Defining the effective upper bound of thin-film thickness for capillary-driven evaporation in micropillar wicks

Capillary-driven evaporation provides efficient thermal management solutions for high-heat-flux electronic devices in confined spaces that require temperature uniformity. Evaporation from the thin-film region with thickness varies from nanometer at the adsorption film region to micrometer at intrinsic meniscus can contribute over 80% of the total evaporation heat transfer. The effective upper bound of thin-film thickness ( δ tf,max ) is a key parameter for determining the evaporation capacity within this region. Although this parameter is conventionally taken to be 1-10 μm, there is a lack of a precise theoretical prediction method. This work combines the theoretical model of evaporation at the thin-film region with computational fluid dynamics (CFD) simulations to determine δ tf,max . Effects of micropillar geometric parameters, contact angles, and liquid-vapor interfacial heat transfer coefficient on δ tf,max are accurately quantified. A high-fidelity semi-empirical correlation for the effective upper bound of thin-film thickness is proposed. This semi-empirical correlation achieves a prediction accuracy of ±20%, and can be adopted to predict the optimal geometric dimensions of wicking structures for capillary-driven evaporation. This work provides an efficient design-and-optimization framework for thermal management of high-heat-flux electronics using capillary-driven evaporation.