Characterization of Marangoni effect in non-isothermal falling liquid films of binary mixtures

The flow characteristics and heat transfer were investigated for the liquid films of water, ethanol aqueous solution, glycerol aqueous solution and aqueous Na2SO4 solution. By using a sensitive thermal imaging system, the temperature and liquid distributions of the film could be carefully studied to reveal the influence of Marangoni effect on the heat transfer and flow of the binary mixture films. Since the surface tension gradient and surface tension counteracted on the film, a surface tension factor was presented to describe the surface tension effect in the films and help designing the experiments. It was concluded to be the thermal Marangoni effect that caused the cooled films expanded and the heated films contracted. A novel expansion was found for the heated ethanol aqueous film, due to the concentration variation and so-caused solutal Marangoni flow that overcame the thermal Marangoni flow in the lateral direction of the film. Theoretical analysis also indicated that the solutal Marangoni effect was more effective than the thermal Marangoni effect. Furthermore, viscosity has a complex influence on the Marangoni flow. Larger viscosity generally caused greater temperature gradient, and thus enhanced surface tension gradient (Marangoni flow), meanwhile, all the liquid flow (including Marangoni flow) were retarded due to the more significant viscous resistance.