黏弹性液滴撞击超疏水表面的反弹行为研究

BOUNCING DYNAMICS OF IMPINGING VISCOELASTIC DROPLETS ON SUPERHYDROPHOBIC SURFACES

水滴撞击超疏水表面后通常会发生完全反弹.大量研究表明,通过在液滴中添加少量聚合物可有效抑制水滴的撞击反弹行为.尽管该方法已被广泛应用于工农业生产中,但对黏弹性液滴撞击反弹行为的抑制机理认识仍不够全面.采用高速摄影技术,开展了聚氧化乙烯(PEO)水溶液液滴撞击不同微纳复合结构超疏水表面的实验研究.研究结果表明:添加高分子可增强流体的黏弹性及其与表面的黏附作用,因而完全反弹行为仅发生在低PEO浓度液滴的撞击实验中,且随着超疏水表面微结构间距的增大,发生完全反弹的上限和下限韦伯数阈值均减小.在低韦伯数反弹区域,撞击液滴始终处在微结构顶部,因而接触时间随韦伯数的增加而减小,这与平整超疏水表面上的现象相似;在高韦伯数反弹区域,液体会部分浸润微结构表面,因而接触时间将随韦伯数的增加而增加.此外,所有反弹液滴的恢复系数随韦伯数的增加呈线性下降趋势,且下降速率随微结构间距的增大而减缓.基于界面力学和能量转换分析,对实验中观测到的黏弹性液滴撞击反弹动力学行为给予了定性解释.

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.