Astronaut Don Pettit narrates some of his experiments with air and water droplets in microgravity in this video. The lack of body forces and buoyancy in microgravity means that surface tension effects frequently dominate. Pettit’s demonstrations also involve some fun basic physics with bubble behaviors inside of water droplets. See more of Pettit’s Saturday Morning Science videos for additional microgravity fluid mechanics.
Videos
Stirring with Electromagnetism
If a fluid is electrically conductive, then magnetohydrodynamics (often abbreviated as MHD) describe its behavior. Electric and magnetic fields can be used to stir such a fluid, as in the video above. By inducing a potential difference across the electrodes lining the walls and the disk-shaped electrodes far from the walls, complicated flow patterns can be produced. #
Wave Pool
This Japanese pool, lined with computer-controlled actuators, uses the principle of wave interference to create complex shapes at the center of the pool. While we may be more familiar with wave interference using light or sound, the principles remain the same for a wave in a fluid. (via Gizmodo and phredgreen)
Seeing Shock Waves with Schlieren
Schlieren photography is actually a pretty commonly used system in high-speed experimental aerodynamics. A typical schlieren system will shine a collimated light source on the target (a wind tunnel test section or, above, a candle), bounce that light off a mirror, block half the light with a knife-edge at the focal point, and then record the subsequent images with a camera (high-speed or otherwise). The density of air is closely related to its index of refraction, so light that hits air of a different density will be bent more or less than a neighboring ray. This uneven bending of the light rays due to density gradients is what causes the light and dark areas on the schlieren images. Since the density of air changes drastically across a shock wave, the schlieren system is perfect for visualizing shock waves and has, in fact, been used for that purpose since 1864!
Convection in Cream and Liqueur
We are used to associating convection with differences in temperature, but what’s actually necessary for a Rayleigh-Taylor-type instability is a density variation (and a gravitational field). The solutal convection seen above when mixing liqueur with cream is caused by the interaction of density and surface tension. When the alcohol of the liqueur mixes with the cream, it forms a less dense alcohol-cream that tries to rise to the surface. The alcohol also breaks the surface tension of the cream, causing it to contract and open cells where the alcohol surfaces. As the alcohol evaporates, the alcohol-cream mixture gets denser and sinks back down where it can pick up more alcohol and start the process again. (via jshoer and io9)
Superhydrophobic Carbon Nanotubes
Carbon nanotubes form a superhydrophobic (super water repellent) surface that interacts with water droplets in interesting ways. The droplet is unable to wet the surface and thus the bounces along. When the impact velocities are too great for surface tension to hold the decelerating mass together, it breaks into many, smaller droplets that also bounce along the surface. # (via @JetForMe and @Vinnchan)
Fluidized Sand
What’s shown in this video are some pretty spectacular demonstrations of fluidization, where a gas is introduced at the bottom of a bed of granular particles–like sand. At the critical gas velocity, the aerodynamic forces exerted on each particle by the gas will balance the gravitational force on the particle and it will become suspended. All of a sudden, the macroscopic behavior of the solid particles will be like that of a fluid; you can even make it “boil”!
Canon Sound Sculptures
In a new series of ads for Canon, colorful paints are placed on a speaker cone and filmed at high speed to create beautiful “sound sculptures”. Paint, like oobleck, is a non-Newtonian fluid but does not react the same when excited by sound because it is shear-thinning. (When painting, you want the paint to run off the brush easily but not drip when it’s on the wall; hence, shear-thinning.) Both the photos and videos are lovely examples of fluid mechanics as art. Watch how they did it. # (Via jshoer, @ftematt, @JetForMe)
Steam Devils
The formation of the ethereal steam devil is quite similar to the formation of a fire tornado. In this case, the first frost of the season cooled air temperatures substantially below the temperature of the water of the lake, creating conditions for steam and for updrafts of rising, warmer air. A slight breeze across the lake is enough to create pockets of vorticity, which stretch due to the updrafts and intensify due to conservation of angular momentum. This creates the narrow spinning vortex filaments that pull steam up and dance across the lake’s surface. #
Non-Newtonian Fluids
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Non-Newtonian fluids have even made it into pop culture. #