The effect of contact angle on droplet impact onto a solid surface

Contact angle has an important effect on the dynamic behavior of a droplet during its impact onto a solid surface; this behavior includes deformation, spreading and even rebounding of the droplet from the surface. In this paper, the effect of contact angle during droplet impact is studied and numerically simulated. First, using molecular-kinetic theory and based on the newly available research in this field, a correlation for determining the dynamic contact angle during droplet impact is obtained. This correlation is then implemented in the numerical model used to simulate the process. The results of the numerical model are compared with those of the experiments and those of the previous models (using an equilibrium contact angle; two advancing and receding contact angles). Model predictions agreed well with experimental measurements; this validated the model results for the droplet deformation during the impact. The results show that using the molecular-kinetic theory in modeling contact angle prevents the spreading-and-recoiling oscillations of the droplet that are seen when an equilibrium contact angle is used. Compared to the model with advancing and receding contact angles, the molecular-kinetic theory results in a better prediction of the time and value of the maximum and minimum spread of the droplet on the solid surface. Based on the results of the developed model, a critical maximum spread and two regions of droplet spreading or rebounding from the surface are defined. Next in the paper, an analytical model is presented to study the effect of contact angle on the droplet maximum spread. The results of this model show that for low Capillary numbers (Ca) with an increase of the equilibrium contact angle or a decrease of We number, the maximum spread of the droplet is decreased and becomes less dependent on Re number. In contrast, the maximum spread for high We numbers or low equilibrium contact angles is increased and becomes more dependent on Re numbe