Nonaxisymmetry and flow transition in evaporating water drops

Understanding the internal flow in evaporating sessile drops is of paramount importance in a myriad of applications such as ink-jet printing, surface patterning, and medical diagnostics. Marangoni flow driven by a gradient in surface tension is an essential internal flow mechanism, whose characteristics in evaporating water drops remain elusive in the literature. Here, by employing infrared thermography and particle image velocimetry, we show that the manifestation of Marangoni flow as a convective cell at the liquid-vapor interface results in a nonaxisymmetric internal flow field. Eventually, during evaporation, the flow transitions to a buoyancy-dominated regime, where an axisymmetric toroidal flow is observed. This transition marks a reversal in the flow along with an order of magnitude decrease in velocity. We corroborate this experimentally observed transition using previously reported analytical and scaling frameworks. Finally, we present hitherto unreported features correlating the three aspects of evaporating water drops, viz., contact line dynamics, thermal field, and internal flow field, which are generally investigated independently.

Automated summary

Understanding the internal flow in evaporating sessile drops is crucial in various applications. This study reveals the nonaxisymmetric internal flow field caused by Marangoni flow at the liquid-vapor interface, and the transition to an axisymmetric toroidal flow during the evaporation process.