About a year ago, we featured a video in which a fluid droplet bouncing on a vibrating pool demonstrated some aspects of the wave-particle duality fundamental to quantum mechanics. Work on this system continues and this new video focuses on studying some of the statistics of such a bouncing droplet–called a walker in the video–when it is confined to a circular corral. Using strobe lighting and capturing one frame per bounce, the vertical motion of these droplets is filtered out and the walking motion and the surface waves that guide it are captured. When the droplet is allowed to walk for an extended time, its path appears complicated and seemingly random, but it is possible to build a statistical picture and a probability density field that describe where the walker is most likely to be, much the way one describes the likelihood of locating a quantum particle. Parallels between the physical macroscale system and quantum-mechanical theory are drawn. (Video credit: D. Harris and J. Bush; submission by D. Harris)
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Superfluid Vortices
Cooling helium to a few degrees Kelvin above absolute zero produces superfluid helium, a substance with some very bizarre behaviors caused by a lack of viscosity. Superfluids exhibit quantum mechanical properties on a macroscopic scale; for example, when rotated, a superfluid’s vorticity is quantized into distinct vortex lines, known as quantum vortices. These vortices can be visualized in a superfluid by introducing solid tracer particles, which congregate inside the vortex line, making it appear as a dotted line, as shown in the video above. When these vortex lines approach one another, they can break and reconnect into new vortices. These reconnections provoke helical Kelvin waves, a phenomenon that had not been directly observed until the present work by E. Fonda and colleagues. They are even able to show that the waves they observe match several proposed models for the behavior. (Video credit: E. Fonda et al.)
Dancing Droplet Clusters
When a fluid surface is vibrated, it’s possible to bounce a droplet indefinitely on the surface without the droplet coalescing into the pool. This is because each bounce of the droplet replenishes a thin layer of air that separates the droplet and the pool. If many droplets are added to the surface, as in the video above, a clustering behavior is observed, with many droplets gathering together. There is a limit, however, to the size of the cluster based on the amplitude of vibration. If vibrational amplitudes are pushed to the point of creating Faraday waves–standing waves on the surface of the pool–then large clusters of droplets can be suspended and sustained. (Video credit: P. Cabrera-Garcia and R. Zenit; via io9; submitted by oneheadtoanother)
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)
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.)
Leidenfrost Dynamics
When a liquid impacts a solid heated well above the liquid’s boiling point, droplets can form, levitating on a thin film of vapor that helps insulate them from the heat of the solid. This is known as the Leidenfrost effect. Here a very large Leidenfrost droplet is shown from the side in high-speed. A vapor chimney forms beneath the drop, causing the dome in the liquid. When the dome bursts, the droplet momentarily forms a torus before closing. The resulting oscillatory waves in the droplet are spectacular. The same behavior can be viewed from above in this video. (Video credit: D. Soto and R. Thevenin; from an upcoming review by D. Quere)
Microgravity Water Balloons
When a water balloon pops in microgravity, waves propagate from the initial point of contact and the final point of contact (where the balloon skin peels away). As these waves come inward toward one another, the water is compressed from its original potato-like shape into a pancake-like one. In most cases, surface tension will provide a damping force on this oscillatory motion, eventually making the water into a sphere. On Earth, in contrast, a water balloon seems to hold its shape after popping. This is because the effect of gravity on the water is much larger than the effect of the propagating waves. This is one reason that it is useful to have a laboratory in space! Without a microgravity environment, it is much harder to study and observe secondary and tertiary-order forces on a physical event. (Video credit: Don Pettit, Science Off The Sphere)
Vapor Cone
This stunning National Geographic photo contest winner shows an F-15 banking at an airshow and a array of great fluid dynamics. A vapor cloud has formed over the wings of the plane due to the acceleration of air over the top of the plane. The acceleration has dropped the local pressure enough that the moisture of the air condenses. Some of this condensation has been caught by the wingtip vortices, highlighting those as well. Finally, the twin exhausts have a wake full of shock diamonds, formed by a series of shock waves and expansion fans that adjust the exhaust’s pressure to match that of the ambient atmosphere. (Photo credit: Darryl Skinner/National Geographic; via In Focus; submitted by jshoer)
Atomizing
High-speed video reveals the complexity of fluid instabilities leading to atomization–the breakup of a liquid sheet into droplets. A thin annular liquid sheet is sandwiched between concentric air streams. As the velocity of the air on either side of the liquid sheet varies, shear forces cause the sheet to develop waves that result in mushroom-like shapes that break down into ligaments and droplets. Quick breakup into droplets is important in many applications, most notably combustion, where the size and dispersal of fuel droplets affects the efficiency of an engine. (Video credit: D. Duke, D. Honnery, and J. Soria)
The Veil Nebula
There is no grander scale for the observation of fluid dynamics than that of the astronomical. Here Hubble astronomers discuss the formation of the Veil Nebula, a supernova remnant formed some 5,000-10,000 years ago. Wisps of gas and plasma remain, creating stunning astronomical landscapes that are the result of shock waves, turbulence, diffusion, and other processes familiar to us here on Earth. (Video credit: ESA/Hubble)
