FYFD

Search results for: “shear”

  • Featured Video Play Icon

    Exascale Simulations

    Capturing what goes on inside a combustion engine is incredibly difficult. It’s a problem that depends on turbulent flow, chemistry, heat transfer, and more. To represent all of those aspects in a numerical simulation requires enormous computational resources. It’s not simply the realm of a supercomputer; it requires some of the fastest supercomputers in existence.

    Exascale computing, like that used for the simulations in this video, is defined as at least 10^18 (floating-point) operations per second. For comparison, my PC has a recent, high-end graphics card, and it’s about a million times slower than that. These are absolutely gigantic simulations. (Image and video credit: N. Wimer et al.)

  • Slab Avalanche Physics

    Slab Avalanche Physics

    Slab avalanches like the one shown here begin after weak, porous layers of snow get buried by fresher, more cohesive snow layers. On a steep slope, the weight of the new snow can be too great for friction to hold the slab in place, causing the upper layer to crack and slide at speeds up to 150 meters per second. Scientists had two competing theories for how slab avalanches began. One theory presumed that the weak layer of snow failed under shear; the other argued that the collapse of the lower, porous layer was at fault.

    In a new study combining large-scale numerical simulation with real-life observations, scientists came to a new conclusion: cracks began to form in the porous layer as the weight of heavier snow crushed down, but once the cracks formed, the shear mechanism took over. Cracks formed by shear could propagate along the existing cracks in the porous layer, allowing faster crack propagation than through undamaged snow. In the end, it’s the combination of the two mechanisms that triggers the avalanche. (Image credit: R. Flück; research credit: B. Trottet et al.; via Physics World)

  • Finger Painting Physics

    Finger Painting Physics

    Spreading paint with a brush or with fingers is familiar activity for most people. It’s also similar to processes used in industry for spreading thin layers of paint and other complex fluids. In a recent study, researchers took a look at how a soft, elastic blade (similar to a paintbrush or one’s fingers) spreads shear-thinning fluids (like paint) and Newtonian fluids (like water). Surprisingly, they found that it actually takes 30% more mechanical work to spread a shear-thinning fluid than the same volume of an equivalent Newtonian one. That’s pretty much the opposite of what we’d expect since the action of spreading (and shearing) the complex fluid should reduce its viscosity. However, they did find that the shear-thinning fluid spreads to a thin layer more consistently than the Newtonian fluid does. (Image credit: A. Kolosyuk; research credit: M. Krapez et al.)

  • Featured Video Play Icon

    Stably Jammed

    Granular materials like sand, gravel, and medications can become a rigid mass when squeezed or sheared. Even with a relatively loose packing, these materials can jam together to act like a solid if the contacts between grains no longer allow particles to shift or rotate. In this video, researchers explore how stable these jammed states are by repeatedly shearing the mixture and observing how it changes.

    Most of the videos are set up as a triptych, where all three panels show the same material. On the left, you see a simple view showing the position of each particle. In the middle, the disks are viewed through polarized filters, so that the material looks brightest where it is stressed. This view lets us see the force chains that run through the material. On the right, UV-sensitive ink on each marker glows to show any rotation particles experience.

    In the first sample, repeated shearing slowly unjams the mixture and allows it to shift and flow once more. We see this from the decreasing brightness in the middle panel. The slow fade to black means that the force chain network has disappeared entirely. In contrast, the second sample ultimately reaches an “ultra-stable” jammed state, in which further shear cycles cause no change to the network. Once again, this is easiest to observe in the middle image, where the bright force network stops changing after 2,000 cycles or so. (Image and video credit: Y. Zhao et al., research pre-print)

  • Asperitas Formation

    Asperitas Formation

    In 2017, the World Meteorological Organization named a new cloud type: the wave-like asperitas cloud. How these rare and distinctive clouds form is still a matter of debate, but this new study suggests that they need conditions similar to those that produce mammatus clouds, plus some added shear.

    Using direct numerical simulations, the authors studied a moisture-filled cloud layer sitting above drier ambient air. Without shear, large droplets in this cloud layer slowly settle downward. As the droplets evaporate, they cool the area just below the cloud, changing the density and creating a Rayleigh-Taylor-like instability. This is one proposed mechanism for mammatus clouds, which have bulbous shapes that sink down from the cloud.

    When they added shear to the simulation, the authors found that instead of mammatus clouds, they observed asperitas ones. But the amount of shear had to be just right. Too little shear produced mammatus clouds; too much and the shear smeared out the sinking lobes before they could form asperitas waves. (Image credit: A. Beatson; research credit: S. Ravichandran and R. Govindarajan)

  • Dispelling Ice

    Dispelling Ice

    In winter weather, delays pile up at airports when planes need de-icing. Our current process involves spraying thousands of gallons of chemicals on planes, but these chemicals are easily removed by shear stress and dissolution, meaning that by the time a plane takes off, there is little to no de-icing agent remaining on the plane. Instead, those chemicals become run-off.

    Researchers looking to change that have developed a family of anti-icing coatings — including creams, sprays, and gels — that are easy to use and apply, non-toxic, and much longer lasting than conventional methods. Ice slides easily off their gel coatings, which remain optically transparent even under freezing conditions — and ice can take 25 times longer to form on the gels compared to current anti-icing tech.

    The team envisions using their coatings on much more than airplanes. Imagine traffic lights that can’t be obscured by ice or snow, a windshield on your car that never freezes over, or even an anti-icing spray that could protect crops from a sudden freeze! (Image, video, research, and submission credit: R. Chatterjee et al.; see also)

  • Mixing the Immiscible

    Mixing the Immiscible

    Immiscible liquids — like oil and water — do not combine easily. Typically, with enough effort, you can create an emulsion — a mixture formed from droplets of one liquid suspended in the other — like the one above. But a team of researchers have taken mixing immiscible liquids to a new level using their Vortex Fluid Device (VFD).

    Longtime readers may remember the group from their Ig-Nobel-winning demonstration of unboiling an egg, but this time the team is used the VFD to mix and de-mix immiscible liquids. As shown in the video below, the VFD is essentially a fast-spinning tube tilted at a 45-degree angle. As it spins, the liquids inside are forced into thin films with very high shear rates — high enough that immiscible liquids like water and toluene are forced together without forming an emulsion. Essentially, the mechanical forces mixing the liquids are strong enough to overcome the chemistry that typically keeps them apart.

    Impressively, the device manages this without using harsh surfactants or catalysts that other methods rely on. As a result, the technique offers a greener method for mixing chemicals for pharmaceuticals, cosmetics, food processing, and more. (Image credit: pisauikan; research credit: M. Jellicoe et al.; video credit: Flinders University; submitted by Marc A.)

  • Beijing 2022: Why Are Ice and Snow Slippery?

    Beijing 2022: Why Are Ice and Snow Slippery?

    Although every Olympic winter sport relies on the slippery nature of snow and ice, exactly why those substances are so slippery has been an enduring mystery. Michael Faraday hypothesized in the nineteenth century that ice may have a thin, liquid-like layer at its surface, something that modern studies have repeatedly found.

    One recent study used an entirely new instrument to probe the characteristics of this lubrication layer and found that it is only a few hundred nanometers thick. But the fluid in this layer is nothing like the water we’re used to. Instead it has a viscosity more akin to oil and its response to deformation is shear-thinning and viscoelastic, more like the complex fluids in our kitchens and bodies than pure, simple water. They found that using a hydrophobic probe modified the interfacial viscosity even further, which finally provides a hint at the mechanism behind waxing skis and skates. 

    Fortunately for us, we’ve found plenty of ways to employ and enjoy water’s slipperiness, even as the mystery of it slowly gives way to understanding. (Image credit: M. Fournier; research credit: L. Canale et al.; via Physics World; submitted by Kam-Yung Soh)

  • Changing with the Flow

    Changing with the Flow

    Chemically-reacting flows are some of the toughest problems to unravel. In this new study, researchers found that the very act of flowing through narrow channels can change the speed of chemical reactions. In particular, they found that protein molecules carried through a capillary tube (comparable in size to human capillaries) changed their local shape as a result of the shear forces they experienced. Those changes actually sped up the proteins’ chemical reactions compared to the reaction speed for the chemicals in bulk.

    That finding suggests two important takeaways: 1) chemicals may be absorbed in the human bloodstream differently in capillaries than in other parts of the cardiovascular system, and 2) mimicking these tiny capillaries in microfluidic devices could be useful in speeding up certain biochemical reactions. (Image credit: top – KazuN, visual abstract – T. Hakala et al.; research credit: T. Hakala et al.; via Science; submitted by Kam-Yung Soh)

    Graphical abstract showing that shear forces in small channels can cause local changes to protein structure that affect the rate of chemical reactions.
  • Driven From Equilibrium

    Driven From Equilibrium

    With the right application of force, liquids can take on shapes that defy our intuition. Here researchers sandwiched two immiscible oils between glass slides and applied an electric field. Because the two oils have different electrical responses, charges build along the interface between them. These charges lead to non-trivial electrohydrodynamic flows and a multitude of bizarre shapes. They observed polygonal droplets, streaming droplet lattices, and spinning filaments among others. As long as the electric field remains on, the wild behaviors continue; once the field is turned off, the oils relax back to typical, rounded drops. (Image, video, and research credit: G. Raju et al.; via Physics World)

More posts