Insights into the evolution of the thermal field in evaporating sessile pure water drops

Abstract

We investigate the evolution of the thermal field during evaporation, a fundamental aspect of evaporating sessile drops. With numerous reports in the literature investigating the contact line dynamics, we aspire to identify generalized features in the evolution of the thermal field and ultimately correlate these with the contact line dynamics. Considering a broad range of experimental parameters such as substrate wettability, substrate temperature, initial volume of the drop, and ambient relative humidity results in a wide range of evaporation rates, in turn affecting the strength of internal convective flows. Infrared thermography is utilized to extract the thermal field at the liquid–vapor interface, and optical imaging is used to record the evolution of drop shape during evaporation. We observe that the onset and presence of a convective cell as a cold spot at the interface highlights a non-axisymmetry in the thermal field. In consequence, a hitherto unreported asymmetry in the internal flow field is observed, as evidenced by the particle image velocimetry. Among the multitude of experiments conducted, we report four distinct trends in the evolution of interfacial temperature difference depending on the presence and duration of the presence of the convective cell, which are elucidated by discussing the evolution of maximum and minimum temperatures at the interface. The interplay between heat conducted into the drop and heat released due to evaporation can result in a momentary decrease in temperature of the drop, which is not reported previously. Lastly, a theoretical estimate for the temperature difference within the drop is extracted using vapor diffusion model and energy balance during evaporation. Comparison of this theoretical temperature difference with experimental observations highlights the influence of internal convective flows in homogenizing the thermal field within the drop.