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. Published by AIP
Publishing.
