There’s more to non-Newtonian fluids than shear-thickening and shear-thinning. The viscosity of some fluids can also change with time under constant shear. A fluid that becomes progressively less viscous when shaken or agitated is called thixotropic. The opposite (and less common) behavior is a fluid that becomes more viscous under constant agitation; this is known as a rheopectic fluid. This video demonstrates both types of fluids using a rotating rod as the agitator. The rheopectic fluid actually appears to climb the rod–similar to the Weissenberg effect–while the thixotropic fluid moves away from the rod.
Category: Phenomena
Liquid Settling
Despite the strange shapes of the arms on this container, the fluid inside will always settle to a common height. This is because each interconnected section is open to the outside air. The fluid’s surface has to reach a static equilibrium with the atmosphere–i.e. the surface of the fluid must be at atmospheric pressure–and the pressure at the lowest level in each section must match because the arms are connected. When fluid is added, the height of the columns oscillates some because the momentum of the added fluid carries the column past its equilibrium position, much like a perturbed mass hanging from a spring will oscillate before settling.
Supercritical Fluids
Supercritical fluids live in the region of a phase diagram beyond the critical point. At these temperatures and pressures, a substance is neither strictly liquid nor a gas but exhibits behaviors from both. A supercritical fluid can effuse through a solid like a gas does but can also dissolve substrates like a liquid. As noted in the video above, supercritical fluids are useful substitutes for organic solvents in many industrial applications. Carbon dioxide, for example, is used as a supercritical fluid in the decaffeination process.
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Freezing Soap Bubbles
This is what it looks like when a soap bubble freezes. Perhaps not strictly fluid mechanical in nature, but it’s a nice thermodynamics demonstration.
Saturnian Storm
Back in mid-December, amateur astronomers discovered an enormous new storm on Saturn. The Cassini spacecraft captured this image early in the storm’s history (it now stretches farther around the planet). The fluid dynamics of Saturn’s atmosphere are incredibly complex and well beyond our current understanding, but we can certainly appreciate the majesty of a swirling, turbulent storm half the size of our entire planet. (via APOD, Martian Chronicles)
Superfluid Helium Leaks from its Container
Below a temperature of 2.17 Kelvin, helium becomes a superfluid, a state of matter boasting several unique properties including zero viscosity (resistance to flow). In this video, scientists demonstrate that property. When they pull the glass “bucket” of helium out at 2:50, the helium starts to leak out. The glass is solid but it contains numerous tiny spaces between its atoms. In its normal state, the viscosity of helium prevents it from escaping through those holes. But as a superfluid, its resistance to flowing goes to zero and it leaks right through the solid glass.
Ferrofluid Labyrinths
Here’s a different take on ferrofluids. Instead of spikes, we get 2D patterns reminiscent of these ones. Most likely the ferrofluid is trapped under glass as part of a Hele-Shaw cell. The results remind me some of chaotic Rayleigh-Benard convection cells, actually.
Levitating Liquid Oxygen
The Leidenfrost effect occurs between a fluid and a solid of vastly different temperatures. In the case of liquid oxygen, a thin layer of the oxygen vaporizes on contact with the room temperature solid, leaving a droplet of liquid oxygen to float along on its own vapor. Oxygen droplets are paramagnetic, meaning that they are susceptible to magnetic fields; in this video, scientists demonstrate how magnets can affect the motion of these droplets.
Vibrating Fluid Interfaces
The Faraday instability forms when a fluid interface is vibrated. This high-speed video shows the differences in the shapes formed by a vibrated fluid interface when the two fluids are miscible–capable of mixing–and when they are immiscible–like oil and water. Note how the miscible interface breaks down quickly into turbulence, but the immiscible interface maintains a complex shape.
Colorful Computational Combustion
Many fluid dynamics problems are so complicated that they require supercomputers to calculate the mathematical and physical details. This image shows a computer simulation of a cold ethylene jet combusting in hot air. Different colors indicate different combustion by-products. Researchers use simulations like this one to investigate ideal flames that improve efficiency in applications like cars or jet engines. #
