Abstract
Water entry studies traditionally employ homogeneous projectiles of varying impactor shape, entry speed, and surface roughness. Surface heterogeneity is yet another means to manipulate splash dynamics. In this experimental study, we systematically investigate the water entry of smooth, free-falling, hemispherically coated spheres for Froude numbers in the range of . Hydrophilic spheres are hemispherically coated with a hydrophobic compound and in-turn produce deep seal cavities, provoke changes in super-surface splash features, and alter sphere trajectories. Generally, flow separation is initialized when hydrophobic surfaces make contact with the fluid, leading to air-entrainment across the range of entry speeds and impact orientations on test. Cavity formation induced by the hydrophobic portion of a hemispherically coated sphere promotes flow separation across the hydrophilic surface at impact velocities well below the threshold of 8 m/s required for air-entrainment by completely hydrophilic spheres. Spheres having partially hydrophilic and partially hydrophobic surfaces entering the fluid simultaneously, experience asymmetric cavities and horizontal forces that result in lateral migration from straight-line trajectories. Such observations augur well for water entry applications where the coupled dynamics of flow separation and passive trajectory control are desirable.
- Received 16 April 2020
- Accepted 1 April 2021
DOI:https://doi.org/10.1103/PhysRevFluids.6.044003
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