Droplet impact dynamics on different wettable surfaces at moderate Weber numbers

Studies on droplets impacting surfaces with different wettabilities are of great significance for understanding interface phenomena and controlling droplet behaviors. In this work, the droplet impact processes on five surfaces from superhydrophilic to superhydrophobic at various Weber numbers are investigated through experimental and theoretical methods. The spreading and height factors and the oscillating cycle time and amplitude are analyzed. The results show that as the Weber number increases and the contact angle decreases, the impact droplet generally spreads faster and thus yields a larger maximum spreading factor. The maximum spreading time shortens with the increasing Weber number and contact angle. The height factors on different wettable surfaces show a similar decreasing trend in the early spreading stage. The larger the contact angle, the smaller the minimum height factor. As the Weber number increases, both the height factor and minimum height factor decrease. During the oscillating stage, the oscillation cycle time increases at a larger contact angle while it first increases and then decreases as the Weber number increases. Besides, the modified empirical formulas for predicting the maximum spreading factor and time are proposed to consider the effect of the surface contact angle following the principle of energy conservation. These findings can provide guidance for various practical applications of different wettable surfaces.