Internal flow in evaporating water drops: dominance of Marangoni flow

Abstract

The internal flow field in evaporating sessile water drops is experimentally investigated in the present work. The interdependency in the prevailing thermal field and the internal flow field is analyzed by simultaneous utilization of infrared thermography and particle image velocimetry. Experiments are conducted on a hydrophobic substrate while varying the substrate temperature between 25 and 60 ∘C, resulting in a significant variation in the strength of internal convection. For the case of a non-heated substrate, a monotonic variation in temperature along the liquid–vapor interface results in an axisymmetric flow field inside the drop. For heated substrates, the presence of a cold spot at the liquid–vapor interface due to the dominance of the Marangoni flow results in a non-axisymmetric flow field. In such a situation, two counter-rotating vortices inside the drop are visualized. Here, the velocities inside the drop are ∼ O(mm/s), where velocities of ∼ O(μm/s) are previously reported for buoyancy-dominated flows. Qualitative features in the internal flow field, such as the duration of the presence of the non-axisymmetric flow and the shift in the center of vortices, highlight more vigorous Marangoni convection in drops evaporating on substrates maintained at a higher temperature. Quantitative analysis of the flow field is presented in terms of the spatiotemporal evolution of velocity and vorticity inside the drop, which are further correlated to the evolution of the thermal field by analyzing the interfacial temperature difference. Further, by observing the deposition pattern of tracer particles formed after the evaporation of drops, the effect of variations in the internal flow field on deposition patterns is deduced.