Krill and other tiny marine zooplankton make daily migrations to and from the ocean surface. Previously, models of ocean mixing ignored these migrations; these animals are tiny, researchers argued, so any effects they could have would be too small to matter. But zooplankton make these migrations in huge swarms, and studies of a laboratory analog of their migrations (using brine shrimp rather than krill) reveal that, when moving en masse, these tiny swimmers create turbulent jets and eddies far larger than an individual. Their collective motion is enough to mix salty water layers 1000 times faster than molecular diffusion alone! Learn more in the latest FYFD video, embedded below. (Image and video credit: N. Sharp; research credit: I. Houghton et al.; h/t to Kam-Yung Soh)
Search results for: “jet”
Nautilus Swimming
The shellbound chambered nautilus is a champion of underwater jet propulsion. It can eke out efficiencies as high as 75%, far outclassing other jet-based swimmers like squid, salps, and jellyfish. That high efficiency is especially important for the nautilus, which spends a great deal of time at depths where the oxygen needed to fuel movement is in short supply. To get around, the nautilus draws water in through an enlarged orifice, then squirts it out little by little. Its this asymmetry between drawing in and expending that keeps efficiency high. By releasing a jet slower and at lower speeds, the nautilus is able to reduce wasteful losses to friction and thereby keep the efficiency high. The drawback is that the nautilus swims relatively slowly at an average of around 8 centimeters–less than one body length–per second. (Image credit: Simon and Simon Photography/University of Leeds; research credit: T. Neil and G. Askew; via NYTimes; submitted by Kam-Yung Soh)
Water Atop Oil
At first glance, this image looks much like the impact of any drop on a pool of the same liquid, but that’s not what you’re seeing. This is the impact of a water droplet on a thin film of oil, and the immiscibility of those two fluids has important effects on the collision. When the water drop impacts, it spreads and forms a compound crown that rises out of the fluid. Eventually, that momentum runs out and the crown falls into the liquid.
Water’s intermolecular forces are strong enough to pull the remains of the droplet back in on itself. As that fluid collides at the center, it gets forced up into a central jet with enough energy to eject a droplet or two at its tip. Even though this looks like a Worthington jet, it’s not. Worthington jets form after the collapse of a cavity in the impacted liquid – in other words, they form on pools, not on films. Despite the visual similarity, this central jet is formed entirely differently! (Image and research credit: Z. Che and O. Matar, source; submitted by O. Matar)
The Fishbone
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)
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)