Hypothesis: Volatile binary liquid samples on wetting substrates are known to undergo either spreading or contraction tendencies, as a result of solutal Marangoni stresses due to differential volatility. Enhanced spreading is commonly thought to occur when the lower surface tension component is more 'volatile', while contraction is expected otherwise. We seek to test the limits of this scenario for various configurations such as sessile drops with free or pinned contact lines, without or with microparticles, and tears-of-wine menisci.
Experiments: We consider isopropanol- and ethanol-water mixtures, important in numerous applications. We conduct interferometric experiments with sessile droplets for multiple combinations of the initial concentration and controlled ambient humidity (water vapour only), essentially covering the entire range of these parameters. Experiments are also carried out for other configurations mentioned above.
Findings: Contraction regimes are found in certain situations where spreading is expected, despite the alcohols being more volatile than water. Furthermore, regime reversals occur between cases with different initial liquid concentrations even at zero humidity, and are not necessarily associated with the existence of an azeotropic composition. Such surprising observations are rationalized by a simple model highlighting the often overlooked role of the diffusion coefficient ratio of the two vapours in conjunction with the non-ideality of the mixture. Our picture of the phenomenon is demonstrated to be universal for all configurations studied.
Keywords: Contact lines; Diffusion-limited evaporation; Interferometry; Non-ideal binary mixtures; Particle deposition; Sessile droplets; Solutal Marangoni effect; Tears of wine.
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