2. Experimental investigation of evaporation from asymmetric microdroplets confined on heated micropillar structures

Droplet evaporation is ubiquitous both in everyday life and numerous engineering, biomedical, and environmental applications. For example, the large amount of latent heat associated with the liquid-vapor phase change process makes droplet evaporation an ideal solution for cooling high-powered electronic devices. Over the past 100 years, many new theories and models of droplet evaporation have been proposed. Still, much of the fundamental transport physics remains elusive, which makes it challenging to describe diverse evaporation behavior by a universal mechanism. In particular, evaporation from asymmetric droplets remains grossly unexplored due to their rarity in nature. However, recent advances in micro- and nanoengineering technology have made it possible to tune and maintain droplets in non-spherical geometries, where the interfacial transport rate becomes highly anisotropic along the circumferential direction. Such a characteristic represents a distinctive feature from evaporation of capped-spherical droplets and has not been comprehensively explained by existing theories. This paper exams the evaporation rate and heat transfer performance of continuously–fed water microdroplets confined on heated silicon micropillar structures with circular, square, and triangular cross-sections. The evaporation experiments are performed at substrate temperatures ranging from 60 °C to 98 °C. The droplets exhibit a capped spherical shape on the circular micropillar but an asymmetric geometry on non-circular micropillars. Our experimental results show that increasing levels of droplet asymmetry significantly enhance evaporative transport. Specifically, for a 60 °C substrate, a triangular micropillar has 45% greater heat transfer coefficient than a circular one; at 98 °C, the increase reaches 71%. This enhancement is validated by multiphase numerical simulations which show agreement with experimental values. The enhanced heat transfer coefficient from asymmetric droplets is attributed to a reduced conduction resistance inside the droplet and smaller diffusion resistance at the droplet interface, both of which result from the non-uniform geometric features of the droplet along the circumferential direction. Finally, a thermal resistance network is developed to demonstrate the variation in the local droplet thickness and interfacial curvature along the circumferential direction for droplets confined on circular, square, and triangular micropillars. Asymmetric droplets yield smaller thicknesses near regions with high curvature, which results in a smaller conduction resistance and higher fraction of heat transport near the contact line region compared to a capped spherical droplet. Furthermore, with increasing droplet temperature, the thermal resistance associated with vapor diffusion is reduced significantly, leading to greater enhancement of evaporative heat transport. These findings not only provide insights in the evaporation behavior of asymmetric droplets, but also provide important guidance for relevant applications, such as the design of evaporative cooling systems.

L. Shan et al. / Experimental Thermal and Fluid Science 109 (2019) 109889