Abstract:
: This article reports the hitherto unreported
phenomenon of arrested evaporation dynamics in pendant droplets
because of electric field stimulus. The evaporation kinetics of
pendant droplets of electrically conducting saline solutions in the
presence of a transverse, alternating electric field is investigated
experimentally. While the increase of field strength reduces the
evaporation rate, increment in field frequency has the opposite
effect. The same has been explained on the solvation kinetics of
ions in polar water. Theoretical analysis reveals that change in
surface tension and the diffusion-driven evaporation model cannot
predict the decelerated evaporation. With the aid of particle image
velocimetry, suppression of internal circulation velocity within the
droplet is observed under electric field stimulus, which directly
affects the evaporation rate. A mathematical scaling model is proposed to quantify the effects of electrohydrodynamic circulation and
electrothermal and electrosolutal advection on the evaporation kinetics. The analysis encompasses major governing parameters,
namely, the thermal and solutal Marangoni numbers, the electrohydrodynamic number, the electro-Prandtl and electro-Schmidt
numbers, and their respective contributions. It has been shown that the electrothermal Marangoni effect is suppressed by the electric
field, leading to deteriorated evaporation rates. Additionally, the electrosolutal Marangoni effect further suppresses the internal
advection, further reducing the evaporation rate by a larger proportion. Stability analysis reveals that the electric body force retards
the stable internal advection. The stability mapping also illustrates that if the field strength is high enough for the electrosolutal
advection to overshadow the solutal Marangoni effect completely, it can lead to improvement in evaporation rates.
