A bubble initiated near a free surface–like the air-water interface here–can generate some spectacular dynamics. Beginning at the far left, the expanding subsurface bubble causes a dome at the surface that sharpens into a spike. By Frame 3, the bubble is collapsing but overshoots and rebounds, which introduces the tiny instability in Frame 4 that grows in subsequent time steps to form the water skirt that surrounds the spike. Although generated entirely differently, the end result is reminiscent of the water sculptures made by artists like Marcus Reugels, Corrie White, Jack Long, and others. (Image credit: A. M. Zhang et al.)
Tag: cavitation
Turning Sound into Light
Sonoluminescence – the creation of light from sound – was discovered in the 1930s, and, due to the difficulty of obtaining direct measurements, the exact mechanism remains highly debated even today. The phenomenon typically takes place within a tiny cavitation bubble inside a liquid. When bombarded with ultrasonic sound, such a bubble will repeatedly expand and collapse. Once a bubble is established, the cycle can be kicked off by increasing the driving acoustic pressure. This will collapse the bubble, drastically increasing its pressure and temperature (up to thousands of degrees Kelvin) and causing the bubble to emit a pulse of light before the pressure imbalance causes it to expand again. Several theories exist as to how the light is generated, the leading one being that the high temperatures in the bubble ionize the noble gases within and that those free electrons emit light via thermal bremstrahlung radiation. Sonoluminescence happens outside the lab, too. Both the previously discussed pistol shrimp and the mantis shrimp generate such light-emitting bubbles when hunting. (Video credit: The Point Studios; suggested by Bobby E.)
Dendritic Designs
Imagine a thin layer of viscous liquid sandwiched between two horizontal glass plates. Then pull those plates apart at a constant velocity. What you see in the image above is the shape the viscous fluid takes for different speeds, with velocity increasing from left to right and from top to bottom. For lower velocities, the fluid forms tree-like fingers as air comes in from the edges. At higher velocities, though, there’s a transition from the finger-like pattern to a cell-like one. The cells are actually caused by cavitation within the fluid. When the plates are pulled apart fast enough, the local low pressure in the fluid causes cavitation bubbles to form just before the force required to remove the plate reaches its peak. (Photo credit: S. Poivet et al.)
Explosions Underwater
Underwater explosions are, in general, much more dangerous than those in air. This video shows an underwater blast at 30,000 fps. During the initial blast, a hot sphere of gas expands outward in a shock wave. In air, some of the energy of this pressure wave would be dissipated by compressing the air. Since water is incompressible, however, the blast instead moves water aside as the bubble expands. Eventually, the bubble expands to the point where its pressure is less than that of the water around it, which causes the bubble to collapse. But the collapse increases the gas pressure once more, kicking off a series of expansions and collapses. Each bubble contains less energy than the previous, thanks to the loss of pushing the water aside. (Video credit: K. Kitagawa)
Gravity’s Effect on Bursting Bubbles
In a gravitational field, the pressure in a fluid increases with depth. You can consider it due to the weight of the fluid above. Outside of scuba diving or hiking at altitude, this effect is not one typically given much thought. But what effect can it have at a smaller scale? This video shows the collapse and rebound of three initially spherical cavitation bubbles inside a liquid. Each bubble is created in a different gravitational field – one in microgravity, one in normal gravity, and one at 1.8x Earth gravity. The bubble in microgravity remains axisymmetric and spherical, but the two bubbles recorded in gravitational fields develop jets during rebound. Even at a scale of only a few millimeters, gravity causes an imbalance in pressure across the bubble that creates asymmetry. (Video credit: D. Obreschkow et al.)
Inside a Blender
The fluid dynamics of a commercial-quality blender amount to a lot more than just stirring. Here high-speed video shows how the blender’s moving blades create a suction effect that pulls contents down through the middle of the blender, then flings them outward. This motion creates large shear stresses, which help break up the food, as well as turbulence that can mix it. But if you watch carefully, you’ll also see tiny bubbles spinning off the blades. These bubbles, formed by the pressure drop of fluid accelerated over the arms of the blades, are cavitation bubbles. When they collapse, or implode, they create localized shock waves that further break up the blender’s contents. This same effect is responsible for damage to boat propellers and lets you destroy glass bottles. (Video credit: ChefSteps; via Wired; submitted by jshoer)
Surface Explosions
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)
Catastrophic Cracking from Cavitation
At your next party, you can break the bottom of a glass bottle with the palm of your hand and the power of fluid dynamics. As shown in the video above, striking the mouth of the bottle accelerates fluid at the bottom, lowering the local pressure below the vapor pressure and causing the formation of cavitation bubbles. When these bubbles collapse, they form very high temperatures and pressures for an instant, and it is this which can break the glass. (Video credit: J. Daily et al., BYU Splash Lab)
Cavitation in a Bottle
Sudden changes in the pressure or temperature in a liquid can create bubbles in a process known as cavitation. Underwater explosions are just one of the ways to induce cavitation in a liquid. As identified in the above video, the shock waves traveling through the liquid force a change in pressure that creates bubbles. When these bubbles collapse, the container is subjected to an enormous oscillation in pressure, which often results in damage. The same phenomenon is responsible for damage on boat propellers as well as this beer bottle smashing trick. Check out these other high-speed videos of cavitation in a bottle: (Video credit: Destin/Smarter Every Day; submitted by Juan S.)
The Supersonic Plonk
Everyone knows the familiar plonk of a stone falling into a pond but few realize the complexity of the physics. When a solid object falls into a pool, a sheet of liquid, the crown splash, is sent upward. Simultaneously, the object pulls a cavity of air down with it. As the water moves inward, this cavity is pinched, creating an hourglass-like shape reminiscent of the shape of a rocket’s nozzle. As the diameter of that pinched cavity shrinks, the velocity of the upward escaping air increases, resulting in the formation of an air jet moving faster than the speed of sound. This air jet is followed by a slower liquid jet that may rebound to a height higher than then original height of the dropped object. So next time you throw a stone into a pond, enjoy the knowledge that you’ve broken the sound barrier. (Photo credit: D. van der Meer; see also Physics World)
