Sweeping by Sessile Drop Coalescence

Abstract

During coalescence of liquid drops contacting a solid, the liquid sweeps wetted and solid-projected areas. The extent of sweeping dictates the performance of devices such as self-cleaning surfaces, anti-frost coatings, water harvesters, and dropwise condensers. For these applications, weakly- and non-wetting solid substrates are preferred as they enhance drop dynamical behavior. Accordingly, our coalescence studies here are restricted to drops with contact angle 90° $\le {\theta }_0\le$ 180°. Binary sessile drop coalescence is the focus, with volume of fluid simulations employed as the primary tool. The simulations, which incorporate a Kistler dynamic contact angle model, are first validated against three different experimental substrate systems and then used to study the influence of solid wettability on sweeping by modifying ${\theta }_0$. With increasing ${\theta }_0$ up to 150°, wetted and projected swept areas both increase as drop center of mass heightens. For ${\theta }_0\ge$ 150°, coalescence-induced drop jumping occurs owing to the decreasing wettability of the substrate and a focusing of liquid momentum due to the symmetry-breaking solid. In this regime, projected swept area continues to increase with ${\theta }_0$ while wetted swept area reaches a maximum and then decreases. The sweeping results are interpreted using the mechanical energy balance from hydrodynamic theory and also compared to free drop coalescence.

17 pages, 10 figures, 3 tables, to be published in Eur. Phys. J. Special Topics: Challenges in Nanoscale Physics of Wetting Phenomena