In this high-speed video, two flexible spheres are dropped from the same height. The one on the left is filled with air, the other is partially filled with a liquid. Although both spheres rebound to nearly the same height after the first bounce, their behavior differs drastically after that. The sloshing of the liquid inside the sphere acts as a damper, absorbing energy that would otherwise cause the ball to continue bouncing. The effects of contained liquids sloshing are important for understanding the dynamics of tankers, fuel on spacecrafts, and even how to walk without spilling your coffee.
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Viscous Fluid Falling on a Moving Belt
In this video a very viscous (but still Newtonian) fluid is falling in a stream onto a moving belt. Initially, the belt is moving quickly enough that the viscous stream creates a straight thread. As the belt is slowed, the stream begins to meander sinusoidally and ultimately begins to coil. Aside from some transient behavior when the speed of the belt is changed very quickly, the behavior of the thread is very consistent within a particular speed regime. This is indicative of a nonlinear dynamical system; each shift in behavior due to the changing speed of the belt is called a bifurcation and can be identified mathematically from the governing equation(s) of the system. (Video credit: S. Morris et al)
Viscoelastic Fluids in Space
In honor of astronaut Don Pettit’s launch to the International Space Station (and in the hope that he’ll do more neat microgravity fluids demonstrations while in space!), here’s a look a the behavior of viscoelastic fluids in microgravity. The elasticity of these fluids means that, when strained, the fluid deforms instantaneously and then returns to its initial shape when the strain is removed. Pettit demonstrates both Plateau-Rayleigh instability behavior, where a column of fluid breaks apart due to surface tension variations, and die swell, where a fluid jet expands beyond the diameter of nozzle from which it was extruded. Such swelling is commonly caused by the stretching and relaxation of polymers in the fluid as they react to forces caused by the nozzle opening.
Sound Sculptures
This is another fun and artistic use of non-Newtonian fluids (paint) vibrating on a speaker cone for advertising purposes. The shear-thinning viscous properties of the paint vie with surface tension to create lovely instantaneous sculptures of color. Check out Canon’s Pixma ads for similar artwork.
Surface Tension Instability
Droplets of oleic acid spread across a thin film of glycerol on a silicon wafer. The shapes here are driven by hydrodynamic instabilities, particularly Marangoni effects due to the differences in surface tension between the two fluids. (Photo credit: A. Darhuber, B. Fischer and S. Troian)
Testing Flames in Space
In microgravity, flames behave very differently than on earth due to a lack of buoyant forces. On earth, a flame can continue burning because, as the warm air around it rises, cooler air gets entrained, drawing fresh oxygen to the flame. In microgravity, both the heat from the flame and the oxygen it needs to burn move only by molecular diffusion, the random motion of molecules, or the background environmental flow (air circulation on the ISS, for example). This video shows a test of the Flame Extinguishment Experiment (FLEX) currently flying onboard the ISS. A fuel droplet is ignited, burns in a symmetric sphere and then eventually extinguishes either due to a lack of fuel or a lack of oxygen. Check out this NASA press release for more, including great quotes like this:
“As a Princeton undergrad, I saw in a graduate course the conservation equations of combustion and realized that those equations were complex enough to occupy me for the rest of my life; they contained so much interesting physics.” – Forman Williams
Ink Sculptures
Dripping ink into water can create fantastic structures as the two fluids mix. In this artwork there are numerous complex mixing phenomena: the eddies and multiple scales of turbulence; the long, thin streams of laminar flow; and the wispy mushrooms and umbrellas of the Rayleigh-Taylor instability. (Photo credit: Mark Mawson; via @thinkgeek)
Superfluid Fountains
Superfluids, a special type of fluid located below the lambda point near absolute zero, exhibit some mind-bending properties like zero viscosity and zero entropy. They are, in essence, a macroscopic manifestation of quantum mechanics. Here their thermomechanical, or fountain, effect is explained. This bizarre state of matter isn’t only found in laboratories, though. Scientists now think that superfluids may exist at the heart of neutron stars.
“Oil in Water”
There’s beauty even in something as simple as two immiscible fluids–oil and water–colliding. (Video credit: Shawn Knol)
