Droplets trapped by a wetting surface with chemical defects in shear flows

Abstract

With the development of techniques to control the wettability on solid surface, directional control of droplets on such surfaces exhibit promising applications in the design of microfluidic devices. The sliding droplet can even be trapped by the heterogeneous surface that is equipped with wetting defects. However, its detailed mechanism and general physical conditions required for trapping droplets displaced by another fluid have never been fully described. In this work, we present numerical simulations based on front-tracking method to investigate the trapping of droplets on a hydrophilic surface with a stripe-like chemical wetting defect displaced by another immiscible liquid. Three kind of equilibrium status are observed for trapped droplets based on the analysis on the dynamic of the contact line. We determine the transition conditions between trapping and escaping by mapping droplet dynamics as a function of nondimensional defect strength and capillary number. To reveal the underlying mechanism of droplets escaping, the deformation and motion including the dynamics of triple-phase contact line are explored as the droplet passing through the defect. It is found that the unbalanced Young’s force acting the contact line could be a driving or resistance force, which is the key factor to dominate the critical condition of the trapping-to-escaping transition.