Non-monotonic effect of ethanol concentration on the spreading of an ethanol-water binary droplet impact on a supercooled surface

We investigate the effect of ethanol concentration on the spreading dynamics of an ethanol-water binary droplet impact on a supercooled surface. The maximum spreading factor, comprised of the maximum internal spreading factor and dimensionless fingering length, decreases and then increases with the ethanol concentration. It is attributed to the change in ethanol concentration which varies liquid viscosity and surface tension directly, and surface cooling which lowers the droplet temperature and thus varies liquid properties that further affect droplet spreading. The maximum internal spreading factor which is also non-monotonic with the ethanol concentration dominates the maximum spreading. The heat transfer analysis on droplet spreading shows that the temperature reduction of the droplet from impact to maximum spreading is non-monotonic dependent on the ethanol concentration, while shows different trends with surface cooling at various ethanol concentration. Deviations of the predicated maximum spreading factor, considering the droplet temperature reduction, obtained by reported correlations from the experimental ones in this study could be mainly due to the difficulty in accurately predicting the temperature-dependent spreading boundary layer thickness of the droplet. The maximum spreading time decreases and then increases with the ethanol concentration, while it keeps decreasing with the surface supercooling. A new correlation of the maximum spreading time is proposed by incorporating the coefficient of surface supercooling and capillary-inertial time of the droplet which involves density and surface tension.