The microgravity environment of space is an excellent place to investigate fluid properties. In particular, surface tension and capillary action appear more dramatic in space because gravitational effects are not around to overwhelm them. In this animation, astronaut Don Petit injects a jet of air into a large sphere of water. Some of the water’s reaction is similar to what occurs on Earth when a drop falls into a pool; the jet of air creates a cavity in the water, which quickly inverts into an outward-moving jet of water. In this case, the jet is energetic enough to eject a large droplet. Meanwhile, the momentum, or inertia, from the air jet and subsequent ejection causes a series of waves to jostle the water sphere back and forth. Surface tension is strong enough to keep the water sphere intact, and eventually surface tension and viscosity inside the sphere will damp out the oscillations. You can see the video in full here. (Image credit: Don Petit/Science off the Sphere)
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Meandering River
When unconstrained by the local topography, rivers tend to meander, as shown in this astronaut photograph of the Arkansas River near Little Rock, AR. The current course of the river is visible in green in the lower right hand corner of the image, but numerous lakes and curved banks show some of the former paths the river took. When rivers develop a bend, flow is faster on the inner bank than around the outer bank. This speed difference causes a vortical secondary flow inside the river that removes sediment from the outer bank and deposits it on the inner side. The end result is that the bend in the river gets sharper and the river meanders further. Sometimes the bends get so sharp they pinch off, leaving behind lakes. (Photo credit: Exp. 38/NASA Earth Observatory)
Vibrating on a Subwoofer
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Vibrating a liquid droplet produces some awesome behavior. The video above shows a water droplet vibrating on a subwoofer at real-time speeds. The behavior and shape of the droplet shifts with the frequency of vibration, which we hear as a change in pitch. To see more clearly the shapes a particular frequency induces, check out this high-speed video of vibrating water droplets. For a given driving frequency, the droplet’s shape, or mode, is distinct and consistent. For a droplet vibrating to a song, though, there is more than one frequency driving its motion. In this case, the droplet’s shape is a superposition of the individual modes, which is just a way of saying adding the shapes together. So frequency determines the droplet’s shape. The vibration amplitude, or audible volume, affects how energetic the drop’s motion is. And the fluid’s surface tension and viscosity act as dampers to the system, controlling how quickly the drop can change shape as well as how well it holds together. (Video credit: A. Read)
Abstract Fluids
Janet Waters’ abstract photography is full of effects created with fluid dynamics. Diffusion merges different fluids, and gradients in surface tension drive interfacial flows. Changes in density and viscosity produce fingers and streaks and all manner of forms. Be sure to check out her photostream for many more examples of fluids as art. (Photo credits: J. Waters)
Cavitation in a Bottle
This high-speed video shows the cavitation that occurs when a bottle of water is struck. The impact accelerates the bottle downward, generating localized vacuums between the glass and the liquid. These are cavitation bubbles, which expand until the pressure of the water surrounding them is too great. This outside pressure triggers an implosion of the bubble, which collapses until the pressure within the bubble makes it expand again. These rapid oscillations in pressure can often shatter the glass bottle. Cavitation can also generate extremely high temperatures and even trigger luminescence. It’s used by both pistol shrimp and mantis shrimp to hunt their prey. (Video credit: P. Taylor)
Balloons in the Car
Destin from Smarter Every Day has just made a video on one of my favorite fluids brain teasers: what happens to a helium balloon when you accelerate in a car? Take a moment to think about the answer before watching or reading further…
Okay, so what happens? Contrary to what you may expect, hitting the accelerator with a balloon in the car will make it shift forward. This is a matter of buoyancy. As Destin demonstrates with the water bottle, when two fluids are accelerated forward, the denser one will shift backwards, which pushes the lighter one forward. Because the helium is lighter than the air filling the car, accelerating pushes the air backward (just as it does the pendulum and the car’s inhabitants) and that shifting of the air pushes the helium in the balloon forward. (Video credit: Smarter Every Day)
“Becoming Harmonious”
Much as I try to keep from getting repetitious, this was just too neat to pass up. This new music video for The Glitch Mob’s “Becoming Harmonious” is built around the standing Faraday waves that form on a water-filled subwoofer. The vibration patterns, along with judicious use of strobe lighting, produce some fantastic and kaleidoscopic effects. (Video credit: The Glitch Mob/Susi Sie; submitted by @krekr)
Tiny Fliers
There’s an apocryphal story claiming that, aerodynamically speaking, honeybees should not be able to fly. Obviously, they can, but it’s true that a small, flapping creature and a large, fixed-wing aircraft will not generate lift exactly the same way. NYU professor Leif Ristroph has a lot of projects exploring flapping flight on smaller scales, as seen in this video. His oscillatory fliers and rotating flapping flight simulator have both been featured previously. Part of the beauty of these projects is their size; in a field that’s historically required giant wind tunnels and room-length wave tanks, Ristroph’s work provides insight into long-standing problems using apparatuses that fit on a countertop. (Video credit: Cool Hunting/L. Ristroph et al.)
“High Ball Stepper”
The recently released music video for Jack White’s “High Ball Stepper” is a fantastic marriage of science and art. The audio is paired with visuals based around vibration effects using both granular materials and fluids. There are many examples of Faraday waves, the rippling patterns formed when a fluid interface becomes unstable under vibration. There are also cymatic patterns and even finger-like protrusions formed by when shear-thickening non-Newtonian fluids get agitated. (Video credit: J. White, B. Swank and J. Cathcart; submitted by Mike and Marius)
Kelvin Wakes
Ducks, boats, and other objects moving along water create a distinctive V-shaped pattern known as a Kelvin wake. As the boat moves, it creates disturbance waves of many different wavelengths. The constructive interference of the slower waves compresses them into the shock wave that forms either arm of the V. Sometimes evenly spaced wavelets occur along the arms as well. Between the arms are curved waves that result from other excited wave components. The pattern was first derived by Lord Kelvin as universally true at all speeds – at least for an ideal fluid – but practically speaking, water depth and propeller effects can make a difference. Recently, some physicists have even suggested that above a certain point, an object’s speed can affect the wake shape, but this remains contentious. (Image credit: K. Leidorf; via Colossal; submitted by Peter)
