When a solid object impacts on a liquid a cavity typically forms, entraining air into the pool. But this behavior varies widely according to the surface of the solid as well as the fluid’s properties. This video shows a sphere impacting a highly viscous liquid. The sphere stops shortly after impact while the cavity continues expanding in its wake. With a fluid like water, a long and thin cavity will typically pinch off before the object is decelerated, causing bubbles to form. No such behavior here. Instead the wide cavity pinches off at the surface of the motionless sphere and begins its rebound upwards. It even appears to pull the sphere partially back towards the surface! (Video credit: A. Le Goff et al.)
Almost everyone has tried skipping rocks across the surface of a pond or lake. Here Professor Tadd Truscott gives a primer on the physics of rock skipping, including some high-speed video of the impact and rebound. In a conventional side-arm-launched skip, the rock’s impact creates a cavity, whose edge the rock rides. This pitches the rock upward, creating a lifting force that launches the rock back up for another skip. Alternatively, you can launch a rock overhand with a strong backspin. The rock will go under the surface, but if there’s enough spin on it, there will be sufficient circulation to create lift that brings the rock back up. This is the same Magnus effect used in many sports to control the behavior of a ball—whether it’s a corner or free kick in soccer or a spike in volleyball or tennis. (Video credit: BYU Splash Lab/Brigham Young University)
Underwater explosions often behave non-intuitively. Here researchers explore the effects of surface explosions by setting off charges at the air/water interface. Initially, an unconfined explosion’s blast wave expands a cavity radially into the water. This cavity collapses back toward the surface from the bottom up, ultimately resulting in a free jet that rebounds above the water level. Confined explosions behave very differently, expanding down the glass tube containing them in a one-dimensional fashion. The cavity never extends beyond the end of the glass tube, likely due to hydrostatic pressure. (Video credit: Adrien Benusiglio, David Quéré, Christophe Clanet)
When a fluid is vibrated, instabilities can form along its surface. With a sufficient amplitude, voids form inside the fluid and their collapse leads to a jet that shoots out from the fluid. A very different process leads to air cavities forming in a vibrated granular medium, but the jets produced are remarkably similar, as seen in this video. (Video credit: M. Sandtke et al.)
Bubbles rising through a viscous fluid deform and interact. As they collapse into one another, the lower bubble induces a gravity-driven jet that projects upward into the higher bubble. The more elongated the bubble, the faster the jet. The same behavior is seen in the rebound of a cavity at the free surface of a liquid. The authors suggest a universal scaling law for this behavior. (Video credit: T. Seon et al.)
