Controlling the wetting and evaporation dynamics of non-ideal volatile binary solutions

Area of Science:

• Fluid dynamics
• Physical chemistry
• Surface science

Background:

• Binary liquid mixtures on wetting substrates can spread or contract.
• Solutal Marangoni stresses from differential volatility typically dictate these behaviors.
• Enhanced spreading is usually linked to the more volatile component having lower surface tension.

Purpose of the Study:

• To investigate the limits of spreading/contraction tendencies in volatile binary liquid mixtures.
• To test these phenomena in various configurations: sessile drops (free/pinned contact lines), with/without microparticles, and tears-of-wine menisci.
• To challenge the conventional understanding of volatility-driven spreading.

Main Methods:

• Experiments with sessile droplets of isopropanol-water and ethanol-water mixtures.
• Utilized interferometric techniques to observe droplet behavior.
• Varied initial concentrations and ambient humidity (water vapor only).
• Examined additional configurations like tears-of-wine and microparticle-laden drops.

Main Results:

• Observed contraction regimes even when the more volatile component (alcohol) was expected to cause spreading.
• Identified regime reversals between different initial concentrations, independent of humidity or azeotropic composition.
• Demonstrated that the diffusion coefficient ratio of vapors and mixture non-ideality are critical factors.

Conclusions:

• The interplay between vapor diffusion coefficients and mixture non-ideality significantly influences spreading/contraction.
• The established model successfully explains the observed phenomena across all tested configurations.
• Challenges the simplistic view that volatility alone determines liquid film behavior.