Water droplet impact on perfluoropolyether-based liquid-like surfaces

Water droplet impact on water-repellent surfaces is of relevance to many important engineering applications. Although the droplet impact behavior on water-repellent surfaces has been extensively investigated, in particular, textured superhydrophobic surfaces, that on water-repellent liquid-like surfaces has been barely reported. In this study, we experimentally investigated the oblique impact of water droplets on a surface with a liquid-like perfluoropolyether coating using a high-speed camera. The direct observation showed that all the impinging water droplets released from various heights exhibited similar outcomes, including an inelastic collision without rebound as well as a continuous sliding over the liquid-like surface. The liquid-like perfluoropolyether molecular chain on the surface was suggested to behave like a high viscosity oil layer that could cause more intense energy dissipation during the impact process. The crucial impact parameters of the maximum tangential droplet extension, the spreading time and the sliding length were further analyzed quantitatively by the methodology widely adopted in the exploration of the impact behavior in previous studies. It was indicated that the process of the impinging droplet spreading on the liquid-like surface was more likely dominated by the competition between the inertial and capillary forces although the increased viscous dissipation arising from the liquid-like layer significantly suppressed the rebound of the impinging droplet. Compared to superhydrophobic surfaces, the liquid-like surface could generate a stronger friction force for the slide motion of the impinging water droplet. The triboelectric effect arising from the water-solid contact was proven to inhibit the retraction of the impinging droplet with relatively higher impact velocities. This study is expected to improve the fundamental understanding of the droplet impact process and facilitate us to make more rational designs of liquid-like surfaces for relevant applications.