A fan’s blade passes through the hot air rising above a flame in this iconic image by high-speed photography pioneer Harold Edgerton. This photo uses an optical technique known as schlieren photography that makes density differences in transparent media like air visible. Because of its lower density, the hot plume of air above the flame rises. When the fan blade swings past, it sheds a vortex off its tip and the rising air from the flame gets pulled into the vortex to make it visible. To the left, a ghostly counter-rotating vortex sits on the opposite side of the fan blade. (Photo credit: H. Edgerton and K. Vandiver)
Tag: buoyancy
Spreading Bubbles Help Nature’s Scuba Divers
How liquid droplets spread on solid surfaces is pretty well understood, but researchers have looked less at the related problem of how a gas spreads. In a recent paper, scientists have examined the spreading dynamics of bubbles impacting an immersed solid. As the bubble contacts the surface, it quickly squeezes out water trapped between the bubble and the gas layer trapped at the solid surface. The bubble squishes as surface tension tries to flatten the liquid-gas interface. Buoyancy also helps flatten the bubble. The spreading is remarkably fast, taking only about 10 milliseconds. That’s good news for the many insects who use trapped air bubbles like these to breathe underwater. Check out the video below to learn about some of these natural scuba divers. (Image credit: H. de Maleprade et al., source; video credit: Deep Look)
Acrylic and Oil
Photographer Alberto Seveso is well-known for ink in water art, some of which FYFD has featured previously (1, 2, 3). More recently, he’s been experimenting with alternative methods, dropping fluids like acrylic paint into sunflower oil. The effect is quite different but no less beautiful. Because the paint and oil are immiscible, the boundaries between the two fluids are much more clearly defined and highlighted in an iridescent sheen. Instead of appearing like billowing waves of silk, the paint forms abstract and alien shapes driven by gravity, inertia, and density differences. For many more great examples, check out Seveso’s website. (Photo credit: A. Seveso)
Dissolving
It looks like the fiery edge of a star’s corona, but this photo actually shows a dissolving droplet. The droplet, shown as the lower dark region in this shadowgraph image, is a mixture of pentanol and decanol sitting in a bath of water. Pentanol is a type of alcohol that is fully miscible with decanol and is water soluble, so that it will dissolve into the surrounding water over time. Decanol, on the other hand, is immiscible with water, so that part of the droplet won’t mix with the surrounding water.
The bright swirls along the droplet’s edge show areas with more pentanol. As the alcohol dissolves into the water, it forms a buoyant plume at the top of the droplet that rises due to pentanol’s lower density. That rising plume draws fresh water in from the sides, shown by the upper white arrows. Inside the droplet, flow moves in the opposite direction, from the top toward the outer edges. This is a result of uneven surface tension within the droplet. Scientists are interested in understanding the dynamics of these multiple component drops for applications like printing, where it’s desirable for pigments in an ink drop to be distributed evenly as the drop dries. (Image credit: E. Dietrich et al.)
“Oil Spill”
In “Oil Spill” artist Fabian Oefner explores the shapes and colors of oil floating atop water. An old adage tells us that oil and water don’t mix, but this is not perfectly true. Especially in low concentrations, oil can mix slightly with water, which is why the edges of Oefner’s creations become fuzzy and break down. For the most part, though, the thin layer of oil spreads across the water’s surface, its slight variations in thickness casting the different iridescent colors we observe – just the same as a soap bubble’s iridescence. The colorful patterns are a snapshot of motion in the oil; in some places it radiates outward, pulled by the stronger surface tension of water. In other places it forms plumes and swirls that may be the result of temperature variations or other disquiet motion in the surrounding water or air. (Image credits: F. Oefner)
Mixing Fresh and Salty
Earth’s oceans are a complex and dynamic environment, but fortunately, we can simulate some of their physics on a smaller scale in the laboratory. The time series of images above show how fresh and salty waters mix. On the right side of the image is fresh water with its top layer dyed green. On the left is salty water dyed pink. Initially, the fresh water spreads horizontally toward the salty region in a smooth and laminar fashion. As the fresh water picks up salt, it gets denser and starts sinking, ultimately forming a turbulent plume that will push all the way back across the tank. For more images, check out the full poster. (Image credit: P. Passaggia et al.)
Oil in Alcohol
A drop of oil impacts and falls through a pool of isopropyl alcohol. Momentum, viscosity, and diffusion combine to deform the drop into a shape that is initially like an upside-down wine glass (top image). Because the oil is both denser than the alcohol and soluble in it, the drop sinks and dissolves as it falls. The drop expands rapidly outward, thinning and formed a concave shape around its denser, sinking core (bottom image). Ultimately, the droplet will deform and fragment as it dissolves into the alcohol. (Image credit: R. La Foy et al.)
A Buoyant Rise
Hold a buoyant sphere like a ping pong ball underwater and let it go, and you’ll find that the ball pops up out of the water. Intuitively, you would think that letting the ball go from a lower depth would make it pop up higher – after all, it has a greater distance to accelerate over, right? But it turns out that the highest jumps comes from balls that rise the shortest distance. When released at greater depths, the buoyant sphere follows a path that swerves from side to side. This oscillating path is the result of vortices being shed off the ball, first on one side and then the other. (Image and research credit: T. Truscott et al.)
Living Fluid Dynamics
This short film for the 2016 Gallery of Fluid Motion features Montana State University students experiencing fluid dynamics in the classroom and in their daily lives. As in her previous film (which we deconstructed), Shanon Reckinger aims to illustrate some of our everyday interactions with fluids. This time identifying individual phenomena is left as an exercise for the viewer, but there are hints hidden in the classroom scenes. How many can you catch? I’ve labeled some of the ones I noticed in the tags. (Video credit: S. Reckinger et al.)
Rio 2016: Synchro Swimming and Water Polo
Both synchronized swimming and water polo require competitors to hold themselves stable above the water’s surface without touching the pool’s bottom. One of the basic techniques for doing so in both sports is known as the eggbeater kick, shown above. The eggbeater kick is very similar to the motion for the breaststroke’s kick, but it’s performed upright and with alternating leg motions, sweeping a clockwise circle with the left leg and a counterclockwise one with the right.
A swimmer typically stays afloat due to a buoyant force equal to the weight of the volume of water the swimmer displaces. Rising further out the water means reducing the buoyant force, so the swimmer must generate other forces to counter their weight. The eggbeater kick does this two ways. First, as the swimmer sweeps their foot around, it acts like a hydrofoil, generating lift that holds the swimmer up. Second, other parts of the kick cycle force water downward, which, by Newton’s third law, pushes the swimmer up.
Keeping a wide stance and sweeping the legs alternately allows the athlete to balance the horizontal forces their motions create while keeping the upward forces generated relatively constant. This gives them a stable, arms-free platform that’s a foundation for everything else their sport requires. (Image credits: GettyImages; The Studio WLV, source)
Previously: How buoyancy helps swimmers
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