Spreading and retraction dynamics of sessileevaporating droplets comprising volatile binary mixtures

TL;DR

This paper develops a new model to predict the spreading and retraction of volatile binary mixture droplets on heated surfaces, validated by experiments showing ethanol-water droplets exhibit superspreading due to preferential evaporation.

Contribution

A novel one-sided lubrication model incorporating concentration-dependent properties and inertia effects for volatile binary droplets on heated substrates.

Findings

Increased solutal Marangoni stress enhances spreading.

Preferential evaporation causes superspreading and contact line instability.

Model shows good qualitative and quantitative agreement with experiments.

Abstract

The dynamics of thin volatile droplets comprising of binary mixtures deposited on a heated substrate are investigated. Using lubrication theory, we develop a novel one-sided model to predict the spreading and retraction of an evaporating sessile axisymmetric droplet formed of a volatile binary mixture on a substrate with high wettability. A thin droplet with a moving contact line is considered, taking into account the variation of liquid properties with concentration as well as the effects of inertia. The parameter space is explored and the resultant effects on wetting and evaporation are evaluated. Increasing solutal Marangoni stress enhances spreading rates in all cases, approaching those of superspreading liquids. To validate our model, experiments are conducted with binary ethanol-water droplets spreading on hydrophilic glass slides heated from below. The spreading rate is quantified, revealing that preferential evaporation of the more volatile component (ethanol) at the contact line drives superspreading, leading in some cases to a contact line instability. Good qualitative agreement is found between our model and experiments, with quantitative agreement being achieved in terms of spreading rate.