  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.
