FYFD

Tag: vibration

  • Bouncing in Lockstep

    Bouncing in Lockstep

    Droplets of silicone oil bounce on a pool of the same thanks to the vibration provided by a loudspeaker. Each droplet’s bounce causes ripples in the pool and the interference between these ripples fixes the droplets in lockstep with one another. As long as the vibration continues to feed the thin layer of air that separates the droplets from the pool during each bounce and no impurities break the surface tension at the interface, the droplets will bounce indefinitely on their liquid trampoline. Such systems can be used to observe quantum-mechanical behavior like wave-particle duality on a macro-scale. (Photo credit: A. Labuda and J. Belina)

  • Dancing Jets

    Dancing Jets

    Vibrating a gas-liquid interface produces some exciting instability behaviors. The photo above shows air and silicone oil vibrated vertically within a prism. For the right frequencies and amplitudes, the vibrations produce liquid jets that shoot up and eject droplets as well as gas cavities and bubble transport below the interface. To see a similar experiment in action, check out this post. (Photo credit: T. J. O’Hern et al./Sandia National Laboratories)

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    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)

  • Fluids Round-up – 9 June 2013

    Fluids Round-up – 9 June 2013

    It’s time for some more fluidsy fun around the Internet! Here are some fun links I’ve come across since our last round-up.

    (Photo credit: L. L. A. Adams et al., multi-fluid double emulsions)

  • Fluids Round-up – 25 May 2013

    Fluids Round-up – 25 May 2013

    Sometimes I come across cool links and stories about fluid dynamics that don’t quite fit into a typical FYFD post, but I’d like to start sharing those semi-regularly with round-up posts. Here’s some fun stuff I’ve seen lately:

    And, yes, that last Specialized video chat includes an FYFD shout-out about 49 minutes in. 🙂

    (Photo credit: Specialized)

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    Bouncing on a Pool

    There’s something wonderfully serene about watching water droplets skate their way across the surface of a pool. Here the pool of water is being vibrated at a frequency just below the Faraday instability – meaning that no standing waves form on the surface. Instead, the bounce is just enough to create a thin layer of air between the droplet and the pool to prevent coalescence. With each bounce, gravity’s effect on the water tries to drain the air away, but each rebound lets more air rush in to hold the droplet up. Eventually, gravity wins and the droplets coalesce into the pool. In high-speed that process is mesmerizing, too. (Video credit: K. Welch)

  • Bouncing to Mix Oil and Water

    Bouncing to Mix Oil and Water

    Mixing immiscible liquids–like oil and water–is tough. The best one can usually do is create an emulsion, in which droplets of one fluid are suspended in another. The series of images above shows a double emulsion consisting of oil and water that’s been formed by bouncing the compound droplet on a vibrating bath. The vibration of the liquid surface keeps the droplet from coalescing with the bath and the deformation provides mixing. The top row shows the initial impact while the bottom row of images shows the droplet after many bounces. As time goes on, the layer of oil around the compound drop becomes a cluster of tiny droplets contained within the water portion of the drop. (Photo credit: D. Terwagne et al.)

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    “Frozen” Water Stream

    We saw previously how vibrating a falling stream of water and filming it with a matching camera frame rate appears to “freeze” the falling liquid. This video shows the same illusion, now with a 24 Hz sine wave, which the falling water mimics. Vibrating the speaker that drives the water stream slightly slower or slightly faster than the camera frame rate makes the water appear to slowly fall or rise relative to its “frozen” wave state. This is a beat effect caused by the slight difference in frequency between the water and the camera.  (Video credit: brusspup; via BoingBoing; submitted by many readers)

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    Tuning Fork Fluids

    This high-speed video shows a liquid crystal fluid vibrating on a tuning fork. As the surface moves, tiny jets shoot upward, sometimes with sufficient energy that the fluid column is stretched beyond surface tension’s ability to keep it intact, resulting in droplet ejection. The jets and surface waves create a mesmerizing pattern of fluid motion. (Video credit: J. Savage) 

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    Hummingbirds Singing with their Tail Feathers

    Aeroelastic flutter occurs when fluid mechanical forces and structural forces get coupled together, one feeding the other. Usually, we think of it as a destructive mechanism, but, for hummingbirds, it’s part of courtship. When a male hummingbird looks to attract a mate, he’ll climb and dive, flaring his tail feathers one or more times. As he does so, air flow over the feathers causes them to vibrate and produce noise. Researchers studied such tail feathers in a wind tunnel, finding a variety of vibrational behaviors, including a tendency for constructive interference–in other words two feathers vibrating in proximity is much louder than either individually. For more, check out the original Science article or the write-up at phys.org. (Video credit: C. Clark et al.)