Regulating complex fluid sessile droplet evaporation kinetics by suppression of internal electro-convection

Abstract

We report the phenomenology of suppression of the evaporation kinetics of electrically conducting saline sessile droplets in the presence of transverse electric field stimulus. Our experimental results show that the evaporation rates (on hydrophilic and superhydrophobic surfaces) of saline droplets are higher than water droplets, but the rates reduce when an alternating electric field is applied across the saline droplets. The reduction is noted to be a direct function of the electric field strength, and the contact line dynamics of the drying droplet are largely affected. After the classical vapour-diffusion-driven evaporation models fail to explain the modified evaporation kinetics, the flow visualization and infrared thermography were performed to diagnose the internal thermo- and solutal-electrohydrodynamics. Suppression of the internal circulation velocities is observed in the saline sessile droplets under the application of the electric field. A scaling analysis model is proposed based on the internal advection mechanisms to quantify the role of electrohydrodynamics, electro-thermal, electro-solutal convection, and the Electrohydrodynamic number on the internal circulation velocity and evaporation rates. The model incorporates the effects played by the governing Marangoni, Capillary, evaporative Jacob, electro-Prandtl, and electro-Schmidt numbers towards morphing the thermo-solutal advection and is in agreement with the experimental results. An interfacial shear modified Stefan flow analysis is put forward to determine the reduced evaporation rates, and a good match with the experimental observations is obtained.