Marangoni Effects Control Flow in a Heated Evaporating Sessile Droplet

This work investigates how internal flow in a steadily fed sessile water droplet on a heated substrate shifts between buoyancy-driven and Marangoni-driven convection. Using COMSOL Multiphysics 6.2, two regimes are simulated: one with only buoyancy forces and one that also includes a temperature dependent surface-tension gradient at the free surface. At substrate temperatures near 30^◦C, the droplet develops a single circulation cell typical of buoyancy-controlled flow. As the substrate temperature increases towards 40^◦C, strong Marangoni stresses appear, producing multiple counter-rotating vortices and increasing the characteristic velocity by roughly an order of magnitude. This clear transition to Marangoni-dominated transport enhances internal mixing, redistributes heat, and modifies the evaporation pattern at the interface. The results identify the temperature range where Marangoni forces overtake buoyancy and provide quantitative guidance for engineering thermally driven droplet flows in microfluidics, thermal management, and heat-assisted deposition processes.