Experimental investigation on the evaporation dynamics of ethanol–water binary sessile droplets on a heated substrate

Abstract

To understand the coupled effect of substrate temperature and ethanol concentration on the evaporation characteristics of binary sessile droplets and their induced flow instability, we conducted an experimental study of the evaporation kinetics of ethanol–water binary sessile droplets on a heated substrate. The substrate temperature varies from 30 °C to 60 °C, while the ethanol volume concentration is from 0 to 90%. The distribution of the droplet surface temperature was observed using infrared thermography. Additionally, the evolutions in droplet surface thermal patterns and droplet morphology were examined. The results suggest that the evaporation of binary mixture droplets (BMD) is influenced by a combination of thermocapillary convection and solute capillary convection, resulting in pronounced flow instabilities, including hydrothermal waves (HTWs) and Bénard-Marangoni instabilities. The surface thermal pattern of BMD is closely related to the ethanol concentration. At low concentrations of ethanol, “three-convective cell” and “four-convective cell” structures are formed on the surface of the droplets, which are not observed at high ethanol concentrations. Increasing the substrate temperature enhances the droplet evaporation, leading to a higher BMD evaporation rate and an increase in the number of HTWs at the droplet surface. Furthermore, low concentration droplets exhibit a mixed evaporation mode, while high concentration droplets predominantly evaporate in a constant contact radius (CCR) mode.