Conjugate heat and mass transfer in a binary sessile droplet on a hydrophobic plate: effects of plate temperature and component composition

The dynamics, heating and evaporation of sessile water-ethanol droplets on a heated aluminum-magnesium alloy AMg6 plate are investigated experimentally and theoretically. The structured hydrophobic surface of the plate was prepared using laser nanosecond texturing. Time dependencies of the diameter of the contact area, dynamic contact angle, and relative droplet volume at three plate temperatures are found. Three modes of droplet evaporation are identified: spreading (contact diameter (Dc) increases with time), pinning (Dc oscillates about an average value), and mixed mode (Dc decreases with time). The criteria for transition between these modes are suggested based on the maximal amplitude of oscillations of Dc in the pinning mode. The analysis of the experimental results is based on the solution of transient transport equations, using COMSOL Multiphysics and the assumption that Dc is constant which is justified by relatively short durations of spreading and mixed modes. It is demonstrated that the observed time dependencies of relative droplet volume for a wide range of ambient air humidities and plate temperatures agree with those predicted by COMSOL, but differ considerably from those predicted by the previously developed simplified one-dimensional model.