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.)
Tag: cavitation
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)
London 2012: Diving Physics
Divers twist and spin gracefully in the air, but the highest marks come when they enter the water with little to no splash. This rip entry–named after paper-ripping sound characteristic of such a dive–is possible thanks to fluid dynamics. Any time a solid object enters a still liquid, it tears a cavity into the liquid. The smaller this cavity is, the less the liquid will rebound and splash when the cavity gets refilled. In diving, achieving a small splash requires a couple items. First, the diver will grab his hands over his head to form a flat surface. This will create the initial small cavity through which his body follows. When entering, the diver will keep his body straight and rigid, with arms pressed against his head; this adds stability to keep the diver from letting the force of striking the water at 35 mph affect his body’s form and create splash. Finally, the perfect dive enters vertical to the water surface. This ensures that all of the diver’s body finds its way into that cavity created by the hands without striking any undisturbed water. Once under the water, divers often extend their arms to generate enough drag to slow down quickly. All in all, the rip entry minimizes the cavity size and thus the splash, adding a great exclamation point to a beautiful dive. (Photo credits: Associated Press, Adam Pretty/Getty Images, Nigel Wade, Jed Jacobsohn)
FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out our previous posts on how the Olympic torch works, what makes a pool fast, the aerodynamics of archery, the science of badminton, and how cyclists “get aero”.
Hydrophobic Water Entry
Many factors can affect the size and shape of the splash when an object impacts water and wettability–the ability of a liquid to maintain contact with a solid–is one of them. Here a sphere coated in a hydrophobic (water-repellent) nano-layer impacts water, creating a large air, streaky air cavity and a substantial splash. Contrast this with the behavior of a hydrophilic sphere entering the water, and you can imagine divers might want to invest in some hydrophilic coatings prior to the London Olympics. (Video credit: L. Bocquet et al)
