Spreading and evaporation of sessile drops comprising binary mixtures*

Spreading and evaporation of a binary mixture sessile drop from a heated substrate is a complex process governed by the delicate balance between capillary stresses, evaporation, hydrodynamic flow, mass diffusion and surface tension, with both thermal and solutal Marangoni stresses present. We examine the behaviour and stability of volatile wetting ethanol-water drops deposited onto heated substrates using both experimental and modelling approaches. Our one-sided model uses lubrication theory to obtain a base state which we then perturb and assess the stability using a linear stability analysis evoking the quasi-steady-state approximation. Evolution equations are derived for the film height, temperature and concentration field assuming that the mixture comprises two ideally mixed volatile components with surface tension linearly dependent on temperature and concentration. Contact line movement is permitted by releasing the drop over a precursor film, avoiding the singularity at the contact line. Our simulations and experiments indicate that concentration gradients give rise to super-spreading and contact line instabilities, not previously seen in pure fluids. Results from our model qualitatively and quantitatively agree with experiments.