FYFD

Tag: biology

  • Grebe Rushing Physics

    Grebe Rushing Physics

    As capable a water-runner as the common basilisk is, the western and Clark’s grebe is even more impressive. Not only do these birds weigh up to three times as much as an adult basilisk, but they start their water-walking from inside the water, which requires overcoming much more hydrodynamic force.

    Like the lizards, grebes must slap the water with their feet to generate upward forces capable of supporting their weight above water. The birds take as many as 20 steps a second – an incredible and unmatched stride rate for a creature their size. Their feet impact the water at up at 4.5 m/s, which generates an impulse equivalent to 30-55% of the grebe’s weight. The rest of the necessary impulse comes from the stroke phase, where the bird pushes its foot down against the water.

    When retracting its foot, the grebe extracts the foot with a sideways motion through the water – unlike the basilisk which pulls its foot out through the air cavity its stroke created. In order to reduce drag, the grebe’s foot collapses into a more streamlined shape as it gets pulled from the water, letting the bird set up for the next step. (Image/video credit: B. Struck, source; research credit: G. Clifton et al.)

    This week FYFD is exploring the physics of walking on water, all leading up to a special webcast on March 5th with guests from The Splash Lab. The live webcast will be open to all FYFD patrons, so be sure to sign up if you want to tune in.

  • Surface-Tension Supported Walkers

    Surface-Tension Supported Walkers

    Nature’s smallest water-walkers use surface tension to keep themselves afloat. This includes hundreds of species of invertebrates like insects and spiders as well as the occasional extremely tiny vertebrate, like the 2-4 cm long pygmy gecko shown above. These animals typically have very thin parts of themselves touching the water – like the spindly legs of the water strider. These skinny appendages curve the air-water interface and that curvature, along with the water’s surface tension, generates the force supporting the animal.

    Staying afloat on surface tension does little good if a raindrop or passing splash submerges these tiny water-walkers. To avoid that fate, these animals are also hydrophobic or water repellent. This adaptation keeps them from drowning and helps them enhance the curvature where their feet meet the water.

    Those tiny indentations can also be important for the animal’s propulsion. Water striders, for example, use their long middle legs like oars to propel themselves. Any rower will tell you that sticks make poor paddles – they’re just not good at transferring momentum to the water. But curving the surface and then pushing off that curvature works remarkably well. It’s how the water strider creates the vortices in its wake in the image above.

    For more on water strider propulsion, I recommend this Science Friday video. If you’d like to see the gecko in action, check out BBC Life’s “Reptiles and Amphibians” episode, which is available on Netflix in the U.S. (Image credits: pygmy gecko, BBC; water strider, J. Bush et al.)

    This week FYFD is exploring the physics of walking on water, all leading up to a special webcast on March 5th with guests from The Splash Lab. You don’t want to miss it!

  • The Basilisk Lizard

    The Basilisk Lizard

    One of the most famous water-walking creatures is the common basilisk lizard. These South American reptiles are far too large to be kept aloft by surface tension and other interfacial effects. They generate the vertical force necessary to stay above water by slapping the water hard and fast. There are three phases to a basilisk’s water running gait: the slap, the stroke, and the retraction.

    In the slap phase, the lizard slams its foot flat against the water surface at a peak velocity of about 3.75 m/s. The impact pushes water down and generates an upward force on the lizard that accounts for between 15-30% of the lizard’s body weight, depending on the size of the lizard. The rest of the upward force comes from the stroke phase, where the lizard pushes its foot downward in the water, causing an air cavity to form.

    The air cavity is vital for the last phase of the lizard’s step. The basilisk must pull its foot out and prepare for the next slap, ideally doing so without generating too much drag. The lizard does this by pulling its foot through the air cavity before it seals. Doing so through air is much easier than through water.

    Water-walking this way requires fast reflexes. Basilisks take up to 20 steps per second when running across water and reach speeds of about 1.6 m/s. Although both juvenile and adult basilisks can run on water, the smaller lizards do better because they can generate more than enough impulse to overcome their weight. (Image credit: T. Hsieh/Lauder Laboratory, source; video credit: BBC; research credits: J. Glasheen and T. McMahon, G. Clifton et al.)

    This week FYFD is exploring the physics of walking on water, all leading up a special webcast March 5th with guests from The Splash Lab.

  • Featured Video Play Icon

    Watching a Sneeze

    What does a sneeze look like? You might imagine it as a violent burst of air and a cloud of tiny droplets. But this high-speed video shows, that’s only part of the story. The liquid leaving a sneezer’s mouth and nose is a mixture of saliva and mucus, and in the few hundred milliseconds it takes to expel this air/mucosaliva mixture, there’s not enough time for the liquid to break into droplets. Instead, liquid leaves the mouth as a fluid sheet that breaks into long ligaments.

    Because mucosaliva is viscoelastic and non-Newtonian, it does not break down into droplets as quickly as water. Instead, when stretched, the proteins inside the fluid tend to pull back, causing large droplets to form with skinny strands between them – the beads-on-a-string instability. The end result when the ligaments do finally break is more large droplets than one would expect from a fluid like water. Understanding this break-up process and the final distribution of droplet sizes is vital for better understanding the spread of diseases and pathogens.  (Video credit: Bourouiba Research Group; research paper: B. Scharfman et al., PDF)

  • Drying Blood Can Reveal Anemia

    Drying Blood Can Reveal Anemia

    Blood is a remarkably complicated fluid, thanks in part to its many constituents. What we see here is an animation of a drop of blood evaporating at several times normal speed. As water from the blood evaporates, it causes relative changes in surface tension. These surface tension gradients cause convection inside the drop and carry red blood cells toward the outer portion of the drop. As the blood evaporates further, it leaves behind different patterns that depend on which parts of the whole blood mixture were deposited in each region. Interestingly, the final desiccation patterns can indicate the healthiness of a patient. Below are images of dried blood patterns from (left) a healthy individual and (right) an anemic individual. (Image credits: D. Brutin et. al., source)

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  • Fluids Round-Up

    Fluids Round-Up

    New year, new (or renewed) experiments. This is the fluids round-up, where I collect cool fluids-related links, articles, etc. that deserve a look. Without further ado:

    (Video credit and submission: Julia Set Collection/S. Bocci; image credit: IRPI LLC, source)

  • Inside APS DFD 2015

    Inside APS DFD 2015

    What do shark scales, underwater robots, blood flow, and art have in common? They’re all a part of the latest FYFD video! Check out my behind-the-scenes look at the latest American Physical Society Division of Fluid Dynamics meeting. Meet the researchers and find out about the science everyone was talking about! (Image/video credit: N. Sharp)

  • Collecting Water in the Desert

    Collecting Water in the Desert

    Desert-dwelling plants like cactuses have to be efficient collectors of water. Many types of cactus are particularly good at gathering water from fog that condenses on their spines. Droplets that form near a spine’s tip move slowly but inexorably toward the base of the spine so that the cactus can absorb them. The secret to this clever transport lies in the microstructure of the spine’s surface. The

    Gymnocalycium baldianum cactus, for example, has splayed scales along its spines. Capillary interactions with the scales result in differences in curvature on either side of the droplet. Curved fluid surfaces generate what’s known as Laplace pressure, with a tighter radius of curvature causing a larger Laplace pressure. Because the curvature of the droplet varies from the base side to the tip side of the spine, the difference in Laplace pressures across the droplet creates a force that drives the droplet toward the spine’s base. (Image credit: C. Liu et al., source)

  • Nectar-Eating Bats

    Nectar-Eating Bats

    Nectar-eating bats have evolved to use several methods to drink. Some bats, like the Pallas’ long-tongued bat (top), use a lapping method. Hair-like papillae on the bat’s tongue increase the contact area with the nectar, helping to draw the fluid up in viscous globs as the bat repeatedly dips its tongue into the nectar. The orange nectar bat (middle and bottom), in contrast, has a tongue with a long central groove. This bat’s tongue stays submerged as it drinks. Researchers hypothesize that muscle action along the tongue, combined with capillary action in the narrow groove, allow the bat to actively pump nectar up to its mouth. It’s worth noting that the edges of the bat’s tongue do not curl around to touch, so the bat is definitely not using suction as one would with a straw. (Image credit: M. Tschapka et al., source)

  • How Plants Move

    How Plants Move

    Though most plants don’t move at speeds that we humans notice, many plants are remarkably active, as seen in the timelapse animations above. Much of this motion is driven by water flow inside the plant. The two plants above are phototropic–they move in response to light. The motion is actuated via a specialized motor cell called the pulvinus, which is located at the base of the leaf where it meets the stem. Unlike animal cells, plant cells have stiff outer walls that allow them to maintain an internal pressure–or turgor pressure–that differs from the outside environment. In fact, it’s not unusual for a plant’s cell to hold a pressure equivalent to 5 atmospheres! The plant manipulates this turgor pressure by controlling the transport of ions across cell membranes. Pump more ions into a cell, and osmosis will cause water to flow into the area of high solute (ion) concentration. This causes the cell to swell and raises the turgor pressure, resulting in the plant’s leaf moving. (Image credit: L. Miller and A. Hoover, source; additional research credit: J. Dumais and Y. Forterre)