FYFD

Category: Research

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    Simulating Better Breaking Waves

    In the ocean, breaking waves trap air into bubbles that then cluster into foam, but conventional simulations don’t capture this foaminess. For bubbles to cluster into foam, there has to be a force preventing — or at least delaying — their coalescence. Typically, this is caused by impurities in the water that help lower the surface tension and thereby lengthen the bubbles’ lifespans. When these features get added to simulation models, bubbles begin to cluster and breaking waves become foamy. (Image and video credit: P. Karnakov et al.)

  • The Wanderings of Micro-Scallops

    The Wanderings of Micro-Scallops

    In the 19th century, botanist Robert Brown observed pollen granules beneath his microscope jittering randomly. Einstein showed that this motion resulted from the impacts of much-smaller atoms against the particles. For small enough objects, the random walk of Brownian motion dominates their dynamics. A new study explores how flexible objects move at this Brownian scale.

    The researchers used trios of colloids — microscopic particles — held together by a lipid fluid layer that allows the three particles to change shape without losing contact. Essentially, each trio forms a tiny hinge. As atoms strike the colloids, they both move and change shape.

    Compared to rigid shapes, the researchers found their flexible hinges moved around in space about 3-15% faster. They also found coupling between the shape changes and motion. When the colloids hinge closed, it propels them in the direction the hinge points. Because this resembles the propulsion of scallops, the researchers refer to this as the “Brownian quasi-scallop mode.” (Image and research credit: R. Verweij et al.; via phys.org)

  • Oil Drops and Filter Feeders

    Oil Drops and Filter Feeders

    Natural oils provide critical nutrients to filter feeders like zooplankton and barnacles. These creatures capture oil droplets on bristle-like appendages such as cilia and setae. But this droplet-catching turns into a disadvantage during petroleum spills, when capturing and ingesting oil can be lethal. A recent study looks at the fluid dynamics of oil droplet capture for these tiny creatures.

    The authors found that filter feeders capture a range of droplets regardless of size and oil viscosity. But not all droplets stay attached long enough to get consumed, and the larger a droplet is, the lower the flow velocity necessary to detach it from the animal. That suggests a method of limiting uptake of spilled petroleum into the marine food chain: use surfactants to break up the oil into droplets large enough that they’ll detach from filter feeders before getting eaten. (Image credit: D. Pelusi; research credit: F. Letendre et al.; submitted by Christopher C.)

  • Testing Granular Gas Theory

    Testing Granular Gas Theory

    When excited, a group of particles can behave much like a gas. These granular gases exhibit many similarities to molecular gases but contain one vital difference: without a constant input of energy, granular gases lose kinetic energy to collisions.

    Over the years, scientists have developed a special theory to describe the behaviors of granular gases, but most of its predictions could only be tested numerically. A new study used a microgravity experiment aboard a sounding rocket to physically test the theory.

    The experiment, shown above, consists of nearly 2800 magnetic particles, which the researchers could stir up using pairs of magnets. Once they shut off the magnets (which occurs at t=0 in the image above), the granular gas begins to “cool” as collisions sap away its energy. With this set-up, the researchers were able to confirm several key predictions of the granular gas theory. (Image and research credit: P. Yu et al.; via APS Physics)

  • Bacterial Turbulence

    Bacterial Turbulence

    Conventional fluid dynamical wisdom posits that any flows at the microscale should be laminar. Tiny swimmers like microorganisms live in a world dominated by viscosity, therefore, there can be no turbulence. But experiments with bacterial colonies have shown that’s not entirely true. With enough micro-swimmers moving around, even these viscous, small-scale flows become turbulent.

    That’s what is shown in Image 2, where tracer particles show the complex motion of fluid around a bacterial swarm. By tracking both the bacteria motion and the fluid motion, researchers were able to describe the flow using statistical methods similar to those used for conventional turbulence. The characteristics of this bacterial turbulence are not identical to larger-scale turbulence, but they are certainly more turbulent than laminar. (Image credits: bacterium – A. Weiner, bacterial turbulence – J. Dunkel et al.; research credit: J. Dunkel et al.; submitted by Jeff M.)

  • Why Slicing Tomatoes Works

    Why Slicing Tomatoes Works

    Picture it: a nice, ripe tomato. Your not-so-recently sharpened kitchen knife. You press the blade down into the soft flesh and… it explodes. Soft solids – like a tomato – don’t react well to cutting, but they slice just fine. Examining why that’s the case is at the heart of this model.

    Tomatoes are essentially a gel encased in a thin skin. Gels are a kind of hybrid material — not quite liquid and not quite solid. They consist of a network of particles or polymers bonded together and immersed in a liquid. To cut that network apart, the downward force of the blade has to strain the gel past its limits, which squeezes out the surrounding liquid.

    The researchers found that this liquid layer is key to how force from the knife’s motion gets transmitted. In particular, they found that the horizontal motion of a slice is necessary to initiate a cut, and that the gel parts most easily when the downward knife velocity is no more than 24% of the horizontal cutting speed. Press down any faster and the strain propagation fluctuates, creating that unfortunate tomato explosion. (Image credit: G. Fring; research credit: S. Mora and Y. Pomeau; via Ars Technica; submitted by Kam-Yung Soh)

  • Shear in Shaken Sands

    Shear in Shaken Sands

    The dynamics inside a shaken granular material, like sand, are fascinatingly complex. In this study, researchers used x-ray radiograms to peer inside a horizontally-shaken container of sand. They found that the sand soon formed bands of lower density (seen as yellow in the radiogram) near the center of the container. Because these bands show a lot of horizontal movement between grains, they’re known as shear bands.

    The shear bands don’t simply stay still, though. One remains more or less stationary at the center, but others split and rise through the upper half of the container. The researchers suggest this migration happens due to gravity; because the shear band is less dense than the material above, it cannot support the weight. Sand sinks into the void, making the less dense region effectively migrate upward. They also suggest that these moving shear bands are responsible for the fluctuations in sand height seen at the surface. (Image credit: beach – RAMillu, radiogram – J. Kollmer et al.; research credit: J. Kollmer et al.)

  • Flexible Filament Reduces Drag

    Flexible Filament Reduces Drag

    Most shapes aren’t streamlined for fluid flow. We call these bulky, often boxy shapes, bluff bodies. Above, we see two examples of a bluff body, a flat plate, in a soap film. On the left, the plate sits perpendicular to the soap film’s top-to-bottom flow. Two large, counter-rotating vortices form behind the plate and a wide wake stretches behind it.

    On the right, we see the same flat plate but now a long, flexible filament is attached to either end. As the flow moves past, it deforms the filament, creating a rounded shape. Researchers found that, under the right conditions, this flexible afterbody could reduce drag on the object by up to 10%. (Image and research credit: S. Gao et al.)

  • Branching Gels

    Branching Gels

    If you sandwich a viscous fluid between two plates, then pull the plates apart, you’ll often get a complex branching pattern that forms as air pushes its way into the fluid. But the exact results depend strongly on what kind of viscous fluid you used. A new study looks specifically at what happens when that fluid is a yield-stress gel.

    Yield-stress fluids behave like a solid until a critical amount of force causes them to flow. Think about your toothpaste. When you take the cap off, the toothpaste stays put until you squeeze the tube enough to make it flow. The gels used in this experiment behave similarly.

    The researchers found that their gels required a critical energy input in order to branch and flow. If the energy applied in pulling the plates apart was too low, no branching occurred (Image 1). But beyond that critical energy, separating the plates created intricate branching patterns consistent with those seen in simpler, Newtonian fluids. (Image, research, and submission credit: T. Divoux et al.; via APS)

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    Crystalline Critters

    In 5th grade, I grew crystals by evaporating solutions of salt water from miniature pie tins. The results were white, boxy crystals whose size depended on how much salt I’d managed to dissolve into the water. But it turns out I could have gotten much cooler results if I’d evaporated my salt water a drop at a time on a hot superhydrophobic surface. That’s how these researchers formed the “crystal critters” shown in the video above.

    Initially, the evaporating salt water drop is what we would expect, but once enough water is gone to leave a shell of salt, the drop grows legs and lifts off the surface. From that point, all growth occurs from the surface up. Because the surface is heated, evaporation happens quickest at that point of contact, and the water that remains is drawn down the legs, providing more fluid for evaporation as well as additional salt to grow the crystal. (Video, image, and research credit: S. McBride et al.)