Local heating effect on thermal Marangoni flow and heat transfer characteristics of an evaporating droplet

• Flow characteristics of evaporating droplets are investigated using numerical simulations. • Uniform and local heating is considered to control the internal flow of the droplets. • Flow motions are analyzed with the liquid–air interface temperature and local surface tension differences. • Flow velocity has a maximum value of 0.048 m/s at the location of the peak region of local surface tension difference. This study numerically investigated the thermal Marangoni flow and heat transfer characteristics of an evaporating droplet. Uniform and local heating methods can control the internal flow patterns of a droplet during evaporation. The present study applied the dynamic mesh method to simulate the behaviors of the liquid-air interface during evaporation. The results revealed that the flow transition inside the droplet appeared in the early stages of evaporation and occurred owing to the temperature variation at the liquid-air interface; these variations eventually yielded surface tension gradients. Moreover, nonuniform evaporation fluxes caused capillary flows that moved from the center of the droplet to the contact line along the substrate. The surface tension gradient along the liquid-air interface had a dominant effect on the internal flow, which induced the thermal Marangoni flow. For the local heating cases, different flow patterns appeared, as compared with those that appeared from uniform heating. The flow directions changed according to the local heating conditions owing to the difference in the local surface tensions at the liquid-air interface.