Evaporation-induced convective transport in confined saline droplets

Deka, N and Saha, S and Dash, S (2022) Evaporation-induced convective transport in confined saline droplets. In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, 639 .

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Abstract

Here, we report on the convection pattern, inside a confined saline droplet sandwiched between two hydrophobic substrates, before crystallization (prenucleation) and at the onset of crystallization (postnucleation). Crystallization occurs inside a saline droplet due to evaporation-induced supersaturation of the droplet. The hydrophobic nature of the substrates leads to the crystallization at a single nucleation site at the triple contact line of the droplet or in the interior of the droplet. The flow field in the prenucleation and the postnucleation stages is attributed to the density gradient established inside the droplet during evaporation and subsequent crystallization. A stark contrast in the convective flow patterns and the flow strength is observed between the two stages of evaporation. While the prenucleation regime is marked by an axisymmetric toroidal vortex pattern, the symmetry breaks in the postnucleation regime. Once the nucleation of crystal initiates, a recirculating flow field directed upward from the location of nucleation, and a 3�4-fold increase in the magnitude of flow velocity is observed. This increase in flow strength is attributed to the large difference in the local concentration arising because of nucleation. In addition, prior to nucleation, the change in the magnitude of maximum velocity is marginal (~10) over an evaporation time of ~15 min. In contrast, the flow strength reduces appreciably with evaporation time (~ 45 over 15 min) in the postnucleation stage due to the growth of the crystal that in turn lowers the solutal concentration inside the droplet. We present a numerical model to determine the flow field inside the droplet in the postnucleation regime by introducing a sink term, accounting for crystallization-induced-depletion of solute, in the species transport equation and show that it agrees well with the experimental observation. © 2022 Elsevier B.V.

Item Type:	Journal Article
Publication:	Colloids and Surfaces A: Physicochemical and Engineering Aspects
Publisher:	Elsevier B.V.
Additional Information:	The copyright for this article belongs to Elsevier B.V.
Keywords:	Crystallization; Drops; Evaporation; Flow fields; Flow velocity; Heat convection; Hydrophobicity; Nucleation, Confinement; Convection; Convection patterns; Convective transport; Evaporation time; Flow strength; Hydrophobic substrate; Pre-nucleation; Saline droplets; Salt solution, Supersaturation
Department/Centre:	Division of Mechanical Sciences > Mechanical Engineering
Date Deposited:	10 Feb 2022 11:50
Last Modified:	10 Feb 2022 11:50
URI:	http://eprints.iisc.ac.in/id/eprint/71147

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