Ionic solubility and solutal advection governed augmented evaporation kinetics of salt solution pendant droplets

Abstract

The presence of dispersed inclusions is known to modify the interfacial characteristics in liquids by adsorption–desorption of the ions at interfaces. The present article reports the influencing role of dissolved ions in a polar fluid on its evaporation dynamics. The evaporation dynamics of pendant droplets of aqueous solutions of variant simple salts and concentrations have been experimentally studied. The presence of salts is observed to enhance the evaporation rate (obeying the classical D2 law), and the enhancement has been found to hold a direct proportionality to the concentration of the dissolved salt. Furthermore, it is observed that the degree of enhancement in the evaporation rate is also directly proportional to the solubility of the salt in question. The phenomenon is explained based on the chemical kinetics and thermodynamics of hydration of the ionic species in the polar fluid. The classical evaporation rate constant formulation is found to be inadequate in modeling the enhanced species transport. Additional probing via particle image velocimetry reveals augmented internal circulation within the evaporating salt based drops compared to pure water. Mapping the dynamic surface tension reveals that a salt concentration gradient is generated between the bulk and periphery of the droplet and it could be responsible for the internal advection cells visualized. A thermo-solutal Marangoni and Rayleigh convection based mathematical formulation has been put forward, and it is shown that the enhanced solute-thermal convection could play a major role in enhanced evaporation. The internal circulation mapped from experiments is found to be in good quantitative agreement with the model predictions. Scaling analysis further reveals that the stability of the solutal Marangoni convection surpasses the thermal counterpart with higher salt concentration and solubility. The present article sheds insight into the possible domineering role of conjugate thermohydraulic and mass transport phenomena on the evaporation kinetics aqueous droplets with ionic inclusions.

Interplay of electro-thermo-solutal advection and internal electrohydrodynamics governed enhanced evaporation of droplets

“…The PIV setup is validated using internal velocity of evaporating water droplets, and the interior of the droplet is observed to exhibit zero advection, with minor drift motion. This is in agreement with reports on the subject matter [8,11,37]. Figures 5 (a)-(d) illustrate the time-averaged velocity contours and vector field for the different electric field configurations mentioned above.…”

“…Figure 4 (b) illustrates a comparison between the model predictions and the experimentally observed evaporation rates. It is noteworthy that the model predicts the evaporation rate for water droplet accurate to ±10 % (not illustrated in figure, however, reported in literature by present authors [8] and in other works…”

“…The selection of salt solution has been made based on previous experiments by the present authors (Jaiswal et al [8]). The report showed that sodium iodide (NaI) salt based droplets showed the best augmentation in the evaporation rates with respect to water, owing to the high solubility of the salt in aqueous systems.…”

“…Due to the internal circulation and the solutal Marangoni advection at the droplet surface, the vapor diffusion layer in contact with the surface of the droplet is also sheared. This shear replenishes the otherwise stagnant diffusion layer with ambient air, leading to improved evaporation [8].…”

“…A pioneering work by Godsave [7] introduces the classical D 2 law for the evaporation of such pendent droplets. In more recent times, studies [8,9] have shown that evaporation characteristic of such pendent droplets are modulated by tuning the interfacial properties employing salts or surfactants or particulate dispersions. Studies on the role of electromagnetic body forces on pendent droplet behaviour have also been reported (Rossow [10]).…”