Micro-particle image velocimetry visualization study of thermal Buoyant-Marangoni flow in microtubes

Abstract

The coupled momenta induced by thermal effects during evaporation near liquid–vapor interfaces cause complex three-dimensional flow structures, called thermal buoyant-capillary flows. In this study, we investigated the coupled flow mechanisms, observing the internal flow structure of evaporating micro-mini scale menisci in microtubes (680–1560 μm) by the micro-particle image velocimetry (PIV) technique. The horizontal and vertical motions of 1-μm fluorescent particles in volatile ethanol was visualized to obtain the shape of the flow in high-resolution (10–20 μm resolution). The evaporation rate (2–1800 ng/s) and environmental temperature (20–50 °C) were controlled using the Joule heating method. We discuss the effects of varying the thermal conditions and tube size on the position and strength of the vortices. In addition to the experimental work, a simplified numerical simulation was also carried out to estimate the thermal properties that were not measured during experiment. As a result, it was explained that the vortices near the wall of the upper tube are initialized by weak thermal effects from the wall, and higher heating condition cause them to move down the tube and enhance their vorticity by dominant buoyancy effect. In addition, vortices in tubes with large diameters are weaker and their motion is delayed due to the increase in gravitational effects.