On jet impingement on the underside of complex surfaces

Interaction of multiphase flow boundary layers with surfaces of complex distributed roughness and hydrophobic properties cannot be reliably predicted, despite their potential for a wide range of applications from frictional drag reduction to heat exchangers. Superhydrophobic surfaces (SHS) in particular are of great interest because of their ability to trap small air pockets (plastrons) between the liquid-solid interface and hence modify heat and momentum transport. However, a thorough understanding of the heterogeneous wetting mechanism on SHS is lacking due to the complicated nature of the surface. To glean physical insight, we first focus on a repeatable experiment observing the water patch topology resulting from a vertically impinging jet on the underside of various surfaces. Weber numbers ranged from 50 to 800, and impingement on 9 distinct surfaces with equilibrium contact angles ranging from 40 to 150° was examined. The equivalent sand-grain roughness (0.06 < k_s < 40 μm) for each surface was also measured. Referencing the Cassie-Baxter theory, the role of surface chemistry and geometry on wetting characteristics was explored through an energy argument that compares the energies associated with the attached and detached states of the previously impinged liquid film.