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.