The simple collision of two liquid jets can form striking and beautiful patterns. Here the two jets strike one another diagonally near the top of the animation. One is slanted into the screen; the other slants outward. At their point of contact, the liquid spreads into a sheet and forms what’s known as a fishbone pattern. The water forms a thicker rim at the edge of the sheet, and this rim destabilizes when surface tension can no longer balance the momentum of the fluid. Fingers of liquid form along the edge, stretching outward until they break apart into droplets. Ultimately, this instability tears the liquid sheet apart. Under the right conditions, all kinds of beautiful shapes form in a system like this. (Image credit: V. Sanjay et al., source)
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A Hot Tub, Turned Fluidized Bed
Fluidized beds continue to be all the rage among science YouTubers, but Mark Rober supersizes his by turning a broken hot tub into a massive bath of bubbling sand. His video includes a nice explanation of how a granular material like sand gets fluidized as well as how to make your own miniature bed. One of my favorite moments is shown in the animation below. When Mark drops a bowling ball into the fluidized bed, it creates a remarkably liquid-like splash. The ball sprays a splash curtain of sand up on impact and sinks into its own cavity. When the cavity seals behind the ball, it shoots up a tall jet of sand, just like a Worthington jet in water. Even with air fluidizing it, the sand doesn’t have surface tension, though, so the jet breaks up quite differently than water! (Video and image credit: M. Rober; submitted by clogwog)
Liquid Sculptures
With patience and timing, one can create remarkable sculptures with fluids. To capture this shot, Moussi Ouissem used two droplets, perfectly timed. The first fell through the soap bubble (which stayed intact thanks to its powers of self-healing) and hit the pool of water. The impact caused a cavity, which then inverted into a Worthington jet. The second drop was timed to impact the column of the jet, creating the saddle-shaped splash seen here. Ripples in the bubble are still visible from the passage of the second drop, and several satellite droplets are signs of the violence of the impacts. (Image credit: M. Ouissem)
Oil Splatters
Most cooks have experienced the unpleasantness of getting splattered with hot oil while cooking. Here’s a closer look at what’s actually going on. The pan is covered by a thin layer of hot olive oil. Whenever a water drop gets added – from, say, those freshly washed greens you’re trying to saute – it sinks through the oil due to its greater density. Surrounded by hot oil and/or pan, the water heats up and vaporizes with a sudden expansion. This throws the overlying oil upward, creating long jets of hot oil that break into flying droplets. These are what actually hit you. This is a small-scale demonstration, but it gets at the heart of why you don’t throw water on an oil fire. (Image credit: C. Kalelkar and S. Paul, source)
Cavitating Inside a Tube
Cavitation – the formation and collapse of low-pressure bubbles in a liquid – can be highly destructive, shattering containers, stunning prey, and damaging machinery. Inside an enclosure, cavitation can happen repeatedly. Above, a spark is used to generate an initial cavitation bubble, which expands on the right side of the screen. After its maximum expansion, the bubble collapses, forming jets on either end that collide as the bubble shrinks. Shock waves form during the collapse, too, although in this case, they are not visible.
Those shock waves travel to either end of the tube, where they reflect. The reflected waves behave differently; they are now expansion waves rather than shock waves. Their passage causes lower pressure. The two expansion waves meet one another toward the left end of the tube, in the area where a cloud of secondary cavitation bubbles form after the first bubble collapses. Pressure waves continue to reflect back and forth in the tube, causing the leftover clouds of tiny bubbles to expand and contract. (Image credit: C. Ji et al., source)
Bouncing Off a Film
Surface tension is the result of an imbalance between intermolecular forces near an interface. Imagine a water molecule far from the surface; it is surrounded on all sides by other water molecules and feels each of those pulling on it. Since all the nearby molecules are water, the tugs from every direction balance and there is no net force. Now imagine that water molecule near the air interface. Instead of being influenced on all sides by water, our molecule now feels water in some directions and air molecules in another. The water molecules tug harder on it than air, leaving a net force that pulls along the interface. This is surface tension, and, for a liquid-gas interface, it behaves somewhat like an elastic sheet. Surface tension is even strong enough to let a jet of soap solution bounce repeatedly off a soap film. Each bounce deforms the interface, like a trampoline dimpling when someone jumps on it, but surface tension keeps the interface taut enough for the jet to skip off without breaking it. (Image credit: C. Kalelkar and S. Phansalkar, source)
Flag Flapping
Everyone has watched a flag flutter in the breeze, but you may not have given much thought to it. One of the earliest scientists to consider the problem was Lord Rayleigh, who wrote an aside on the mathematics of an infinite flag flapping in a paper on jets (pdf). Today researchers consider the problem in terms of fluid-solid interaction; in other words, to study a fluttering flag, you must consider both the properties of the flag – its flexibility, length, elasticity, and so on – and the properties of the fluid – air speed, viscosity, etc. The combination of these factors governs the complicated shapes taken on by a flag. The image above is a composite of several photos of a string (a 1-d flag) flapping in a flow that moves from left to right. By combining photos, the image highlights the envelope of shapes the flag takes and demonstrates at a glance just how far the flag flutters in either direction along its length. (Image credit: C. Eloy)
Galapagos Week: Pistol Shrimp
One of the most striking things about snorkeling in the Galapagos was how loud it was underwater. There were hardly any boats nearby, but every time my ears dipped below the surface, I could hear a constant cacophony of sound. Some it came from waves against the sand, some of it was the sound of parrotfish nibbling on coral, but a lot of it was likely the work of a culprit I couldn’t see hidden in the sand: the pistol shrimp.
These small crustaceans hunt with an oversized claw capable of snapping shut at around 100 kph. When the two halves of the claw come together, they push out a high-speed jet of water. High velocity means low pressure – a low enough pressure, in fact, to drop nearby water below its vapor pressure, causing bubbles to form and expand. These cavitation bubbles collapse quickly under the hydrostatic pressure of the surrounding water, creating a distinctive pop that makes the pistol shrimp one of the loudest sea creatures around. (Image credit: BBC Earth Unplugged, source; research credit: M. Versluis et al.)
All week we’re celebrating the Galapagos Islands here on FYFD. Check out previous posts in the series here.
Paintball Collisions
In their latest video, the Slow Mo Guys collide paintballs in mid-air, creating some pretty great paint splashes. The high-speed video does a nice job of revealing some of the typical stages a splash goes through. Initially, the paint spreads in a liquid sheet. As it expands and (necessarily) thins, holes form and grow, driving the paint into string-like ligaments. These ligaments are also stretching and eventually break up into an spray of droplets, much like the jet dripping from your faucet does. If you’d like to see some more awesome high-speed liquid collisions, check out what happens when a solid projectile hits a falling drop and then look at when a laser pulse hits a droplet. (Image and video credit: The Slow Mo Guys; submitted by Omar M.)
Rocket Launch Systems
If you’ve ever watched a rocket launch, you’ve probably noticed the billowing clouds around the launch pad during lift-off. What you’re seeing is not actually the rocket’s exhaust but the result of a launch pad and vehicle protection system known in NASA parlance as the Sound Suppression Water System. Exhaust gases from a rocket typically exit at a pressure higher than the ambient atmosphere, which generates shock waves and lots of turbulent mixing between the exhaust and the air. Put differently, launch ignition is incredibly loud, loud enough to cause structural damage to the launchpad and, via reflection, the vehicle and its contents.
To mitigate this problem, launch operators use a massive water injection system that pours about 3.5 times as much water as rocket propellant per second. This significantly reduces the noise levels on the launchpad and vehicle and also helps protect the infrastructure from heat damage. The exact physical processes involved – details of the interaction of acoustic noise and turbulence with water droplets – are still murky because this problem is incredibly difficult to study experimentally or in simulation. But, at these high water flow rates, there’s enough water to significantly affect the temperature and size of the rocket’s jet exhaust. Effectively, energy that would have gone into gas motion and acoustic vibration is instead expended on moving and heating water droplets. In the case of the Space Shuttle, this reduced noise levels in the payload bay to 142 dB – about as loud as standing on the deck of an aircraft carrier. (Image credits: NASA, 1, 2; research credit: M. Kandula; original question from Megan H.)