FYFD

Tag: Plateau-Rayleigh instability

  • Using Sound to Print

    Using Sound to Print

    Inkjet printing and other methods for directing and depositing tiny droplets rely on the force of gravity to overcome the internal forces that hold a liquid together. But that requires using a liquid with finely tuned surface tension and viscosity properties. If your fluid is too viscous, gravity simply cannot provide consistent, small droplets. So researchers are turning instead to sound waves. 

    Using an acoustic resonator, scientists are able to generate forces up to 100 times stronger than gravity, allowing them to precisely and repeatably form and deposit micro- and nano-sized droplets of a variety of liquids. In the images above, they’re printing tiny drops of honey, some of which they’ve placed on an Oreo cookie for scale. The researchers hope the technique will be especially useful in pharmaceutical manufacturing, where it could precisely dispense even highly viscous and non-Newtonian fluids. (Image and research credit: D. Foresti et al.; via Smithsonian Mag; submitted by Kam-Yung Soh)

  • Spinning Droplet Galaxies

    Spinning Droplet Galaxies

    Water flung from a spinning tennis ball takes on a shape reminiscent of a spiral galaxy. As it detaches, water leaves the surface with both the tangential velocity of the spinning ball and a radial velocity due to the centrifugal force flinging it. The continued spin of the ball makes the thin ligaments of water still attached to it spiral and stretch. Eventually, surface tension can no longer hold the water together against the centrifugal forces, and the ligaments split into a spray of droplets. (Image credit: W. Derryberry and K. Tierney)

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    Paint Balloons

    The Slow Mo Guys have a history of personal sacrifice in the name of cool high-speed footage, and their Super Slow Show is no exception. In a recent segment, both Dan and Gav were knocked flat by giant swinging balloons of paint, and, as you might expect, the splashes are spectacular. The speed is just right for some of the paint to form nice sheets before momentum pulls them into long ligaments. Eventually, that momentum overcomes surface tension’s ability to keep the paint together, and the paint separates into droplets, which, as you see below, rain down on the hapless victims. (Video and image credit: The Slow Mo Guys)

  • The Fishbone

    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)

  • Oil Splatters

    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)

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    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.)

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    Burning a Rocket Underwater

    In a recent video, Warped Perception filmed a model rocket engine firing underwater. Firstly, it’s no surprise that the engine would still operate underwater (after its wax waterproofing). The solid propellant inside the engine is a mixture of fuel and oxidizer, so it has all the oxygen it needs. Fluid dynamically speaking, though, this high-speed footage is just gorgeous.

    Ignition starts at about 3:22 with some cavitation as the exhaust gases start flowing. Notice how that initial bubble dimples the surface when it rises (3:48). At the same time, the expanding exhaust on the right side of the tank is forcing the water level higher on that side, triggering an overflow starting at about 3:55. At this point, the splashes start to obscure the engine somewhat, but that’s okay. Watch that sheet of liquid; it develops a thicker rim edge and starts forming ligaments around 4:10. Thanks to surface tension and the Plateau-Rayleigh instability, those ligaments start breaking into droplets (4:20). A couple seconds later, holes form in the liquid sheet, triggering a larger breakdown. By 4:45, you can see smoke-filled bubbles getting swept along by the splash, and larger holes are nucleating in that sheet.

    The second set of fireworks comes around 5:42, when the parachute ejection charge triggers. That second explosive triggers a big cavitation bubble and shock wave that utterly destroys the tank. If you look closely, you can see the cavitation bubble collapse and rebound as the pressure tries to adjust, but by that point, the tank is already falling. Really spectacular stuff!  (Video and image credit: Warped Perception)

  • Inside Ink Jet Printing

    Inside Ink Jet Printing

    Inkjet printers produce droplets at an incredible rate. A typical printhead generates droplets that are about 10 picoliters in volume – that is, ten trillionths of a liter – moving at about 4 meters per second. Resolving the formation of those droplets would require ultra-high speed imaging at millions of frames per second. Instead researchers devised an alternative method to capture droplet formation, based on stroboscopic techniques. In this case the strobe is a 7 nanosecond laser pulse (7 billionths of a second) that illuminates a given droplet twice. By doing this for many droplets, the researchers can create a highly detailed time series like the one above, which shows the inkjet breakup and droplet formation. Here each droplet is 23 micrometers wide – about one-third the width of a human hair. (Image credit: A. van der Bos et al., source)

  • Blue Man Group in Slow Mo

    Blue Man Group in Slow Mo

    In their latest video, the Slow Mo Guys team up with the Blue Man Group for some high-speed hijinks, some of which make for great fluidsy visuals. Their first experiment involves dropping a bowling ball on gelatin. The gelatin goes through some massive deformation but comes out remarkably unscathed. Gelatin is what is known as a colloid and essentially consists of water trapped in a matrix of protein molecules. This gives it both solid and liquid-like properties, which means that the energy the bowling ball’s impact imparts can be dissipated through liquid-like waves ricocheting through the gelatin before the elasticity of the protein matrix allows it to reform in its original shape.

    The video ends with buckets of paint flung at Dan. The paints form beautiful splash sheets that expand and thin until surface tension can no longer hold them together. Holes form in the sheet and eat outward until the paint forms thin ligaments and catenaries. As those continue to stretch, surface tension drives the paint to break into droplets, though that break-up may be countered to some extent by any viscoelastic properties of the paint. (Image and video credit: The Slow Mo Guys + Blue Man Group, source)

  • Putting Out Fires

    Putting Out Fires

    Fires in large, open spaces like aircraft hangers can be difficult to fight with conventional methods, so many industrial spaces use foam-based fire suppression systems. These animations show such a system being tested at NASA Armstrong Research Center. When jet fuel ignites, foam and water are pumped in from above, quickly generating a spreading foam that floats on the liquid fuel and separates it from the flames. Since the foam-covered liquid fuel cannot evaporate to generate flammable vapors, this puts out the fire. 

    The shape of the falling foam is pretty fascinating, too. Notice the increasing waviness along the foam jet as it falls. Like water from your faucet, the foam jet is starting to break up as disturbances in its shape grow larger and larger. For the most part, though, the flow rate is high enough that the jet reaches the floor before it completely breaks up. (Image credit: NASA Armstrong, source)