FYFD

Month: April 2020

  • Sliding Foams

    Sliding Foams

    What happens when a foam interacts with a sliding surface? That’s the question at the heart of this study, which finds three major regimes of foam-surface interaction. On smooth surfaces (Image 1), foams will simply slide against the wall without sticking or deforming. When surface roughness is about as large as the foam’s wall thickness (Image 2), the foam will stick to individual asperities, then slip to the next rough spot as the wall moves. But when the surface roughness is large compared to the foam wall (Image 3), the foam will remain anchored to the surface and all the shear from the wall’s movement goes into deforming the bulk of the foam.

    Researchers thus found they could change foam’s behavior by changing the surface roughness. They also looked at the reverse situation: a surface with fixed roughness — like, say, a human tongue — and how tuning the size of foam bubbles might alter perception and ease of swallowing. That’s what we’re looking at in the last image, where a spoon slides a foam along a surface with roughness similar to the human tongue. (Image and research credit: M. Marchand et al.)

  • Bristling Sharkskin Fights Separation

    Bristling Sharkskin Fights Separation

    The speedy shortfin mako shark has a secret weapon to fight drag: bristling denticles that line its fins and tail. Denticles are tiny, anvil-shaped enamel scales on the mako’s skin. In the photo above, each one is about 100 microns across. Under normal conditions, with flow moving over the shark from nose to tail, the denticles lie flat, providing no interference.

    But when sudden changes in flow near the shark’s skin cause water to begin moving in the opposite direction, the denticles flare up. Their rise interferes with the reversed flow, trapping it in small eddies beneath each denticle. Since that flow reversal is a precursor to the flow separating from the shark’s body, the bristling effectively cuts off flow separation before it can begin. The result is much less separation and much lower drag. Once the flow stops trying to move upstream, the denticles settle back into their original place. (Image credit: mako shark – jidanchaomian, denticles – J. Oeffner and G. Lauder, illustration – A. Lang, bristling – A. Lang et al.; research credit: A. Lang and A. Lang et al.; submitted by Kam-Yung Soh)

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    Expanding Water Beads

    In this timelapse, we see hydrogel beads expanding as they absorb water. There are some interesting subtleties to the physics here. Notice how, in the Petri dish segments, the beads shift from a single crystalline structure to several smaller structures. I suspect those shifts are driven by the dropping water level, which changes how surface tension interacts with the beads’ shape to create attractive forces between beads.

    Another interesting point comes as the beads expand through and out of the glass of water. Initially, the water level doesn’t change in the glass. This is because the water beads are taking up the same volume as the water that they’ve absorbed. But once the beads emerge past the water’s initial height, the water level drops dramatically. That’s because the beads are still absorbing what little water is left and continuing to expand in volume. (Image and video credit: Temponaut)

  • Icy Swirls

    Icy Swirls

    Rafts of sea ice follow swirling eddies in this satellite image of the Gulf of St. Lawrence. Just as with phytoplankton blooms and sediment, this thin sea ice can be moved by wind and currents to reveal hidden flow patterns. Experimentalists use many similar diagnostics that introduce bubbles, particles, smoke, and other tracers into flows to visualize motion that’s otherwise invisible. (Image credit: J. Stevens/NOAA/NASA; via NASA Earth Observatory)

  • Steering as a Boxfish

    Steering as a Boxfish

    Coral reefs are full of odd-looking denizens, but one of the funniest-looking ones must be the boxfish. This family of fish lives up to its name; their bodies feature an angular, bony carapace that helps protect them. But you don’t have to be a fluid dynamicist to wonder how in the world they swim with that kind of shape.

    There’s actually disagreement in scientific circles as to whether the basic shape of a boxfish is stabilizing or destabilizing, in other words, whether the fish’s body shape will try to automatically turn or roll when flow moves past. A new study focuses instead on the role the fish’s tail fin serves. Through experiments (on a fish model) and simulations, the researchers showed that boxfish rely on their tail fins both as rudders and course-stabilizers.

    Living around coral reefs means that boxfish need to be highly maneuverable, and this research indicates that the fish’s body shape, combined with the stabilizing power of its tail, are key to its ability to quickly and easily turn in any direction. (Image credits: boxfish – D. Seddon, simulation – P. Boute et al.; research credit: P. Boute et al.; via NYTimes; submitted by Kam-Yung Soh)

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    Mixing Leidenfrost Drops

    When placed on a very hot, patterned surface, droplets will self-propel on a layer of their own vapor. Here, researchers use this to drive droplets to coalesce so that they can observe how well they mix. After their head-on collision, the merged droplets have two major forces fighting in them: surface tension, which tries to minimize the overall surface area; and gravity, which tries to flatten the large droplet. Together, these forces drive the large oscillations we see in the merged drop, and those oscillations help mix the liquid from the two original drops together. (Image, video, and research credit: Y. Chiu and C. Sun)

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    Choosing Swimming Over Flight

    When studying modern birds it quickly becomes apparent that they can either be good at swimming or at flying, but not at both. The characteristics that make wings good for flying are diametrically opposed to those that make for a good swimmer. So most species have chosen to invest in one strategy or the other. Penguin ancestors chose the swimming route tens of millions of years ago, in the aftermath of the extinction event that emptied our oceans of the large reptilian predators that had ruled them during the age of the dinosaurs. This video explores what we know about the fossil record of these birds, and it’s pretty incredible. Did you know there used to be 2-meter-tall penguins? (Image and video credit: PBS Eons)

  • Breaking Up Granular Rafts

    Breaking Up Granular Rafts

    Particles at a fluid interface will often gather into a collection known as a granular raft. The geometry of the interface where it meets individual particles, combined with the surface tension, creates the capillary forces that attract these particles to one another. Colloquially, this is called the Cheerio’s effect; it’s the same physics that draws those cereal chunks together in your bowl.

    Once together, these granular rafts can be surprisingly difficult to break up. That’s the focus of a new study on erosion in granular rafts. As seen in the top image, the raft has to be moving quite quickly before individual beads get pulled away. The experimental set-up here is pretty neat, and it’s not apparent from the video, so I’ll take a moment to explain it. The particles you see are gathered at an interface between water and oil. To generate the movement we see, researchers take the metal cylinder seen at the left of the image and pull it downward. That curves the oil-water interface, effectively creating a hill for the raft to accelerate down.

    To focus in on the forces necessary to separate individual particles, the researchers also looked at a pair of particles (bottom image). With this set-up, they could more easily track the geometry of the contact line where the oil, water, and bead meet. What they found is that the attractive forces generated between the beads are two orders of magnitude larger than predicted by classical theory. To correctly capture the effect, they needed a far more precise description of the contact line geometry around a sphere than is typically used. (Image and research credit: A. Lagarde and S. Protière)

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    Shock Waves Drive Nova Brightening

    New observations of nova V906 Carinae have provided some of the first direct evidence that the observed brightening of these stellar objects is driven by shock waves. Novae form when hydrogen from a companion star settles onto a white dwarf. Once enough material accumulates, the white dwarf blows out the excess hydrogen in a donut-shaped shell moving about the speed of a typical solar wind.

    Next, another outflow — likely triggered by residual nuclear reactions on the dwarf’s surface — slams into the denser shell at about twice the speed. This collision triggers shock waves that emit light in the gamma and visible wavelengths. Weeks later, a third, even faster outflow expanded into the cloud, generating more shock waves and measurable flares. (Video credit: NASA Goddard; research credit: E. Aydi et al.)

  • Cavitation Through Acceleration

    Cavitation Through Acceleration

    Cavitation refers to the formation of destructive bubbles of vapor within a liquid. Traditionally, we think of it as occurring when the velocity in a flow becomes high enough for the pressure to drop below the local vapor pressure, causing bubbles to form. This is what we see around turbine blades and ship propellers.

    But cavitation also occurs in situations where the overall velocity is relatively low, provided there’s a sudden acceleration. That’s the situation we see above. The impact — either of a mallet off-screen or of the tube striking the floor — causes the liquid inside suddenly accelerate upward. Notice in the second image how the liquid interface moves upward as the first bubbles form.

    Each of these cavitation bubbles has such a low pressure that they’re basically a vacuum, and their collapse can cause shock waves that reverberate through the container, causing it to break. Check out that test tube in the last image. Notice that there’s no sign of cracking when the test tube hits the floor; in fact, the researchers demonstrate in their paper that an empty test tube dropped from the same height doesn’t break. Fractures only form after the cavitation bubbles do. (Image and research credit: Z. Pan et al.; submitted by A.J.F.)