Fizzy drinks like soda and champagne have many bubbles which rise to the surface before bursting. When the film separating the bubble and the air drains and bursts, it leaves a millimeter-sized cavity that collapses on itself. That collapse creates an upward jet of fluid which can break into tiny aerosol droplets that disperse the aroma and flavor of the drink. Similar bubble-bursting events occur in sea spray and industrial applications, too. Researchers find that droplet ejection depends on bubble geometry and fluid properties such as viscosity. More viscous liquids, for example, generate smaller and faster droplets. Learn more and see videos of bubble-bursts at Newswise. (Image credit: E. Ghabache et al.)
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Raindrops on Sand
Here is a high-speed look at the impact of a raindrop on a sandy beach. In this case, a water droplet is falling on a bed of uniform glass beads, but the situation is effectively the same. Depending on the speed of the drop at impact, many types of craters are possible. The higher the impact velocity, the greater the momentum of the drop at impact and the more likely the drop is to tear apart when surface tension can no longer hold it together. Interestingly, there is remarkable similarity between the shape and behavior of these liquid drop impacts and those of a catastrophic asteroid impact. (Video credit: R. Zhao et al.)
Sound Interactions
Sound waves often interact with many objects before we hear them. Understanding and controlling those interactions is a major part of acoustic engineering. The animations above show shock waves–sound–from a trumpet interacting with different objects. The sound is made visible using the schlieren optical technique, allowing us to observe the reflection, absorption, and transmission of sound as it hits different surfaces. Fiberboard, for example, is highly reflective, redirecting the sound waves along a new path without a lot of damping. In contrast, the metal grid is only weakly reflective and a small portion of the incoming sound wave is transmitted through the grid. To see more examples, check out the full video, and, if you want to learn more about acoustics, check out Listen To This Noise. (Image credits: C. Echeverria et al., source video)
Supercooling Water
Supercooling is the process of lowering a fluid’s temperature below its freezing point without the fluid becoming solid. Though this may sound bizarre, it’s an effect you can recreate easily in your refrigerator, as detailed in the video above. Supercooling shows up in nature as well, particularly with water droplets at high altitudes. If a plane flies through supercooled water droplets, it can create icing problems on the aircraft’s wings. Alternatively, flying through supercooled water vapor can cause a hole-punch cloud to form when the vapor flash-freezes into snow. (Video credit: SciShow)
Van Gogh and Turbulence
Turbulence is one of the great unsolved mysteries of classical mechanics. Many physicists and engineers have spent their careers trying to further our understanding of the subject and find the mathematical pattern that underlies its complex motions. But understanding turbulence and representing it artistically may be two different things. This video discusses some neat research that found that some of Vincent van Gogh’s paintings, like “The Starry Night”, display mathematical patterns like those of turbulence. (Video credit: TED Ed)
DFD Reminder
Reminder: APS DFD is starting today. Follow along on Twitter at @fyfluiddynamics and #APSDFD. Later today at 12:30 PT you can follow our science communication workshop and ask questions at #DFDSciComm.
APS DFD 2014
It’s that time of year again! Sunday marks the start of the 67th Annual Meeting of the American Physical Society Division of Fluid Dynamics. I’ll be in San Francisco for the full conference. On Sunday at 15:30 ET/12:30 PT I’ll be co-teaching a workshop on science communication alongside Flora Lichtman, David Hu, Rachel Levy, and Jason Bardi. We’ll be live-tweeting the event with the hashtag #DFDSciComm, and you are welcome to join in with comments and questions, even if you’re not attending the workshop in person. We’ll do our best to answer.
For those coming to the conference, keep an eye out and come say hello. I’ve got special FYFD stickers for those who do.
I expect to do some photos and short updates from the conference here, but for up-to-the-minute info on what I’m up to, your best bet is to follow the FYFD Twitter account. See you in California! (Image credits: N. Sharp/FYFD)
Fine-Tuning Flight
We humans generally use fixed wings for flight, but in nature, flapping flight dominates. As an animal flaps, it extends or draws in its wings during key points of the cycle in order to change its aerodynamics. But this control can be more than just a matter of stretching their wings. Recent work on bats shows that they can fine-tune the stiffness of their wings’ membrane using tiny, hair-thin muscles. Each muscle is too slight to change a wing’s shape on its own, but by firing synchronously–tensing on the downstroke and relaxing on the upstroke–the bat can manipulate its membrane stiffness and thereby affect its wing shape. Moreover, the timing of the muscles’ action changes with flight speed, suggesting that the bats are actively controlling their aerodynamics during flight. (Video credit: Swartz-Breuer lab/Brown University; via Futurity; submitted by Boris M)
Barchan Dunes
Crescent-shaped barchan dunes are common on both Earth (top image) and Mars (bottom image). They form in areas where the wind comes predominantly from one direction. As the wind blows, it deposits sand on the gently sloping windward face of the dune. The leeward face of the dune is steeper; its shape is set by the sand’s angle of repose–essentially the steepest angle the sand can maintain without an avalanche. Barchan dunes are very mobile, moving between one and a hundred meters per year. They have also been seen moving through one another or moving along in formation. (Image credits: Google Earth, NASA/JPL/University of Arizona)
Inside a Water Blob
This new video from the Space Station shows once again that astronauts have the most fun job on–or off–the planet. In it, the Expedition 40 crew members submerge a GoPro camera in a microgravity water blob. Here on Earth, we’re used to surface tension being a minor or secondary force with most fluids we experience daily. This is because gravity often provides the overwhelming effect. But in microgravity, those effects are absent, and forces like surface tension and adhesion dominate water’s behavior. This both why the crew can make such a large water sphere hold together, and why one astronaut eventually gets his hands stuck in the sphere. (Video credit: NASA; submitted by jshoer)