BYU Splash Lab–those breakers of bottles, skippers of rocks, spinners of eggs, students of soap films, masters of splashes, and all-around cool fluid dynamicists–have some fluids-themed, high-speed holiday greetings. Likewike, here at FYFD we’ll be spending the next week celebrating the physics and fluid dynamics of the winter holiday season! In the meantime, you can whet your appetite by brushing up on your cookie dunking techniques, watching how icicles form, and enjoying a good beverage. Stay tuned and happy holidays from FYFD! (Video credit: BYU Splash Lab/BYU News)
Videos
Huddling Penguins and Traffic Jams
Male emperor penguins have the unenviable task of incubating their eggs in temperatures as cold as -50 deg Celsius and winds of up to 200 km/h. To stay warm, the penguins form huddles of up to thousands of individuals. Observations in the wild show that these huddles move in a stop-and-go fashion, with changes propagating through the penguins like waves. Researchers adapted a model used for heavy traffic flow to describe the penguins’ motion. They found that motions like those found in observed penguin huddles could be initiated by slight movements of any penguin in the model huddle, regardless of its position; in other words, the huddle has no leader. They also found that the wave that travels through the penguins can align the huddle to uniform density or help two huddles merge. To learn more, check out the researchers’ video or their paper. (Video credit: D. Zitterbart et al./New Scientist; via J. Ouellette)
Collapsing Soap Bubbles
The colors of a soap film are directly related to their thickness. If a film becomes thin enough (~10 nanometers), it appears black. (Here’s why.) This video shows the thinning of a vertical soap film. Normally, this is a linear process, with gravity pulling the fluid downward and progressively thinning the film from top to bottom at a constant rate. At 0:20 a cold rod slowly contacts the film, adding a thermal driver for the film’s thinning. Two large counter-rotating convection cells form underneath the rod, with weaker secondary vortices in the lower corners of the film. This drastically increases mixing in the film. Gradually small black spots, indicating very thin areas of the film, form and advect. Eventually these spots stretch, forming long tails. The thinning of the film kicks up to an exponential rate until the film becomes uniformly thin. (Video credit: M. Winkler et al.)
Fluctuating Ferrofluids
Ferrofluids–liquids seeded with magnetically sensitive ferrous nanoparticles–demonstrate some beautiful and bizarre behaviors when exposed to magnetic fields. This video shows the reaction of a pool of ferrofluid to the magnetic field generated by an alternating current through a simple wire coil. At 1 Hz, the fluid response is not unlike the normal-field instability–the characteristic spikes–the fluid develops when exposed to a permanent magnet. But because field is fluctuating, the spikes pop out and fade again. At 10 Hz, the behavior gets even more interesting. As the frequency of the magnetic field’s oscillation increases, the time the fluid has to respond to changes in the magnetic field decreases. Eventually, one can imagine a point where the magnetic field oscillates faster than the molecules in the fluid can rearrange themselves to respond. It’s unclear if such a mismatch in timescales is the cause of the increasing violence of the ferrofluid’s response in the later clips or whether this results from an unmentioned change to the current through the coil. For something even wilder, check out Nick’s video of the ferrofluid’s response to music. (Video credit: N. Moore)
Vibrating Paint
Paint is probably the Internet’s second favorite non-Newtonian fluid to vibrate on a speaker–after oobleck, of course. And the Slow Mo Guys’ take on it does not disappoint: it’s bursting (literally?) with great fluid dynamics. It all starts at 1:53 when the less dense green paint starts dimpling due to the Faraday instability. Notice how the dimples and jets of fluid are all roughly equally spaced. When the vibration surpasses the green paint’s critical amplitude, jets sprout all over, ejecting droplets as they bounce. At 3:15, watch as a tiny yellow jet collapses into a cavity before the cavity’s collapse and the vibration combine to propel a jet much further outward. The macro shots are brilliant as well; watch for ligaments of paint breaking into droplets due to the surface-tension-driven Plateau-Rayleigh instability. (Video credit: The Slow Mo Guys)
Mushrooms Make Their Own Breeze
Mushrooms don’t rely on a stray breeze to spread their spores; they generate their own air currents instead. The key is evaporation. The mushroom cap contains large amounts of water, and, as this water evaporates, it cools the mushroom and the air next to it. This cool air is denser than the surrounding air, and so tends to spread out and convect. At the same time, though, the water vapor that evaporated from the mushroom is less dense than nearby air, which helps it rise. This combination of spreading and rising air carries spores away from the mushroom cap and, as seen in the video above, can combine to form beautiful and complex currents that spread the spores. (Video credit: E. Dressaire et al.)
North Dakota Ice Disk
Cold weather can create some wild fluid dynamics, so pay attention to your local rivers and waterfalls during the next cold snap. The video above comes from North Dakota where a combination of cold dense air and a stable river eddy created a spinning ice disk, roughly 16 meters in diameter. The disk forms as a collection of ice chunks–not one solid, spinning piece–because the ice formed gradually. As ice pieces form, they get caught in the river eddy and begin to spin as part of the disk, rather like dust and ice do in the rings of Saturn. Such formations are rare but not unheard of; here’s a video showing a similar disk as it grows. (Video credit: G. Loegering; via Yahoo and io9; submitted by Simon H and John C)
Put the Lid Down When You Flush
Hospital-acquired infections are a serious health problem. One potential source of contamination is through the spread of pathogen-bearing droplets emanating from toilet flushes. The video above includes high-speed flow visualization of the large and small droplets that get atomized during the flush of a standard hospital toilet. Both are problematic for the spread of pathogens; the large droplets settle quickly and contaminate nearby surfaces, but the small droplets can remain suspended in the air for an hour or more. Even more distressing is the finding that conventional cleaning products lower surface tension within the toilet, aggravating the problem by allowing even more small droplets to escape. To learn more, see the Bourouiba research group’s website. (Video credit: Bourouiba research group)
Pathlines vs. Streaklines
When considering fluid motion, there are many ways to describe trajectories through the flow. One is the pathline, the trajectory followed by an individual fluid particle. Imagine releasing a rubber duck down a stream. Following the duck’s position over time would give you a pathline. Now imagine that instead of releasing a single rubber duck you release lots of them – say one every half-second from the exact same starting spot. You would end up with a line of rubber ducks stretching downstream, each of them sharing the same origin but with a different starting time. This is called a streakline. Would the streakline of rubber ducks follow the same trajectory as the lone duck? Not if the flow is time-varying! In fact, for unsteady flows, pathlines and streaklines can give completely different pictures of a flow, as illustrated in the video above. Knowing and understanding the difference between these types of trajectories is extremely important when it comes interpreting flow visualizations in unsteady flows because some visualization methods produce pathlines and others produce streaklines. (Video credit: V. Miller and M. Mungal)
Liquid Crystal Films
Smectic liquid crystals can form extremely thin films, similar to a soap bubble, that are sensitive to electrically-induced convection. Here an annular smectic film lies between two electrodes. When a voltage is applied across it, positive and negative charges build up on the surface of the film near their respective electrodes. The electrical field surrounding the fluid pushes on the surface charges, causing flow inside the film. Above a threshold voltage, an instability forms and the film develops into a series of counter-rotating vortices, which spin faster as the voltage increases. The color variations in the video above are due to differences in the film’s thickness, much like iridescence of a soap bubble. (Video credit: P. Kruse and S. Morris)