FYFD

Month: August 2020

  • Featured Video Play Icon

    Molten Thermite

    This glowing, molten liquid captured by the Slow Mo Guys is thermite. The chemical reaction behind thermite is highly exothermic, hence its intense glow. There’s some great fluid dynamics hiding in this video. First, there’s the dripping thermite (Image 1), which breaks up into droplets via the Plateau-Rayleigh instability before shattering when it hits the ground.

    Then there are the sequences (Images 2 and 3) of thermite dripping into water. The heat of the reacting thermite vaporizes a layer of water around it, creating a bubble that completely envelops the thermite. In other words, the falling thermite is supercavitating! That layer of air significantly reduces drag on the thermite and it insulates the thermite from the cooler temperature of the water. (Video and image credit: The Slow Mo Guys)

  • Droplets From Jets

    Droplets From Jets

    On the ocean, countless crashing waves are creating bubbles. When they burst, those bubbles generate jets and droplets that spray into the sky, carrying sea salt, dust, and biological material into the atmosphere. Researchers know these droplets and their evaporation are important for understanding environmental processes, but figuring out how to capture that importance in models continues to be a challenge.

    In a new study, researchers concentrated on a simplified problem: the bursting of a single bubble in pure water. By studying a wide range of conditions, the team found that jets from these bubbles could eject as many as 14 droplets apiece. And though existing models have mostly ignored all but the first droplet, their work showed that all of the droplets should be accounted for in any evaporation models. (Image credit: C. Couto; research credit: A. Berny et al.)

  • Featured Video Play Icon

    “Vorticity 3”

    Mike Olbinski’s “Vorticity 3” is a stunning view of storm chasing in the American West. I’ve learned after years in Colorado to always look up because dramatic skies are common here, as is seeing rain falling miles away. Olbinski’s film captures all of that grandeur and more, giving all of us a glimpse inside the incredible storms that mark the summer months in this region. You’ll see spinning supercell thunderstorms, bulbous mammatus clouds, towering cumulus clouds, and more. (Video and image credit: M. Olbinski)

  • The Power of a Penguin’s Rectum

    The Power of a Penguin’s Rectum

    When brooding their eggs, penguins can rarely leave the nest, but answering nature’s call is still necessary. To keep the nest clean, Adélie penguins project their feces up to more than a meter away. A new study refines previous calculations on this subject and finds that the penguin’s rectum develops far higher pressures than that of humans.

    In one hypothetical calculation, the authors estimate that a human of average height, capable of developing penguin-like rectal pressures, would project excrement more than 3 meters. In the authors’ words, “He/she should not use usual rest rooms.”

    Knowing the likely range of contact for penguins is important primarily for zookeepers, who understandably would like to avoid such projectiles. (Image credit: H. Neufeld; research credit: H. Tajima and F. Fujisawa; via phys.org)

  • Featured Video Play Icon

    The Explosive Vaporization Derby

    When pressurized, liquids can be superheated to temperatures well above their normal boiling point. When the pressure is released, the liquid will start boiling, sometimes explosively. In this video, researchers explore that dynamic by “racing” a series of liquids against one another. Each racer has been heated to a different temperature beyond the expected boiling point.

    The clear winner is the liquid with the highest overheat; as explained in the latter part of the video, beyond a critical overheat temperature, vaporization waves in the fluid enhance the boiling, helping vaporization take place faster. (Video and image credit: K. Jing et al.)

  • Artificial Microswimmers

    Artificial Microswimmers

    Tiny organisms swim through a world much more viscous than ours. To do so, they swim asymmetrically, often using wave-like motions of tiny, hair-like cilia along their bodies. Mimicking this behavior in artificial swimmers is tough; how would you actuate so many micro-appendages? A new study offers a different method: inducing cilia-like waves using magnetic fields.

    The researchers’ microswimmers are actually arrays of ferromagnetic particles. The Cheerios effect helps draw the particles together, while magnetic repulsion pushes them apart. Together, these forces help the particles assemble into crystal-like arrays.

    To make the particles swim, the researchers shift the magnetic field. All of the outer particles of the array behave like individual cilia. As the magnetic field moves, the cilia-particles move in waves, much like their natural counterparts. Using this technique, the researchers were able to demonstrate both rotational and straight-line (translational) swimming. (Image, research, and submission credit: Y. Collard et al.)

  • Ghostly Chandeliers

    Ghostly Chandeliers

    Highlighter ink sinks from the surface of water, like upside-down green mushrooms.

    Under a black light, highlighter fluid creates ghostly trails as it drips through water. The vortices that form and break into this chandelier-like shape are the result of density differences between the ink and water. Since ink is heavier than water, it sinks, but as the two fluids flow past, they shear one another, forming elaborate shapes. Formally, this is known as the Rayleigh-Taylor instability. While you may be most familiar with it from pouring cream into coffee, it’s also a key to mixing in the ocean and the explosions of supernovas. (Image credit: S. Adams et al.; via Flow Vis)

  • Featured Video Play Icon

    Popping an Oil Balloon

    Oil and water don’t mix — or at least they won’t without a lot of effort! In this video, we get to admire just how immiscible these fluids are as oil-filled balloons get burst underwater.

    Visually, the two bursts are quite spectacular. In the first image, the initial balloon has a sizeable air bubble at the top, which rises even more rapidly than the buoyant oil, creating a miniature, jelly-fish-like plume that reaches the surface first. The large oil plume follows, behaving similarly to the balloon burst without an added air bubble.

    The last of the oil in both cases comes from a cloud of smaller droplets formed near the bottom of the balloon. Being smaller and less buoyant, these drops take a lot longer to rise to the surface and remain much closer to spherical as they do. I suspect these smaller droplets form due to the forces created by the fast-moving elastic as it tears away. (Video and image credit: Warped Perception)

  • Undulating Keeps Flying Snakes Steady

    Undulating Keeps Flying Snakes Steady

    Flying snakes undulate through the air as they glide. But, unlike on land, these wiggles aren’t for propulsion. A new study shows instead that they are key to the snake staying stable in flight.

    Upon take-off, a flying snake flattens its body, forming a wing-like shape that helps them generate lift and control drag. But while they glide, they also slither and pitch their tail.

    Researchers recorded more than 150 flights by live snakes, then used that data to construct their own digital snake. The model could fly like a real snake or be tested without undulations to see what would happen. The researchers discovered that, without that mid-air slithering, the snake quickly lost control and rolled to the side. (Image and research credit: I. Yeaton et al.; via NYTimes; submitted by Kam-Yung Soh)

  • Mimicking Insect Flight

    Mimicking Insect Flight

    There’s an oft-repeated tale that science cannot explain how a bumblebee flies. And while that may have been true 80 years ago, when engineers assumed they could apply their knowledge of fixed-wing aircraft to insects, it’s very far from the truth now.

    Being small, insects use aerodynamic tricks that are very different from the physics used by aircraft or even birds. Insects like fruit flies use a forward-and-backward sweeping motion at a very high angle of attack as they flap. This motion creates a vortex at the leading edge of the wing that provides the lift keeping the insect aloft. It still requires fast reflexes — most insects flap their wings hundreds of times a second — but the mechanism is robust enough to keep insects aloft and maneuverable. (Image credits: Robobee – K. Ma and P. Chirarattananon, simulation – F. T. Muijres et al., illustration – G. Lauder; via APS Physics)