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.
