Mechanism of sessile water droplet evaporation

The energy transport mechanisms during the evaporation of sessile water droplets have been investigated. Steady-state evaporation experiments were conducted on substrates of Cu, Au(111) and PDMS. The buoyancy-driven convection was suppressed by maintaining the droplets' base temperature just less than 4$^{\circ}$C while evaporation cooled the liquid-vapor interface to lower temperatures. The temperature fields were measured in solid (only in Au(111) experiments), liquid and vapor phases. On all three substrates, the energy balance at the liquid-vapor interface showed that thermocapillary convection transported the major portion of the energy required for the evaporation. It transported up to 98\% for Cu, up to 87\% for Au(111) and up to 72\% for PDMS. The role of thermocapillary convection is dominant close to three-phase contact line where most of the evaporation occurs. The experiment on Au (111) showed that of the energy supplied by the solid substrate, only a small portion is transported perpendicular to the solid-liquid interface to the bulk liquid phase. A much larger proportion is conducted through the adsorbed layer at the solid-liquid interface to the three-phase contact line where it is distributed by thermocapillary convection over the liquid-vapor interface and consumed by the phase change process.