Universal spatio-topological control of crystallization in sessile droplets using non-intrusive vapor mediation

Abstract

The present work showcases a mechanism of asymmetric solvent depletion using vapor-mediated interaction that can non-intrusively regulate the site of crystal precipitation. In general, the flow pattern inside a drying sessile saline droplet leads to circumferential deposition of salt crystals at the end of evaporation. Instead, we show that our proposed approach can manipulate the spatial location of crystal precipitation. The introduction of a pendant ethanol droplet near the sessile saline droplet's vicinity creates an asymmetric ethanol vapor gradient around the sessile drop. The differential adsorption of ethanol vapor on the surface of the saline droplet gives rise to a spatial surface tension gradient. This, in turn, enhances the internal convection [∼O (102-103)] within the saline droplet. The vigorous and non-uniform flow promotes targeted contact line depinning, ensuring preferential segregation of the salt crystals. Using this methodology, we can inhibit crystal formation at selected locations and favorably control its deposition in definite regions. The interplay of flow hydrodynamics and the associated contact line motion governs this phenomenon marked by the inception and growth of crystals at a preferential site. The universal character of such a phenomenon is verified for a variety of salt solutions on the glass substrate. Tweaking of contact line motion using vapor mediation can be used as a strategic tool for controlling the salt crystallization sites for various potential applications starting from water purification to portable diagnostics, fouling, and scaling, particularly in cases that require the separation of unwarranted solutes from solvents.