Evolution of thermal patterns during steady state evaporation of sessile droplets

The steady state evaporation process of sessile isopropanol droplets was experimentally investigated by utilizing the infrared thermography. The steady state was achieved by replenishing the droplet from its base. Results indicate that once the evaporation is initiated, the interfacial temperature closed to the triple line is higher than that at the apex, which induces a thermocapillary flow along the interface from the contact line to the apex. With the increase of the evaporation rate, the temperature gradient is enlarged; the flow is enhanced and loses its stability. Then, several thermal patterns, including the forked pattern, the curved hydrothermal waves, the rosebud pattern and the petal-like pattern, were observed. The evolution of thermal pattern is highly correlated with the droplet height and the thermal wave number increases with the substrate temperature. Meanwhile, the global evaporation rate is higher and the surface flow is more intense on copper substrate than that on aluminum substrate.