Evaporation of sessile drops is of interest in academic and industrial research due to applications such as ink-jet printing, biological and chemical assays, thin-film coatings, DNA depositions, efficient electronic cooling. Understanding these sessile drops' evaporation dynamics involves a comprehensive investigation of contact line dynamics, thermal field, and flow field. The interplay between heat conduction in the drop and evaporative cooling due to the release of latent heat can result in significant temperature difference within the drop leading to internal and external convective flows. The internal fluid motion can be either buoyant convection (due to density gradient) or Marangoni convection (due to surface tension gradient).
In this study, we utilize infrared thermography to study thermal field evolution during evaporating pure water drops. First, we systematically study the role of Marangoni convection in the evaporation kinetics of pure water drops, considering the influence of the substrate temperature and wettability. For a substrate temperature of 30 oC, the thermal field reveals a colder liquid at the apex of the drop with hotter liquid near the contact line, similar to previously reported studies. However, for a substrate temperature of 80 oC, the higher evaporation rate will result in larger evaporative cooling, leading to a more vigorous emergence of internal convective flows, a cold spot as convective cell the liquid-vapor interface is observed. The thermal field differences with varying substrate temperatures are attributed to stronger Marangoni flow at high substrate temperatures. An essential outcome of the present study is the correlation between the evolution of contact line dynamics and the thermal field, which generally are studied independently. Following this, we consider a broad range of experimental parameters such as substrate wettability, substrate temperature, the initial volume of the drop, and ambient relative humidity resulting in a wide range of evaporation rates, affecting the strength of internal convective flows. We report four distinct trends in the evolution of interfacial temperature difference depending on the presence and duration of the convective cell's presence, which are elucidated by discussing the evolution of maximum and minimum temperatures at the interface. Lastly, we demonstrate the limitation of the previously reported diffusion-only model in describing the evaporation kinetics of heated drops.