Oscillatory solutothermal convection-driven evaporation kinetics in colloidal nanoparticle-surfactant complex fluid pendant droplets

Abstract

To elucidate the pure physics of evaporation which is free from surface effects, the pendant mode of evaporation is employed in the present study. The present study brings out the evaporation kinetics of a combined surfactant and nanoparticle colloidal system. We also segregate the contributing effects of surfactants alone, particle alone, and the combined effect of surfactant and particles in modulating the evaporation kinetics. It is observed that the rate of evaporation is a strong function of the particle concentration for nanocolloidal suspensions of particle alone and concentration of surfactant molecules up to the micellar concentration and thereafter insensitive to concentration for an aqueous surfactant solution. The combined colloidal system of nanoparticles and surfactant exhibited the maximum evaporation rate, and the rate is a strong function of the concentration of both the particle and surfactant. The theoretical classical diffusion-driven evaporation falls short of the experimentally observed evaporation rate in aqueous surfactant and colloidal solutions. Evidence of convective currents was observed in flow visualization studies in aqueous surfactant solutions, nanocolloidal solution of particle alone, and an oscillatory convective circulation in a combined surfactant-impregnated nanocolloidal solution. Thermal Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found not potent enough to induce strong circulation currents in such systems from a stability map. Scaling analysis of solutal Marangoni is observed to be capable of inducing circulation from a stability map in all the systems and the enhanced thermophoretic drift and Brownian dynamics, and enhancement in the diffusion coefficient of the nanoparticles is also contributing to the enhanced evaporation rate for only nanocolloidal solutions. The oscillatory convective current arising out of two opposing driving potential enhances the evaporation rate of surfactant-impregnated nanocolloids. The present findings could reveal the effect of surfactants in tuning the evaporation rate of colloidal solutions.