Mechanism of Sessile Water Droplet Evaporation: Kapitza Resistance at the Solid–Liquid Interface

The cooling of a Au(111) substrate by an evaporating water droplet has been quantitatively studied. Each droplet was maintained in a thermal steady state and of constant size by heating the droplet and injecting water at its base while removing vapor from an enclosing chamber. The thermal energy transported to the solid–liquid interface of the droplet was measured with thermocouples embedded in the substrate. Because of a large thermal (or Kapitza) resistance at the solid–liquid interface, only a small portion of the thermal energy was transported perpendicular to the solid–liquid interface to the bulk liquid. A much larger fraction—up to 87%—was transported parallel to the solid–liquid interface to the three-phase line where thermocapillary convection distributed this energy along the liquid–vapor interface to be consumed by the phase change process. For a given substrate, the Kapitza resistance is found to increase with the magnitude of the solid–liquid interfacial temperature discontinuity and the pressure in the liquid phase at the solid–liquid interface.