When a drop impacts a pool at very low velocity, a thin layer of air can be trapped between the drop and the pool. When this air film ruptures, a ring of microbubbles forms and expands. Multiple “bubble necklaces” can form if the film ruptures at several points. These rings travel outward until the film is completely destroyed, leaving a chandelier-like shape of microbubbles. See the phenomenon in action with one of the videos linked here. (Photo credit: S. T. Thoroddson et al.; see video at arXiv)
Tag: bubbles
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)
Jets from Hollows
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.)
Donut-Shaped Bubbles
Here researchers simulate rain-like droplet impacts with large drops of water falling into a tank from several meters. The momentum of such an impact is significantly higher than many other droplet impact examples we’ve featured. In this case, the coronet, or crown-like splash, caused by the collision collapses quickly, closing the fluid canopy around a trapped bubble of air. The remains of the coronet fall inward, preventing the development of the usual Worthington jet associated with droplet impacts. Instead, the air bubble takes on an unstable donut-like shape. (Video credit: M. Buckley et al.)
Following a Breaking Wave
It’s fascinating to sit on the beach and watch the waves roll in and break, but rarely do we get a view like the one in this video. Here researchers have created a breaking wave in a wave tank and recorded the wave as it travels the length of the tank with a high-speed camera moving at the same speed as the wave crest. This perspective, moving alongside the fluid, is a Lagrangian coordinate system; if one instead stood still and watched the wave roll past, it would be an Eulerian measurement. Traveling with the wave, we can see how a lip forms on the wave crest, then rolls down, capturing a tube of air. As water begins to flow over the lip, perturbations grow, causing ripples in the laminar curtain. Then the water strikes the main wave and rebounds turbulently, creating a familiar white cap. In the second half of the video, the process is shown from above, highlighting the entrainment of air and the creation of the bubbles that form the white cap of a breaking wave. (Video credit: R. Liu et al)
Peering Inside the Kettle
Here natural convection is explored experimentally in a quasi-2D environment. The researchers demonstrate how this phenomenon, which is much like that seen in a boiling pot, can be investigated by measuring the refractive distortions caused by the thin heated fluid layer. They also demonstrate types of boiling that can occur. Typically, bubbles nucleate at the heated surface and then rise to pull hot fluid with them. At high enough temperatures above the liquid’s boiling point, however, an unstable layer of vapor can form over the heated surface. This “boiling crisis” or critical heat flux produces a marked reduction in heat transfer due to the insulation provided by the vapor layer. (Video credit: S. Wildeman et al.)
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.)
Air Entrainment
When a liquid jet falls into a pool, air is often entrained along with the liquid, creating a cavity and, often, bubbles. Shown above is video of a low-speed laminar jet entering a quiescent pool. The jet appears to entrain a thin film of gas, which then breaks up in a three-dimensional fashion, despite the symmetry of the incoming jet. As the speed of the incoming jet is increased and turbulence is introduced, the resulting air entrainment becomes violent and chaotic. For additional information and videos, see Kiger and Duncan 2012 and their supplemental videos. (Video credit: K. Kiger and J. Duncan)
Liquid Lenses
Here astronaut Andre Kuipers demonstrates fluid dynamics in microgravity. A roughly spherical droplet of water acts as a lens, refracting the image of his face so that it appears upside down. The air bubble inside the droplet refracts the image back to our normal perspective again. (Photo credit: Andre Kuipers, ESA; via Bad Astronomy)
Boiling Without Bubbles
Water droplets sprinkled on a sufficiently hot frying pan will skitter and skate across the surface on a thin layer of vapor due to the Leidenfrost effect. When a solid object is much warmer than a liquid’s boiling temperature, the surface is surrounded by a vapor cloud until the solid cools to the point that the vapor can no longer be sustained. Then the vapor breaks down in an explosive boiling full of bubbles. Unless, as researchers have just published in Nature, the solid is treated with a superhydrophobic coating. The water-repellent surface prevents the bubbling, even as the sphere cools. The technique could be used to reduce drag in applications like the channels of a microfluidic device. (Video credit: I. Vakarelski et al.; see also Nature News; submitted by Bobby E)
