Heat transfer to wetting and non-wetting liquid droplets deposited onto a heated microgroove surface

Abstract

Evaporation of liquid droplets on a heated substrate is an important process in numerous engineering applications, during which the energy transport is dependent upon, among many others, the liquid/substrate wetting characteristics. In this work, transient heat transfer to a liquid droplet deposited on a heated metallic surface with multiple parallel microgrooves were experimentally investigated from distributed temperature measurements underneath the microgrooves. The initial heat conduction following deposition of a liquid droplet causes a sharp decease in the substrate temperature, during which a fleeting but notable temperature plateau for both cases of alcohol and water droplets is likely attributed to a thin layer of vapor formed in between the droplet and the substrate. Depending on the wetting characteristics, the transient heat transfer process is drastically different for the cases of alcohol and water. Deposition of an alcohol droplet is followed by the droplet instantaneous spreading on the microgroove fins and liquid penetration into the microgrooves, leading to continued temperature decrease in the substrate as a result of formation and evaporation of liquid thin films. It takes only nearly half a second for complete evaporation of the deposited alcohol, at which the substrate reaches its lowest temperature in the process. As a water droplet is deposited, it takes about 7 minutes for its complete evaporation, during which the substrate temperature experiences four distinct stages corresponding to evolution of the water droplet on the substrate. The results in this work provide insights into the fundamental physics of heat transfer during evaporation of liquid droplets with different liquid/substrate wetting characteristics.