LAUSR

Abstract In this experimental investigation we use a localised heating source on the bottom side of a horizontally positioned square capillary tube of 1 mm side length, inside which an ethanol meniscus pinned at one of the tube mouth undergoes evaporation in still air. We use Infra-Red thermography to map the temperature distribution along the meniscus interface and Particle Image Velocimetry in the meniscus liquid phase to characterise the fluid motion. As already reported in the literature, the unheated case shows that there is an asymmetry in the temperature profile in vertical cross sections of the tube and this in turn distorts the Marangoni vortex structure. This asymmetry brought by gravity was thought to operate along the meniscus interface inside the tube; instead, we show in this study that the descending cold plume that develops around the capillary tube seems to be ultimately responsible for the temperature distribution at the meniscus wedge around the tube. When we impose a localised heating on the bottom side of the capillary tube, the temperature profile along the vertical diametrical section of the tube becomes symmetrical and the resulting flow pattern becomes also symmetrical with respect to the tube axis, as confirmed by the particle image velocimetry measurements. When more heating is introduced at the bottom side of the tube, the symmetry in the flow is lost again and the toroidal Marangoni vortex spins in the direction opposite to that of the unheated case. This heat and mass transfer problem seems to be diffusion controlled by the amount of vapor in the air surrounding the capillary tube for what concerns the evaporation; the shape and intensity of the Marangoni vortex in the meniscus liquid phase seems also dictated by what happens around the tube rather than inside of it.