Evaporation kinetics of pure water drops: Thermal patterns, Marangoni flow, and interfacial temperature difference

We report a systematic study on the role of Marangoni convection on the evaporation kinetics of pure water drops, considering the influence of heating regime and surface wettability. The Marangoni flows were induced via heating under constant wall temperature (uniform heating) and constant heat flux (local heating) regimes below the drops. To visualize the thermal patterns/flows emerging within the water drops we employed infrared (IR) thermography and we captured the evolution of the drop profile with a CCD camera to follow the evaporation kinetics of each drop. We observed a strong correlation between the temperature difference within the drop and the evolution of drop shape during different modes of evaporation ({i.e.} constant radius, angle or stick-slip) resulting in different Marangoni flow patterns. Under uniform heating, stable recirculatory vortices due to Marangoni convection emerged at high temperature which faded at later stages of the evaporation process. On the other hand, in the localized heating case, the constant heat flux resulted in a rapid increase of the temperature difference within the drop capable of sustaining Marangoni flows throughout the evaporation. Surface wettability was found to also play a role in both the emergence of the Marangoni flows and the evaporation kinetics. In particular, recirculatory flows on hydrophobic surfaces were stronger when compared to hydrophilic for both uniform and local heating. To quantify the effect of heating mode and the importance of Marangoni flows, we calculated the evaporative flux for each case and found to it to be much higher in the localized heating case. Evaporative flux depends on both diffusion and natural convection of the vapor phase to the ambient. Hence, we estimated the Grashof number for each case and found a strong relation between natural convection in the vapor phase and heating regime or Marangoni convection in the liquid phase. Subsequently, we demonstrate the limitation of previously reported diffusion-only} model in describing the evaporation of heated drops.