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Search results for: “shear”

  • Making Drops Stick

    Making Drops Stick

    As you may have noticed when washing vegetables, many plants have superhydrophobic leaves. Water just beads up on their surface and slides right off. This is a useful feature for plants that want to direct that water toward their roots, but it’s a frustration in agriculture, where that superhydrophobicity means extra spraying of pesticides in order to get enough to stick to the plant.

    But that may not be the case for much longer. Researchers have found that adding a little polymer to water droplets (right) can suppress their ability to rebound (left) from superhydrophobic surfaces. Above a critical concentration, the high shear rate of the droplet as it tries to detach activates the viscoelastic properties of the polymer. That viscoelasticity suppresses the rebound, keeping the droplet attached. That’s good news for everyone, since it means less spraying is needed to protect crops. (Image and research credit: P. Dhar et al.)

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    Driving Instabilities with a Twist

    Imagine that you want to study how two fluids mix when a lighter fluid is pushed into a denser one. Conceptually, it’s a straightforward situation. It would be like having a layer of oil under a layer of water and watching what happens. But how do you do that experimentally? Oil won’t naturally stay under water. If you flip the container over to start the experiment, you’ve added a bunch of extra motion from the rotation. And if you use a barrier to separate the two layers and then pull it out, you’ve added extra shear where they meet.

    To deal with challenges like these, researchers at Lehigh University spent five years designing and building the rotating wheel apparatus you see in the video above. Instead of relying on gravity to force the lighter fluid into a denser one, this set-up uses centrifugal force. The test fluids start out on the loading wheel, spinning in their naturally stable configuration. Then once both sides are rotating at the desired speed, the track flips, transferring the experiment onto the other wheel, which rotates in the opposite sense. This gives the fluids a sudden change in the direction of the centrifugal force and, once the apparatus completes shake-down, should give us new insight into the sort of mixing seen in fusion. (Video credit: Lehigh University; see also Turbulent Flow Design Group)

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    Modeling Oobleck

    Oobleck – that peculiarly behaved mixture of cornstarch and water – continues to be a favorite of children and researchers both. Oobleck flows like a liquid when deformed slowly, but try to move it quickly and it will seize up like a solid. This sudden change depends on the tiny particles of cornstarch suspended in the liquid. When they’re given time, electrostatic forces between the particles help them repel one another and keep the liquid flowing. But under sudden impacts, the particles get jammed together and the friction between neighboring grains makes the viscosity of the fluid increase by orders of magnitude. 

    Researchers are now able to model these particle interactions numerically, which will help them predict how oobleck and similar substances will behave in applications like body armor or pothole repair. (Video credit: MIT; via MIT News; research credit: A. Baumgarten and K. Kamrin)

  • Waves on a Supercell

    Waves on a Supercell

    This Colorado supercell thunderstorm features an unusual twist. Notice the sawtooth-like protrusions along the outer cloud wall. These are Kelvin-Helmholtz waves, like these fair-weather clouds we’ve seen before, but instead of occurring vertically, they project horizontally! That implies that the invisible layer of air just outside the cloud wall is moving faster than the wall itself. That creates shear along the outer edge of the cloud wall and causes these waves to form. This is the first time I’ve ever seen this sort of thing. What an awesome photo! (Image credit: M. Charnick; submitted by jpshoer)

  • Waves in the Sky

    Waves in the Sky

    Even when the sky is mostly blue, there’s a lot going on at different altitudes. The winds do not move in a consistent direction or at the same speed, something which becomes apparent when watching clouds move relative to one another. When different layers of air move past one another, there is shear between them, not unlike the friction you feel when running your hand along a table. Under the right circumstances, this shear creates Kelvin-Helmholtz waves like the ones in this image over Helena Valley, Montana. Fast-moving winds (blowing right to left in the image) above a layer of clouds created these breaking wave-like curls. The same phenomenon creates many of the ocean’s waves from the shear caused by wind blowing across water. (Image credit: H. Martin, via EPOD)

  • Asymmetric Wakes

    Asymmetric Wakes

    When a ship moves through water, it leaves a distinctive V-shaped wake behind it. In the nineteenth century, Lord Kelvin made some of the earliest theoretical studies of this phenomenon, calculating that the arms of the V should have an angle of about 39 degrees, known as the Kelvin angle. But that theoretical result doesn’t always hold in practice.

    More recently, researchers calculated and experimentally verified an extension to Kelvin’s theory, one which accounts for what’s going on below the water. They found that any shear in the currents below the surface can strongly affect the shape of a boat’s wake, altering angles and creating asymmetry between the two sides. The results have practical consequences, too: they help predict the wave resistance ships will encounter when traversing areas with substantial subsurface shear, like near the mouths of river deltas. (Image credit: M. Adams; research credit: B. Smeltzer et al.; submitted by clogwog)

  • Storing Memory in Bubbles

    Storing Memory in Bubbles

    Soft systems like this bubble raft can retain memory of how they reached their current configuration. Because the bubbles are different sizes, they cannot pack into a crystalline structure, and because they’re too close together to move easily, they cannot reconfigure into their most efficient packing. This leaves the system out of equilibrium, which is key to its memory. 

    By shearing the bubbles between a spinning inner ring (left in image) and a stationary outer one (not shown) several times, researchers found they they could coax the bubbles into a configuration that was unresponsive to further shearing at that amplitude. 

    Once the bubbles were configured, the scientists could sweep through many shear amplitudes and look for the one with the smallest response. This was always the “remembered” shear amplitude. Effectively, the system can record and read out values similar to the way a computer bit does. Bubbles are no replacement for silicon, though. In this case, scientists are more interested in what memory in these systems can teach us about other, similar mechanical systems and how they respond to forces. (Image and research credit: S. Mukherji et al.; via Physics Today; submitted by Kam-Yung Soh)

  • Floccing Particles

    Floccing Particles

    Adding particles to a viscous fluid can create unexpected complications, thanks to the interplay of fluid and solid interactions. Here we see a dilute mixture of dark spherical particles suspended in a layer of fluid cushioned between the walls of an inner and outer cylinder. Initially, the particles are evenly distributed, but when the inner cylinder begins to rotate, it shears the fluid layer. Hydrodynamic forces assemble the particles together into loose conglomerates known as flocs. Once the particles form these log-like shapes, they remain stable thanks to the balance between viscous drag on particles and the attractive forces that pull particles toward one another. (Image and research credit: Z. Varga et al.; submitted by Thibaut D.)

  • Using Bubbles to Keep Clean

    Using Bubbles to Keep Clean

    Keeping produce clean of foodborne pathogens is a serious issue, and delicate fruits and vegetables like tomatoes cannot withstand intense procedures like cavitation-based cleaning. But a new study suggests that simple air bubbles may have the power to keep our produce free of germs.

    In particular, researchers studied air bubbles injected into water as they bounced and slid along an inclined solid surface. They found that as a bubble approaches a tilted surface, it squeezes a thin film of liquid between itself and the surface. That flow creates a shear stress that pushes contaminants like E. coli away from the point of impact. When the bubble bounces away, fluid gets sucked back into the void left behind, creating more shear stress. In their experiments and simulations, the team measured shear stresses greater than 300 Pa, more than double what’s needed to remove foodborne bacteria like Listeria. (Image credit: Pixabay; research credit: E. Esmaili et al.)

  • Viscoelasticity and Liquid Armor

    Viscoelasticity and Liquid Armor

    One proposed method for improving bulletproof armor is adding a layer of non-Newtonian fluid that can help absorb and dissipate the kinetic energy of impact. Thus far researchers have focused on shear-thickening fluids – like cornstarch-based oobleck – filled with particles that jam together if anything tries to deform them quickly. But is it really the shear-thickening properties that matter for high-speed impacts?

    To test this, researchers studied projectile impact on three fluids: water (left), a cornstarch mixture (not shown), and a shear-thinning polymer mixture (right). Water is Newtonian, and it slows down the projectile but doesn’t stop it. Both the shear-thickening cornstarch and the shear-thinning polymer mixture do stop the projectile. And by modeling the impacts, researchers concluded that the key to that energy dissipation isn’t their shear-related behaviors: it’s the fact that both fluids are viscoelastic.

    That means that these fluids show both viscous (fluid-like) and elastic (solid-like) responses depending on the timescale of an impact. The high speed of the impact triggered a strong viscous response in both fluids, bringing the projectile to a halt. And if, as the researchers suggest, it’s a fluid’s viscoelasticity that matters most, that widens the field of candidates when it comes to developing a fluid-based armor. (Image and research credit: T. de Goede et al.)

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