Evaporation Process of Sessile Liquid Droplet and Layer in Steady State

The ethanol liquid droplet with constant contour and liquid layer with constant height in steady-state contained in the heated substrate under evaporation have been experimentally investigated utilizing infrared thermography and volume control technique. The heat and mass transfer and thermal flow patterns at the gas–liquid interface during droplet and layer evaporation are analyzed through their heat flux curves and interface temperature fields. Experimental results reveal that the steady-state evaporation of liquid droplet and layer at surface emerges after the evaporation rate is equal to replenishment rate by continuous liquid refilling and temperature filed becomes uniform. For evaporating droplets, there are two main stages during the evaporation evolution: unstable evaporation with decreasing heat flux and steady evaporation with constant heat flux. In the steady stage, the interface temperature gradually becomes uniform, and convection cells at the interface disappear. For evaporating layer, three main stages are present: first, the formation and splitting of Marangoni convection cells, then the hydrothermal wave propagating from "source" to "sink", and finally the stable flow stage without vortex cells. The results also show that the direction of the temperature gradient at evaporating droplet interface is opposite under different substrate materials (Aluminum and PTFE). The heat flux density of the evaporating droplet on Aluminum substrate is larger than that on PTFE, resulting in a higher evaporation rate. These new findings help to gain a better understanding of the coupling mechanism between phase change and thermal convection inside evaporating liquid droplets and layers.