On the dynamics of coalescence between droplets and partially filled microgrooves

Abstract

When water vapor condenses on a hydrophobic surface equipped with microgrooves, a unique phenomenon of coalescence between droplets growing on the ridges and the microgrooves partially imbibed with a condensate is manifested. Such coalescence is distinctly different from droplet–droplet coalescence and can trigger rapid removal of the condensate from the surface, a critical requirement for high thermodynamic efficiency of condensation. In this work, we investigate the dynamics of this coalescence process. We develop an experimentally validated, three-dimensional, volume of fluid method-based numerical modeling framework that accounts for dynamic contact angle variation during contact line motion. The condensate wetting the microgroove forms a liquid column with a meniscus pinned to the microgroove edges. We show that ridge droplet coalescence with this pinned meniscus triggers capillary ripples that traverse the microgroove in transverse and longitudinal directions and can trigger the depinning of the contact line from the opposing edge depending on the size of the coalescing droplet. The contact line depins when the local contact angle at the opposing edge reaches ∼180° and, simultaneously, the kinetic energy converted from the excess surface energy available reaches a maximum. Additionally, we show that the overall coalescence process is significantly affected by the microgroove aspect ratio. For the same liquid volume, relatively shallower microgrooves cause the condensate to overflow, thus attaining a morphology akin to large droplets in the Wenzel state. As a result, the coalescence dynamics on such microgrooved surfaces are similar to that on a planar surface.