In the ocean, many forces compete in driving convection, including the temperature and salinity of the water. In the laboratory, it’s possible to mimic these characteristics of oceanic circulation using two different fluids driven by temperature and concentration differences. Recently, researchers were exploring this problem–with the added twist of tilting the fluids ~1 degree–when they discovered a surprising result. After an extended time, the convection self-organized into alternating parallel columns of ascending (dark) and descending (light) fluid. The researchers nicknamed this behavior super-highway convection. Read more about it here or in their paper. (Video credit: F. Croccolo et al; submitted by A. Vailati)
Videos
Fire-Breathing Physics
One of the most dangerous stunts for any fire-eater is breathing fire. Dr. Tim Cockerill explains some of the science behind the feat in this video. Volatility–the tendency of the liquid fuel to vaporize–is actually the enemy of a fire-eater. Use a fuel that is too volatile and it will catch fire too easily when the vaporous fuel mixes with the air. Instead fire-eaters use less volatile fuels and spray a mist of fine droplets to mix the air and fuel. This atomization of the fuel creates a spectacular fireball without endangering the fire-eater (as much). To see a similar fireball in high-speed, check out this post. (Video credit: T. Cockerill/The Ri Channel; via io9)
“Adrift”
Sometimes the time scales of a flow can mask its similarities to other flows. Simon Christen’s “Adrift,” a video of timelapsed fog in the San Francisco Bay area, shows just how these low clouds undulate and flow over the land the way a stream of water flows over and around stones. From the flow of gases in a stellar nursery down to the channels of a lab-on-a-chip, the same physics governs fluids everywhere, and there are always similarities to be found and exploited in our efforts to understand and explain fluid dynamics. (Video credit: S. Christen; via io9)
What is Pressure?
Pressure is a critical concept in fluid dynamics – a driving force behind everything from weather patterns to lift on a wing. But where does pressure come from? Like many macroscopic forces dealt with in fluid dynamics, pressure can be traced to the effects of individual molecules within a fluid. Kinetic theory describes gases as a collection of small particles which are all in constant, random motion. These particles’ collisions with each other and with their container create a multitude of tiny forces, as in the demonstration in the video above. When all of these collisions are summed together, their net effect is expressed as pressure, a force per area. (Video credit: Sixty Symbols)
H Booms
Holidays involving fireworks deserve high-speed videos of hydrogen explosions. Although Periodic Table of Videos focuses on the chemistry involved in setting hydrogen on fire, there are some lovely fluid dynamics on display, too. There’s turbulence, combustion (obviously), and, if you watch closely, you can even see the initial vorticity caused by the rubber’s burst twisting the growing flames. (Video credit: Periodic Table of Videos)
Self-Siphoning Stream of Beads
Pull a bit of a long chain out of a container, and you’ll quickly find the beads take on a life of their own, siphoning out of the jar while leaping and looping in the air. Some of the dynamics are clear – the ever-growing free end of the chain has weight enough to pull the rest of the chain out, much like the pressure difference that drives a siphon. But a lot of the rest of the dynamics are unclear and have generated a lot of discussion. It turns out that the same behavior is observed for chain laid out on a horizontal surface (video links on the right of that page) and even the dynamics of that simpler version of the problem are complex. All part of the beauty inherent in Newton’s second law. (Video credit: Steve Mould/Earth Unplugged; Research credit: J. A. Hanna et al.; submitted by Elin R)
Hydrophobia
Hydrophobic literally means water-fearing, and, once a surface is treated with a hydrophobic coating, the effect on water droplets is stark. The tendency of the non-polar hydrophobic molecules to repel the polar water molecules leads to high contact angles – which make the droplets almost spherical as they glide along the surface. The droplets dance across the surface, colliding and bouncing and coalescing. (Video and submission credit: M. Bell)
Studying Coughs
Bioaerosols–tiny airborne fluid droplets generated by coughing or sneezing–are a major concern for the spread of contagions like influenza. It may be possible, however, to mitigate some of these effects by manipulating biological fluid properties. The video above shows an experimental model of a cough, complete with the generation of bioaerosols from some fake human lung mucus. Contrast this with a cough where the model’s mucus has been treated to increase its viscoelasticity. The treated mucus generates substantially fewer droplets during a cough. The results suggest that drugs that increase viscoselasticity of biofluids may help stem the spread of disease. (Video credit: K. Argue et al.; research credit: M. D. A. Hasan et al.)
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)
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)