Marangoni convection in an evaporating water droplet

Abstract

The existence of Marangoni flow in an evaporating pure water drop has remained an open question. In this study, we report the occurrence of Marangoni convection within a water drop resting on a copper rod as it evaporates into the air. Through careful preparation of the test liquid suspension, we have observed convective structures inside the water drop. We also developed a mathematical model representing the evaporation system and solved it numerically to further strengthen support for the experimental data. Our study is different from previous studies on droplet evaporation in the sense that we allowed the droplet to evaporate spontaneously in the normal laboratory condition without heating the substrate. The substrate was neither externally heated nor did radiation from the illumination light stimulate a Marangoni flow. The velocities in the midsection of the droplet were measured using particle tracking velocimetry in both the sessile and pendant configurations. The typical velocities in the center of the droplet were found to be 10 mm/s experimentally and even higher velocities (about double that) were predicted by the mathematical model. This high value stands out among the thermocapillary-driven flows observed in an autonomously evaporated drop of water under ambient conditions.