Anomalous Motion of a Sphere upon Impacting a Quiescent Liquid: Influence of Surface Wettability

The present study investigates the motion of a rigid sphere in a quiescent liquid following an impact from the air. An extensive set of experiments are conducted by varying parameters such as sphere diameter (d), density ratio (ρ/ρl), and impact Weber number (We). High-speed imaging technology is used to study the underwater behaviour of the sphere. According to prior reports, high-density (ρ/ρl \(\ge \) 7.926) spheres travel in a straight line. However, spheres with a lower density ratio (ρ/ρl \(\le \) 2.771) are observed to deviate from their vertical impact position. It is observed that the resulting deviation in the radial direction is random. Interestingly, the penetration depth is repeatable across various experiments, irrespective of the deviation in the radial direction. This anomalous deviation in the trajectory of the spheres is attributed to the rotation of the sphere, leading to asymmetric vortices in the wake. In the present work, we also show that this anomalous motion of the sphere can be controlled by modifying the wettability of the surface. By making the surface superhydrophobic, we observe the formation of a cavity in the wake of the sphere due to air entrapment, resulting in a straight trajectory for the sphere.