Thermocapillary Migration of a Volatile Evaporating Droplet on a Non-Uniformly Heated Substrate

We investigate the thermocapillary migration of volatile alcohol droplets on a nonuniformly heated glass surface using a combined experimental and theoretical analysis. While the migration of nonvolatile droplets has been widely studied, the migration of volatile droplets─where thermocapillary motion is coupled with evaporation-induced mass loss─remains largely unexplored, to the best of our knowledge. Side-view imaging is used to capture droplet motion and evaporation, while infrared thermography records the temperature field from above. Experiments were performed using ethanol and isopropanol under identical conditions. On a uniformly heated surface, droplets remain stationary due to the absence of a surface-tension gradient along the contact line, resulting in zero net thermocapillary driving force. However, on a nonuniformly heated substrate, droplets consistently migrate from the hotter to the cooler region, driven by asymmetric thermocapillary stresses and resisted by viscous dissipation. Meanwhile, evaporation continuously reduces the droplet size, thereby influencing the migration dynamics. We systematically varied the droplet volume and the imposed temperature gradient to characterize the migration behavior. As the temperature gradient increases, droplet velocity initially increases to a maximum and subsequently decreases due to enhanced evaporation. A theoretical model was developed to predict the observed droplet migration dynamics. In addition, we also proposed a scaling law that collapses the droplet position and velocity onto universal master curves when normalized by the evaporation time scale and a thermocapillary migration length. The fundamental insights gained from this study are relevant to the design of microfluidic systems and hotspot cooling technologies.