Directional migration of an impinging droplet on a surface with wettability difference

On designed heterogeneous surfaces, the lateral motion of a droplet can be controlled without requiring an external energy input. However, it is still a challenge to elaborately manipulate the self-migration behaviors of an impinging droplet because of a lack of fundamental understanding of these behaviors. Through direct numerical simulation, a wide range of parametric studies are conducted to investigate the effect of the Weber number (We), wettability difference, and offset impinging on the four phases-asymmetric spreading, directional retracting, detaching, and migrating-of droplets impacting heterogeneous surfaces. The results indicate that asymmetric spreading and directional retracting toward the hydrophilic area are induced by the unbalanced net forces acting on the three-phase contact line. The increase in the wettability difference leads to the increase in the migration distance from the borderline, the decrease in the droplet bouncing height, and the decrease in the contact time between the liquid and the nonwetting area. An increasing We influences the spreading diameter and the migration distance, but no significant difference can be observed during the retracting and detaching stages for moderate We owing to the combined effect of the spreading rate and the surface tension force. In addition, offset impinging plays a major role in droplet spreading and deformation. These physical insights provide developing guidelines for the design of surfaces to manipulate the behaviors of impinging droplets.