Eur. Phys. J. Special Topics 229, 1739–1756 (2020)
https://doi.org/10.1140/epjst/e2020-900265-5

Regular Article

Sweeping by sessile drop coalescence^★

Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY, 14853, USA

^a e-mail: phs7@cornell.edu

Received: 9 November 2019
Accepted: 6 July 2020
Published online: 14 September 2020

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° ≤ θ₀ ≤ 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 θ₀. With increasing θ₀ up to 150°, wetted and projected swept areas both increase as drop center of mass heightens. For θ₀ ≥ 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 θ₀ 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.