Journal of Colloid and Interface Science, Vol.581, 334-349, 2021
Evaporation mediated translation and encapsulation of an aqueous droplet atop a viscoelastic liquid film
Hypothesis: Viscoelastic liquids could be used as potential substrates in the microfluidics paradigm. The theoretical and experimental investigation of an evaporating aqueous droplet, over a viscoelastic liquid substrate, could provide a fundamental perspective of the complex interplay amongst capillarity, viscosity, and elasticity, resulting in a wide array of intriguing dynamics, which could be important in several microscale processes. Experiments: The evaporation dynamics of a water droplet atop an un-crosslinked polydimethylsiloxane film (polymeric liquid substrate) are examined using an optical goniometer and a laser scanning confocal microscopy, to discern the interfaces. The recorded videos were analyzed to estimate the contact angles, velocities, and other parameters of relevance. Findings: The viscoelasticity of the film, in conjunction with evaporation, triggered a self-propulsion in the droplet, leading to crumpling of the polymeric film, and finally culminating in the encapsulation of the water drop by the polymer. The evaporation caused a dynamic variation in both the radius and contact angle of the droplet. The physics of the hitherto unreported phenomena is explained via the development of a semi-analytical model, considering all the relevant forces. We postulate that this symbiotic and self-sustained dynamics would pave the path towards the comprehension of micro-swimmers and surface encapsulation, to name a few. (C) 2020 Elsevier Inc. All rights reserved.