Interfacial Properties Modulate Water Entry Dynamics For Spherical Projectiles

Rigid bodies impacting liquid pools incite splashing and air-entraining cavities that depend on impactor shape, entry speed, surface texture, and free surface conditions. In this body of work, we investigate the influence of interfacial properties on water entry dynamics for free-falling spherical projectiles. Our investigations translate to everyday fluid-structure interactions that are complex and generally conclude beyond the temporal acuity of the naked eye. High-speed videography between 1000 -- 3200 fps is used to digitize the water entry process and measure salient splash features under different initial conditions. We assess splashes arising from the impacts of free-falling hydrophilic spheres with thin, non-woven fabrics resting atop a liquid bath to ascertain alterations to splash crowns, air-cavities, and Worthington jets, when compared to impacts onto an unaltered, quiescent free surface. The inclusion of fabrics promotes air-entrainment for hydrophilic spheres, well below the impact-velocity threshold of 8 m/s otherwise required for cavity formation. Meager amounts of fabric amplifies splash metrics while providing the drag-reducing benefits of flow separation. Punctured fabrics suppress splash crowns normally seen for cavity-producing impacts while intact fabrics generate deeper cavities, higher Worthington jets, and more pronounced splash crowns. We proceed to modulate super-surface splash features, and alter sphere trajectories with impactor surface texture. When fluid flowing around the impactor encounters the hydrophobic surface, flow separation is tripped and air entrained across all entry speeds and impact orientations. We conclude this work by replacing solid impactors with liquid drops impacting passive supersurface particles, an experimental system inspired by the survival of water striders during rainfall. We show the stride\u27\u27s locomotive response, low density, resistance to wetting when briefly submerged, and the ability to regain super-surface rest state, render it impervious to impacting water drops. The compendium of new observations from our work augur well for water entry applications where the coupled dynamics of flow separation and passive trajectory control are desirable