The way a sphere enters water depends on its size, speed, and surface properties. A hydrophilic (water-attracting) sphere behaves differently than a hydrophobic (water-repelling) one. But what happens when the object’s surface properties aren’t uniform?
That’s the situation we see above. The dark line marks the two hemispheres of the sphere and their differing surface properties. To the left, the sphere is hydrophilic; to the right, it is hydrophobic. When the sphere hits the water, both the splash and underwater cavity quickly become asymmetric. On the hydrophobic side, the cavity wall is smooth, but the cavity is rough on the hydrophilic side. In the end, the asymmetries create a horizontal force that pushes the sphere sideways. (Image and research credit: D. Watson et al.)
