BOUNCING DYNAMICS OF IMPINGING VISCOELASTIC DROPLETS ON SUPERHYDROPHOBIC SURFACES

Impinging water droplets on superhydrophobic surfaces always completely rebound. A substantial amount of research indicates that adding tiny amounts of polymers into liquid droplets can effectively suppress the rebound behavior of water droplets upon impact. Though this method has been widely applied in industrial and agricultural areas, the understanding of the suppression mechanism for the bouncing behavior of viscoelastic liquid droplets remains uncomprehensive. Herein, high-speed photography techniques were employed to experimentally study the impact of aqueous poly(ethylene oxide) (PEO) droplets on superhydrophobic surfaces with different micro-nano composite structures. It is indicated that adding polymer can enhance the viscoelasticity of the fluid and its adhesion to the surface as well. Therefore, the complete rebound behavior only occurs for the droplets of low PEO concentration. Moreover, both the upper and lower Weber number (We) thresholds for complete rebound decrease with the increasing microstructure spacing of the superhydrophobic surface. In the first rebound region of low We, the droplets always remain on the microstructure, leading to a decrease in contact time with increasing We, similar to that observed on flat superhydrophobic surfaces. In the other rebound region of high We, the liquid partially penetrates the microstructure surface, causing an increase in contact time with increasing We. The restitution coefficients of all rebounding droplets linearly decrease with the increasing We, and the decreasing rate slows down with the increasing microstructure spacing. By analyzing the interfacial mechanics and energy conversion during the impact, the dynamic behaviors of rebounding viscoelastic liquid droplets were qualitatively rationalized.