When coalescing droplets jump: A unified energy conversion model incorporating droplet size and surface adhesion

Abstract

Jumping-droplet condensation pushes the boundary of condensation heat transfer by enabling microdroplet shedding via coalescence-induced droplet jumping. The latter is empowered by surface-to-kinetic energy conversion. Regardless of extensive studies of droplet jumping on ideally non-wetting surfaces, a quantitative description of droplet jumping from realistic surfaces remains a challenge due to limited insight into the complex energy conversion process that is strongly coupled with droplet-droplet and droplet-substrate interactions. Here, we use a three-dimensional (3D) pseudopotential multiphase multiple-relaxation-time lattice Boltzmann method (MRT-LBM) to simulate binary-droplet coalescence with various droplet sizes and surface wettability. Then, we developed a comprehensive and unified energy conversion model, derived by rigorously analyzing the dynamic droplet-surface interaction and quantifying the roles of droplet size scale, droplet size mismatch, and surface wettability. Our simulations capture coalescence and jumping dynamics of arbitrary-sized droplets on surfaces having various wettability and reveal the effect of droplet size and surface wettability. Validated by experiments, the energy model is then used to define the jumping/non-jumping boundaries for coalescing droplets on nanostructured surfaces. Our work demonstrates the key physics and a universal criterion governing self-propelled droplet shedding, key to the design of surfaces for enhanced condensation heat transfer, anti-frosting/icing, self-cleaning, and water/energy harvesting.