Objects falling into a liquid pool create a beautiful splash, and, in this beautiful, award-winning video, the Splash Lab explores a peculiar instability that occurs just as the splash closes. The buckling instability they describe involves distinctive ridges that form along the splash’s ejecta sheet as it domes over and closes. The number of ridges depends both on the object size and the liquid’s properties. (Video credit: J. Marston et al.)
Year: 2014
Beverage Bubbles Bursting
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.)
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)
Wave Clouds
Coming home from APS DFD, I looked out the window as we flew east over the last of the Rockies and caught these wave clouds. Air flowing west to east gets disturbed by the mountains, which creates internal waves in the atmosphere. Generally, these are invisible–though they can cause some of the turbulence you feel when flying. In this case, water vapor has condensed at the crests of the internal waves, creating a pattern of cloudy and clear stripes to mark the waves. The internal waves damped out by the time we flew a couple hundred miles east of Denver, but for awhile conditions were just right. (Photo credit: N. Sharp)
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)
Piazza del Popolo
The lions of the fountain in Rome’s Piazza del Popolo eject a turbulent sheet of water. Random fluctuations in the water sheet cause holes to form. Driven by surface tension, these holes grow and merge, leaving behind ligaments of water which quickly break up into a spray of unevenly-sized drops. (Image credit: E. Villermaux)
Coalescence in Microgravity
Microgravity is a wonderful playground for fluid dynamics. Here astronaut Reid Wiseman demonstrates the interplay of forces involved in coalescence. When smaller droplets hit with insufficient force, they bounce off the water sphere. But if they hit hard enough to overcome surface tension, they coalesce with the sphere. I think the space station needs a high-speed video camera; I’d like to see this behavior at a few thousand frames per second! (Video credit: R. Wiseman/NASA)
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.
