Abstract:
The present article experimentally and theoretically probes the evaporation kinetics of sessile saline
droplets. Observations reveal that presence of solvated ions leads to modulated evaporation kinetics,
which is further a function of surface wettability. On hydrophilic surfaces, increasing salt concentration
leads to enhanced evaporation rates, whereas on superhydrophobic surfaces, it first enhances and
reduces with concentration. Also, the nature and extents of the evaporation regimes (constant contact
angle or constant contact radius) are dependent on the salt concentration. The reduced evaporation on
superhydrophobic surfaces has been explained based on observed (via microscopy) crystal nucleation
behaviour within the droplet. Purely diffusion driven evaporation models are noted to be unable to
predict the modulated evaporation rates. Further, the changes in the surface tension and static contact
angles due to solvated salts also cannot explain the improved evaporation behaviour. Internal advection
is observed (using PIV) to be generated within the droplet and is dependent on the salt concentration.
The advection dynamics has been used to explain and quantify the improved evaporation behaviour
by appealing to the concept of interfacial shear modified Stefan flows around the evaporating droplet.
The analysis leads to accurate predictions of the evaporation rates. Further, another scaling analysis
has been proposed to show that the thermal and solutal Marangoni advection within the system leads
to the advection behaviour. The analysis also shows that the dominant mode is the solutal advection
and the theory predicts the internal circulation velocities with good accuracy. The findings may be of
importance to microfluidic thermal and species transport systems.
