FYFD

Tag: jets

  • Crowns On Impact

    Crowns On Impact

    Dropping a partially-filled test tube of water against a table makes the meniscus at the air-water interface invert into a jet of liquid. In some cases, the impact is strong enough to generate splashing crowns of water around the base of the jet. These crowns come in two forms – one with many splashes layered upon one another and the other with only a few splashes and a faster jet. 

    The many-layered splash crowns come from the pressure wave that reflects back and forth from the bottom of the tube to the surface and back. This pressure wave moves at the speed of sound and vibrates the water surface, creating the many splashes. The same reflected pressure wave occurs in the second type of splash crown, but it gets disrupted by cavitation bubbles that form in the water (visible in the lower left image). Instead the splash crowns form from the shock waves generated when the cavitation bubbles collapse. (Image credits: A. Kiyama et al.)

  • Reflecting in a Stream

    Reflecting in a Stream

    Total internal reflection traps three lasers in a stream of falling water. When light tries to pass from the water – a material with a high refractive index – to the air – which has a lower index of fraction – it can only do so if its angle of incidence is smaller than the critical angle. Here, the light impacts the water-air boundary at a large angle and rather than refracting across the interface – like the distorted view of a straw in a glass of water – the laser light is completely reflected. Instead of escaping, the laser light is trapped, becoming a ribbon of light that swirls inside the water stream until the light is diffused. (Image credits: L. Yarnell et al.; F. Batrack et al.)

  • Spore Squirting

    Spore Squirting

    The fungus Pilobolus spreads its spores with a squirt cannon. Each spore sits on the end of a round fluid-filled pod. Like many plants, the fungus uses a process called osmosis to pump water into the pod. Through osmosis, the fungus increases the concentration of certain molecules inside the pod, which draws water into the pod and increases its pressure. Eventually, the pod ruptures, sending the spore aloft on a jet of fluid that accelerates it at 20,000+g! (Image credit: BBC Earth Unplugged, source; research credit: L. Yafetto et al.)

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    Starting a Lighter

    Lots of fluids are transparent, which makes it hard for us to appreciate their motion. One technique for making these invisible motions visible is schlieren photography, which makes differences in density visible. Here it’s combined with high-speed video to show what happens when you use a lighter (minus the spark!). When the fuel starts flowing, it’s unstable and turbulent, but after that initial start-up, you can see the jet settle into a smooth and laminar flow. Wisps of fuel diffuse away from the jet as the fluid disperses. As the valve shuts off, the flow becomes unstable again, and the remains of the lighter fluid diffuse away. (Video credit: The Missing Detail)

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    Flamethrowing

    Humans have long been fascinated by staring into flames, and the Slow Mo Guys carry on the grand tradition here with 4K, high-speed video of a flamethrower. Like firebreathers, a flamethrower’s fire is the result of a spray of tiny, volatile droplets of fuel. Once ignited, the spray becomes a turbulent jet of flames. Turbulent flows are known for having both large and small-scale structure, and there’s some really great close-ups showing this around the 2:00 mark. Also watch the edges of the flame, where the nearby air has gotten hot enough to shimmer. You can see how the trees in the background ripple and blur as the fire heats up the air and changes its density and refractive index. (Video credit: The Slow Mo Guys)

  • When Lasers Strike

    When Lasers Strike

    Lasers are a great way to deliver a lot of energy very quickly. In this animation, you see a jet of water get struck by a pulse from a powerful X-ray laser. The energy from that laser pulse gets absorbed by the water in a matter of picoseconds – that’s trillionths of a second. All that energy in so little time makes the water vaporize explosively. It’s this vapor explosion that breaks the jet in two. As the vapor expands outward, it forces water from the jet into a thin film that forms a cone. The conical film bends back on itself until it strikes the jet and coalesces. For more, check out this video of a similar experiment that looked at laser impacts on droplets. (Image credit: C. Stan et al., from Supplementary Movie 5; via Gizmodo)

  • Vortex Ring Roll-Up

    Vortex Ring Roll-Up

    Vortex rings are endlessly fascinating, and they appear throughout nature from dolphins to volcanoes and from splashes to falling drops. One way to form them is to inject a jet into a stationary fluid. Viscosity between the fast-moving jet and the quiescent surrounding fluid slows down fluid at the jet’s edge. That slower fluid slips to the rear, only to get sucked into the faster -moving flow and pushed forward again. The result is a spinning toroid, or ring. A similar method generates vortex rings by pushing a fluid out a round orifice. In this case, interaction between the fluid and the wall provides some of the force necessary to form the vortex ring. (Image credit: Irvine Lab, source)

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    Pearls of Mezcal

    Mezcal is a traditional Mexican liquor distilled from agave. (The more commonly known tequila is actually a special type of mezcal.) As a part of the production process, distillers pour a stream of mezcal into a bowl, creating a flotilla of small bubbles called pearls. Strange as it sounds, these pearls let the distiller judge the alcohol content of the liquor! When the ratio of alcohol and water in the mixture is just right, the bubbles will have a longer lifetime before they coalesce. If there’s too little or too much alcohol, the bubbles won’t last as long. The effect depends on both the viscosity and the surface tension of the liquor, but it’s the odd way that viscosity changes in water/alcohol mixtures that creates this Goldilocks behavior. It’s a fascinating demonstration of how traditional techniques often have true scientific underpinnings. (Video credit: M. Wilhelmus et al.)

  • Emulsion Impact

    Emulsion Impact

    Emulsions – mixtures of two immiscible fluids – are quite common; the oil and vinegar combination used in many salad dressings is one. The image sequence above shows the first 800 microseconds of the impact of a similarly emulsified droplet. The outer drop, seen on the left, consists of a water/glycerin mixture, and inside the drop are 20 smaller perfluorohexane droplets. These smaller droplets are denser and tend to settle toward the bottom of the outer drop. When the compound droplet hits a solid surface, it spreads in a spectacular starburst pattern that depends on the number and location of interior droplets. You can see a similar impact in motion here. (Image credit: J. Zhang and E. Li; source: C. Josserand and S. Thoroddsen)

  • Blowing Through a Straw

    Blowing Through a Straw

    As kids, most of us got in trouble at some point for blowing through a straw into our nearly-empty drinks. What you see here is a consequence of such misbehavior, though in this case the fluid is silicone oil and the straw is a metal needle (not shown) through which helium is continuously injected beneath the liquid surface. Depending on the angle of the straw, different behaviors are observed, as seen in this video. The photo above shows an intermediate regime, in which tiny jets form at the surface and eject a stream of drops. Each drop sails in a little parabolic arc and briefly bounces on the surface, like the drops on the right, before coalescing into the pool. (Image credit: J. Bird and H. Stone; video)