Directional migration of impacting droplets on hydrophobic-superhydrophobic boundary

Directional transportation of droplets plays a crucial role in the fields of anti-fog, anti-icing, material transportation, and several other applications. Herein, the total directional transport of droplets on a surface with controllable wettability was numerically investigated by the level set method coupled with the volume of fluid method. The accuracy of the numerical simulation results was validated via high-speed photography experiments. The directional migration and morphological evolution mechanisms of droplets impacting on the superhydrophobic–hydrophobic interface were revealed. Moreover, the effects of the impacting position on the directional migration velocity, rebound height, and transport distance of droplets were systematically analyzed. Theoretical models were derived for predicting the transport distance and rebound height of droplets. The simulation results reveal that, for a droplet impacting on the superhydrophobic and hydrophobic interface, four stages exist, i.e., spreading, contraction, rebound, and directional migration, which differ from those for a droplet impacting on a normal surface. It is thus deduced that the adhesion length is a significant factor that affects the directional migration parameters. Moreover, there exists an optimal adhesion length for the impacting droplet, under which the transport distance can be maximized. The maximum transport distance of the droplet under the optimal adhesion length is 12 mm. Furthermore, the values predicted by using the theoretical models agree well with the actual ones, proving the feasibility of the prediction models. The results contribute to the fundamental theory of droplet directional migration and are valuable for related engineering applications.