts the internal circulation velocities with good accuracy. The findings may be of importance to microfluidic thermal and species transport systems.