Self-induced Marangoni flow in evaporating alcoholic solutions

The self-induced Marangoni convection in alcoholic solutions is the subject of the present experimental investigation. Pure ethanol and its mixtures with 5%, 10% and 20% in weight of water are presented and discussed. In particular, Marangoni flow in horizontal pipes from 100 to 1000 μm inner diameter is studied. Vortex spinning frequency, average particle tracers velocity and evaporation rate are measured and discussed. The evaporation rate increases and the evaporation flux decreases at bigger tube sizes in line with previous investigations; pure ethanol has higher evaporation rate and flux than ethanol/water mixtures. The spinning frequency and the average tracer particles velocity decrease for increasing water content in the mixtures. All of these findings are due to evaporative cooling effect which is higher at the meniscus wedge (where the triple-line region is found) than at the meniscus center; this causes a difference in temperature between the wedge and the center that generates a gradient of surface tension driving vigorous Marangoni convection, that has been reported and analyzed. The experimental results are explained on the basis of a numerical model including evaporation, vapor diffusion, heat and mass transfer from the liquid to the surrounding ambient and the Marangoni effects.