Marangoni effects in the spreading of liquid mixtures on a solid

The spreading of binary liquid mixtures on Pyrex glass was experimentally studied, qualitatively and quantitatively, for a large number of liquids. The mixtures were classified according to the spreading behavior of the individual components and according to the expected direction of the Marangoni flow, which may be induced by the differences in volatilities and surface tensions between the components. The general trends of the results agree with previously reported data for other solid surfaces. The quantitative measurements led to novel conclusions regarding the rates of spreading of mixtures. Binary mixtures of completely spreading components spread faster than the individual components when the Marangoni flow is expected to coincide with the spreading direction. Spreading of such mixtures is delayed or arrested when the Marangoni flow is expected to oppose spreading, until a sufficient amount of the volatile component evaporates. The spreading rate of mixtures of only one completely spreading component may be insensitive to concentration when the Marangoni flow compensates for a decreased tendency for spreading. Many mixtures of two incompletely spreading components exhibit complete spreading when the Marangoni flow is expected to be in the direction of spreading. The effect of the component concentration is unsymmetric. Small concentrations of the less volatile component induce complete spreading of the mixture, while relatively high concentrations of the volatile liquid are required for the same purpose.