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Fluids Round-up – 13 October 2013
There were so many good fluids links this week that I decided for an off-week fluids round-up. Here we go!
- Jefferson Lab has a cool demo on how to make a cloud chamber using dry ice, isopropyl alcohol, and a radioactive source. There is all kinds of fun physics to explain in this one!
- io9 has a great article and videos on the efficiency of jellyfish propulsion (spoiler alert: there are vortex rings).
- Half-blimp, half-jet transport option could change shipping landscape. In a similar vein, Jalopnik takes a look back at the golden age of the dirigible.
- For the armchair daredevils, check out this 360-degree view of BASE jumping with a wingsuit off a Swiss mountain. (via Janeen M)
- Also from io9, an article on my favorite fluids demo: reversible laminar flow. If you’d like to try this at home, here’s a DIY version.
- National Geographic talks about the differences between hurricanes, cyclones, and typhoons.
- Finally, our lead video comes from #5facts and Sesame Street’s Grover who bring us 5 DIY science experiments, 4 of which are fluids demos. Sit back and enjoy!
(Video credit: #5facts/Sesame Street)
October 13, 2013
Hydraulic Bumps
If you’ve ever noticed the circular jump in your kitchen sink when you turn on the faucet, you’re familiar with what a jet does when it plunges into a horizontal layer of liquid. If the liquid is deep enough, the jet will perturb the surface into a circular depression, as in Figure (a) above. As the flow rate increases, a recirculating vortex ring and hydraulic bump forms (Figure b photo and flow schematic). At a critical flow rate, the bump will become unstable and form polygons instead of circles. At even larger flow rates, the system will shift toward a hydraulic jump, with a larger change in fluid elevation. Like bumps, these jumps can also appear in a variety of shapes. (Image credit: M. Labousse and J. W. M. Bush)
October 4, 2013
Ferrofluid Thrusters
Ferrofluids–magnetically-sensitive fluids made up of a carrier liquid and ferrous nanoparticles–may soon have a new application as a miniature thruster on nanosatellites. Microspray thrusters use tiny hollow needles to electrically spray jets of liquid that propel a satellite. But manufacturing the fragile microscopic needles used to disperse the propellant is expensive. Instead researchers are now using ferrofluids to create both the needle-like structures and to serve as the propellant. A ring of ferrofluid is placed on the thruster surface and a magnetic field applied to create the ferrofluid’s distinctive spikes. Then, when an electric force is applied, tiny jets of ferrofluid spray out from each tip, creating thrust. Unlike the conventional needles, the ferrofluid spikes are robust and can reform after being disturbed. (Photo credit: L. B. King et al.; submitted by jshoer)
September 11, 2013
Spiraling Break-up
Instabilities in fluids are sometimes remarkable in their uniformity. Here we see a hollow spinning cup with a thin film of fluid flowing down the interior. The rim of fluid at the cup’s lip stretches into long, evenly spaced, spiraling threads. These filaments stretch until centrifugal forces overcome surface tension and viscous forces and break the liquid into a multitude of tiny droplets. This process is called atomization and is vital to everyday applications like internal combustion and inkjet printing. (Photo credit: R. P. Fraser et al.)
September 9, 2013
Shocking Instabilities
The Richtmyer-Meshkov (RM) instability occurs when the interface between two fluids of different density is impulsively accelerated – usually by the passage of a shock wave. The image above shows a thin layer of gaseous sulfur hexafluoride embedded in air. Each vertical line, from left to right, shows the distortion of the two fluids at subsequent time steps after a Mach 1.2 shock wave passes through the gases. The interface’s initial waviness grows into mushroom-like shapes that mix the two gases together, ultimately leading to turbulence. Scenarios involving the RM instability include supersonic combustion ramjet engines, supernovas, and inertial confinement fusion. The RM instability is closely related to Rayleigh-Taylor instability and shares a similar morphology. (Photo credit: D. Ranjan et al.)
August 27, 2013
Vibrating Droplets
When still, water drops sitting on a surface are roughly hemispherical, drawn into that shape by surface tension. But on a vibrating surface, the same water drop displays many different shapes, like those in the video above. Researchers have observed more than 30 different mode shapes by varying the driving frequency. The metal mesh placed beneath the glass on which the drops sit helps the researchers determine the drop’s shape. As the drop deforms, the mesh appears to distort due to the refraction of light through the changing shape of the drop’s water-air interface. The distortion allows observers to visualize (and in some experiments even reconstruct) the shape of the drop’s surface. Understanding this kind of droplet behavior is valuable for many applications, including ink-jet printing and microfluidic devices. (Video credit: C. Chang et al.; via Science)
August 16, 2013
Drop-Tower Droplets
A microgravity environment can cause some nonintuitive behaviors in fluids. Many of the effects that dominate fluid dynamics in space are masked by gravity’s effects here on Earth. As a result, it can be very difficult to predict how seemingly straightforward technologies like heat exchangers, refrigeration units, and fuel tanks will behave. The photos above show two bubble jets–created by injecting a liquid-gas mixture into a liquid–colliding in microgravity. This particular experiment was conducted in a drop tower rather than on-orbit, which produced some side effects like the large bubbles seen in the images. These were created by the coalescence of smaller bubbles that congregated near the top of the tank shortly before the experiment attained free-fall. (Photo credit: F. Sunol and R. Gonzalez-Cinca)
July 17, 2013
Reader Question: Non-Coalescing Droplets
Reader ancientavian asks:
I’ve often noticed that, when water splashes (especially as with raindrops or other forms of spray), often it appears that small droplets of water skitter off on top of the larger surface before rejoining the main body. Is this an actual phenomenon, or an optical illusion? What causes it?
That’s a great observation, and it’s a real-world example of some of the physics we’ve talked about before. When a drop hits a pool, it rebounds in a little pillar called a Worthington jet and often ejects a smaller droplet. This droplet, thanks to its lower inertia, can bounce off the surface. If we slow things way down and look closely at that drop, we’ll see that it can even sit briefly on the surface before all the air beneath it drains away and it coalesces with the pool below. But that kind of coalescence cascade typically happens in microseconds, far too fast for the human eye.
But it is possible outside the lab to find instances where this effect lasts long enough for the eye to catch. Take a look at this video. Here Destin of Smarter Every Day captures some great footage of water droplets skittering across a pool. They last long enough to be visible to the naked eye. What’s happening here is the same as the situation we described before, except that the water surface is essentially vibrating! The impacts of all the multitude of droplets create ripples that undulate the water’s surface continuously. As a result, air gets injected beneath the droplets and they skate along above the surface for longer than they would if the water were still. (Video credit: SuperSloMoVideos)
June 25, 2013
Protruding Fingers
Instability is a common feature of fluid flows and can generate a near infinite set of patterns. The video above shows the Saffman-Taylor instability, an interface instability that occurs when a fluid of lower viscosity is injected into a higher viscosity fluid. In this case, the fluids inhabit a thin space between two glass plates. The less viscous fluid displaces the more viscous one in a series of branching finger-like shapes. If the situation were reversed, with a more viscous fluid injected into a less viscous one, the interface would be stable and expand radially without any pattern formation. (Video credit: William Jewell College)
June 21, 2013
Stretching to Break
Have you ever wondered what happens inside a jet of fluid as it breaks into droplets? Such events are not commonly or readily measured. This video uses a double emulsion–in which immiscible fluids are encapsulated into a multi-layer droplet–to demonstrate interior fluid flow during the Plateau-Rayleigh instability. The innermost drops and the fluid encapsulating them have a low surface tension between them, thanks to the addition of a surfactant to the inner drops. As a result, the inner drops are easily deformed by motion in the fluid surrounding them. Flow on the left side of the jet is clearly parabolic, similar to pipe flow. Closer to the pinch-off, the inner droplets shift to vertical lines, indicating that the interior flow’s velocity is constant across the jet. After pinch-off, the inner droplets return to a spherical shape because they are no longer being deformed by fluid movement around them. The coiling of the inner drops inside the bigger one is due to the electrical charges in the surfactant used. (Video credit: L. L. A. Adams and D. A. Weitz)
June 19, 2013

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