Droplet Impact Dynamics on Lubricant-Infused Superhydrophobic Surfaces: The Role of Viscosity Ratio.

In this study, the spreading and retraction dynamics of impacting droplets on lubricant-infused PTFE surfaces were investigated through high-speed imagery. Superhydrophobic Polytetrafluoroethylene (PTFE) surfaces with randomly rough microstructures were prepared by sanding PTFE. Several silicone oils with different viscosities were infused into the structures of superhydrophobic PTFE surfaces. A glycerin and water solution was used for the impacting droplets. The viscosity ratio between the impinging droplet and infused oil layer was varied from 0.06 to 1.2. The droplet impact dynamics on lubricant-infused surfaces were found to change as the viscosity of the infused silicone oil layer was decreased. These changes included an increase in the spreading rate of the droplet following impact, an increase to the maximum spreading diameter, and an increase to the retraction velocity after the droplet reached its maximum diameter. These variations in the impact dynamics were most significant as the viscosity ratio became larger than one and are likely due to the reduction of viscous losses between the oil and water phases during the spreading and retraction of the impacting droplet. Using a scaling analysis which takes into account the role of energy dissipation in the impact dynamics, all the data for the maximum diameter of the droplet on lubricant-infused PTFE surfaces were found to collapse onto a single master curve. Finally, measurements of the dynamic advancing and receding contact angle were made during spreading and retraction of the droplet. These measurements showed the expected Cox-Voinov-Tanner scaling of contact angle for the high oil viscosity, low viscosity ratio lubricant infused surfaces. However, like the superhydrophobic surface, little changes in either the dynamic advancing or receding contact angle were observed for droplets spreading on the surface infused with the lowest viscosity oil.