Modelling ridge enhanced droplet jumping with varying mismatches and scales for vapor chamber performance estimations

Abstract

The coalescence-induced jumping detachment of droplets during condensation on superhydrophobic surfaces proves to be a promising strategy for addressing critical issues in various phase-change related applications. Protruding structures such as ridges have been reported to enhance the jumping probability and jumping kinetic energy. However, there is a lack of comprehensive investigations into the coupling physics involved in this process, as well as a unified model for accurately predicting droplet jumping, which is presented in this work. The model can predict droplet jumping under varying ridge heights, mismatches and scales of droplets. With the aid of ridges, we achieve a maximum energy conversion efficiency of 43.1% for micro-droplets, which is 18 times of that on a planar surface. Our phase maps illustrate the critical conditions necessary for successful jumping detachments, validated by experimental data within a 20% margin of error. Applications of our model on the droplet-jumping-based vapor chambers are elaborated. It is found that mere detachment is inadequate. Furthermore, jumping needs to be fast enough to overcome air entrainment. By introducing ridges, we demonstrate that the upper limit of heat flux of vapor chamber at the hotspot can be expanded by an order of magnitude to 1000 W/cm2, corresponding to a vapor flow velocity of 1.48 m/s perpendicular to the condensing surface. Our model helps in the design and assessment of condensation equipment by providing the necessary parameter space required to realize valid condenser-to-evaporator jumping.