FYFD

Category: Phenomena

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    How Canal Locks Work

    For thousands of years, boats have been a critical component of trade, efficiently enabling transport of goods over large distances. But water’s self-leveling creates challenges when moving up and downstream through rivers and canals. To get around this, engineers use locks, which act as a sort of gravity-driven elevator to lift and lower boats to the appropriate water level. In this video from Practical Engineering, we learn about the basic physics behind locks as well as some of the methods engineers use to limit water loss through the lock. (Image and video credit: Practical Engineering)

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    Why Aren’t Trees Taller?

    Trees are incredible organisms, with some species capable of growing more than 100 meters in height. But how do trees get so big and why don’t they grow even taller? The limit, it turns out, is how far fluid forces can win over gravity.

    To live and grow, trees must be able to transport nutrients between their roots and their highest branches. As explained in the video, there are three forces that enable this transport inside trees: transpiration, capillary action, and root pressure. Of these, you are probably most familiar with capillary action, where intermolecular forces help liquids climb up the inside of narrow spaces, like the straw in your drink. Capillary action can’t lift liquids more than a few centimeters against gravity, though.

    Similarly, root pressure is limited in how far it can raise liquids. Functionally, it’s pretty similar to the way a column of water or mercury can be held up by atmospheric pressure acting at the base of a barometer. But atmospheric pressure can only hold up 10.3 meters of water, so what’s a tree to do?

    This is where transpiration — the most important force for sap transport in the tree — comes in. As water evaporates out of the tree’s leaves, it creates negative pressure that — along with water’s natural cohesion — literally drags sap up from the roots. It’s this massive pull that drives the flow and enables most of a tree’s height. (Image and video credit: TED-Ed)

  • Two Views of Ocean Eddies

    Two Views of Ocean Eddies

    Colorful, sediment-laden eddies swirl off the Italian coast in this satellite image. These small-scale eddies — less than 10 km in diameter — can be short-lived and are often difficult to capture in numerical models, but remote sensing can help scientists better understand their impact on oceanic mixing, especially when we capture more than one view of the same event.

    The image below shows the same eddies in an infrared (thermal) view. The resolution on this instrument is not as fine as the natural color one, but we can still make out some of the same swirling motions. It’s also worth comparing the features we don’t see in both images. For example, the Cornia River discharges in infrared as a bright, white plume of cooler water, but it’s barely visible in the color-image, suggesting that the river is not contributing much sediment to the bay. (Image credit: USGS; via NASA Earth Observatory)

    Infrared satellite image of waters off the coast of Italy.
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    Digging Droplets

    A droplet on a surface much hotter than its boiling point will skate on a layer of its own vapor, thanks to the Leidenfrost effect. But if that surface is, instead, a granular mixture like this glass powder, the droplet will dig itself a hole.

    As in the usual Leidenfrost situation, the heat of the powder causes part of the drop to vaporize. But as that vapor flows away, it carries powder with it. At the same time, the vaporization process causes the droplet to vibrate violently, which frees more powder and helps the drop dig deeper. Eventually, the drop will vaporize completely, leaving a volcano-like crater in the powder. (Image and video credit: C. Kalelkar and H. Sai)

    A water droplet falls on heated glass powder, which it then digs its way into.
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    Ventilation and Respiratory Disease

    In 1977, one passenger with the flu infected 38 people onboard a flight with malfunctioning ventilation. In this video, Dianna digs into the physics of respiratory disease transmission and just why ventilation is so key to preventing it.

    There are three primary modes of transmission for respiratory diseases like influence or SARS-CoV-2: 1) touching an infected surface and then oneself, i.e., self-inoculation; 2) inhaling virus-filled droplets larger than 5 nm; and 3) inhaling virus-filled droplets smaller than 5 nm. That size cut-off may seem a little arbitrary, but it’s how scientists distinguish between droplets that fall quickly to the ground and ones that can persist on buoyant air currents.

    That airborne persistence is one of the reasons ventilation — in other words, replacing the air — is so important. So many people on that 1977 flight got sick because there was no system removing the infected air and bringing in fresh air. For more on the fluid dynamics disease transmission, check out these posts. Curious about those bacterial bubble bursts? I’ve covered that, too. (Video and image credit: Physics Girl)

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    Sundews Weaponize Viscoelasticity

    In nutrient-poor soils, carnivorous plants like the cape sundew supplement their diets by eating insects. To entice their prey, the cape sundew secretes droplets of sugary water. But unwary insects who land to feed soon find themselves unable to pull away from this viscoelastic liquid. Complex molecules in the fluid grant it elasticity, so when insects pull against it, the liquid stretches and pulls back instead of breaking up. Other carnivorous plants, like the pitcher plant, use similar non-Newtonian tricks to trap insects. (Video and image credit: Deep Look)

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    Ultrasound in Medicine

    When you hear the term “ultrasound,” your brain likely jumps to grainy black and white images of unborn babies, but this technology has a lot more medical uses than just that! Ultrasound is used to image many parts of the body — earlier this year, I got to see my own heart in action through an echocardiogram, for example. But the technology has therapeutic uses as well. At higher energies, ultrasound is used to break up kidney stones (through cavitation), treat tremors, and alleviate some sources of pain. To learn more, check out Explore Sound’s page on biomedical acoustics. (Video and image credit: Acoustical Society of America)

  • Speeding Sedimentation

    Speeding Sedimentation

    Did you know that particles settle faster in an inclined container instead of a vertical one? This sedimentation phenomenon is known as the Boycott effect, after the researcher who first described it. Boycott noticed that red blood cells settled out of samples faster when the test tubes were inclined.

    The inclined walls give particles a much larger area to settle on. As the particles gather on the wall, it creates a buoyant, particle-free layer of fluid above. That fluid quickly rises to the top of the container, helping to push the sediment further toward the bottom. As you can see in the video below, the Boycott effect drastically reduces settling time. (Video and image credit: C. Kalelkar)

  • Sediment and Coral

    Sediment and Coral

    As rivers wash sediment toward the sea, they carve elaborate deltas like that of the Rio Cauto in Cuba. Over time these sediments build up marshes, swamps, lagoons, and other wetlands that provide critical habitat and flood control. Sediment also washes into the bay, where it interacts with the coral reefs (light green lines on the lower left) and the species that live there. (Image credit: L. Dauphin/USGS; via NASA Earth Observatory)

    Satellite image of Cuba's Gulf of Guacanayabo. The green curves in the lower left are the upper portions of coral reefs in the bay.
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    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)