Evaporation-triggered directional transport of asymmetrically confined droplets

[摘要] Hypothesis: When a liquid droplet is confined between two non-parallel hydrophobic surfaces with dihedral angle a, its behavior is largely influenced by the asymmetric confinement. During evaporation, the droplet morphology under confinement will continuously evolve, leading to the directional transport of the droplet towards the cusp. Experiments and Simulations: During the evaporation process, droplets at different initial locations l(0) from the cusp were experimentally observed to transport towards the cusp. A series of simulations using Surface Evolver were performed to obtain the three-dimensional morphologies of the confined droplets. Force and energy analyses were conducted to unveil the mechanisms dominating the evaporation triggered actuation and transport. Findings: The asymmetrically confined droplet of volume V would drift towards an equilibrium location of l(e) from the cusp with the lowest energy. Its directional motion results from the consecutively decreasing l(e), which is scaled as l(e) similar to alpha V--1(1/3) during evaporation. Herein, the creeping and slipping modes of transport could be characterized as the quasi-stable and unstable self-relaxation processes of droplet from the stretched regime to the equilibrium regime, respectively. Our findings on the intrinsic mechanism of droplet actuation shed light on a novel approach to manipulating the confined droplet behaviors in a passive and decisive fashion. (C) 2021 Elsevier Inc. All rights reserved.

[发布日期] 2021-12-15

[关键词] Droplet evaporation;Evaporation-triggered droplet actuation;Self-relaxation process;Stretched/squeezed droplet state;Unidirectional transport;Asymmetric confinement