Impact dynamics of water droplets on oil-covered dielectrowetting substrate: Effects of oil film thickness and surface wettability

Droplet impacting on solid or liquid film is a ubiquitous phenomenon in relevant processes such as spray cooling and inkjet printing on primer layers. In this study, the dynamics of water droplets impacting a solid substrate covered by a silicon oil film is investigated under the influences of the surface wettability of the dielectrowetting substrate (69.3°-117.4°) and the thickness of the silicon oil film (7.79–21.8 μm) at $\mathrm{We}=6.81$. A phase diagram is established, which reflects the relationships between the impact dynamic behavior, equilibrium contact angle, and the oil film thickness. The maximum spreading factor ${\beta }_{\max }$ and maximum out-of-plane height $H_{\max }/D_0$ for rebound of sub-droplet behaviors was smaller than 13 % and greater than 3 times than those droplet deposition behaviors, respectively. Finally, the retraction dynamics of the droplet after reaching its maximum spreading diameter is analyzed and discussed based on retraction rate ${\gamma }_w=v_r/D_{\max }={\sigma }_w(cos{\theta }_{\mathrm{rmin}}-cos{\theta }_{\mathrm{eq}})/f_k{D}_{\max }$ through the force balance at the three-phase contact line. Results of the study indicate that the surface wettability of the dielectrowetting substrate and thickness of the silicon oil film significantly affect the impact dynamics of water droplets on a solid substrate covered by the silicone oil film, especially in the retraction stage. This study sheds new light on controlling the dynamic impact of the droplets on silicon oil films, which is of importance to the control or optimization of processes such as inkjet printing on a primer layer.