Influence of superhydrophobic area occupancy and impact angle on the water entry dynamics of spheres

Abstract

In this work, we reported a kind of deflecting air cavity generation strategy by controlling the superhydrophobic (SHB) area occupancy and the impact angle of water entry. The influence of SHB area occupancy and the impact angle on the water entry dynamics and drag reduction characteristics of spheres are also explored through both experimental and theoretical analysis. For a hemispherically coated sphere, it is found that the formed air cavity would reach a maximum deflecting angle when the impact angle of water entry is 90°. With the increasing impact angle, the deflection displacement of the hemispherically coated sphere in the horizontal direction first increases and then decreases. When the impact angle is 90°, the deflection displacement reaches the maximum. The deflection displacement of the SHB region-modulated sphere in the horizontal direction has the same variation trend. Moreover, the SHB region-modulated sphere exhibits different air cavity morphologies (no cavity, transition state seal, deep seal, and surface seal) at different impact velocities for impact angles of 0° and 180°. The air cavity pinch-off depth and pinch-off time first increase and then stabilize as the SHB area occupancy increases, regardless of whether the impact angle is 0° or 180°, and the value of ( Zpinch − Zp)/ Zpinch is not affected by the SHB area occupancy (α ∼ 0°). Finally, we demonstrate that SHB region-modulated spheres all move faster than the original spheres, and the sphere with a SHB area occupancy of 0.25 ( α ∼ 180°) is able to reduce the drag reduction coefficient to 0.055.