Influence of substrate flexibility and temperature on thermal patterns of sessile droplet evaporation

The evaporation of sessile droplets on flexible substrates is widely used in various fields, including wearable devices, soft robotics, and flexible electronic devices. Although the evaporation behavior of sessile droplets on flexible substrates has been studied, there is still insufficient research on the evolution of droplet thermal patterns during evaporation. This study explored the thermal pattern evolution and mode transitions of evaporating isopropanol droplets on PDMS substrates under varying flexibility and temperature conditions. The results show that with the increase of substrate flexibility and temperature, the thermocapillary convection within the droplet gradually strengthens, leading to a higher Marangoni number and an increased number of convective cells. At a PDMS temperature of 50 °C with a curing ratio of 10:1, only Bénard-Marangoni convection is observed. As the substrate flexibility increases, hydrothermal waves begin to appear and gradually dominate during the evaporation process. At curing ratio of 10:1, Bénard-Marangoni convection is the only pattern at 35 °C and 50 °C, while longitudinal rolls appear at 65 °C and become the only pattern at higher temperatures. This study identifies the critical conditions for transitions between different thermal patterns, providing new insights into thermal pattern within evaporating droplet on flexible surfaces.