FYFD

Category: Research

  • Dune Invasion

    Dune Invasion

    Migrating sand dunes can encounter obstacles both natural and manmade as they move. Dunes — both above ground and under water — have been known to bury roads, pipelines, and even buildings. A recent experimental study looks at which obstacles a dune will cross and which will trap it in place. Their set-up consists of a narrow channel built in a ring, essentially a racetrack for dunes. Flow is driven by a series of paddles that rotate opposite the tank’s rotation.

    The team studied obstacles of different shapes and sizes relative to their dunes, and they found that dunes were generally able to cross obstacles that were smaller than the dune. Obstacles larger than the dune would trap it in place, and, for obstacles close to the same size as the dune, round obstacles were easier to cross whereas sharp-angled ones tended to trap the dune.

    The idealized nature of their experiment means that their results aren’t immediately applicable to the complex dunes of the outside world, but the study will be an important touchstone for those predicting dune behavior through numerical simulation. Studies like those require experimental cases to validate their baseline simulations. (Image credit: top – J. Bezanger, figure – K. Bacik et al.; research credit: K. Bacik et al.; via APS Physics)

    A quasi-2D underwater dune interacts with an obstacle.
  • Marshland Wave Damping

    Marshland Wave Damping

    Coastal marshes are a critical natural defense against flooding. The flexible plants of the marsh both slow the water’s current and help damp waves. As a result of that hydrodynamic dissipation, marshes help protect against erosion and reduce the magnitude of flooding events. But coastal managers looking to maintain or improve their marshes in order to mitigate climate-change-driven storms need to be able to predict what level of vegetation they need.

    To that end, a team of researchers has built a new model to better capture the flow effects of marsh grasses. Building from an individual, flexible plant (as opposed to a rigid cylinder, as grass is often represented), the authors constructed a model able to predict wave dissipation for many marsh configurations, which should help better predict the infrastructure changes needed in different coastal regions. (Image credit: T. Marquis; research credit: X. Zhang and H. Nepf; via APS Physics)

  • Cloud-Making Waves

    Cloud-Making Waves

    As sea ice disappears in the Arctic Ocean, it leaves behind higher waves on the open water. These large waves help inject sea salt and organic matter into the atmosphere, where they can serve as nucleation sites for ice crystals. A recent field expedition in the Chukchi Sea observed high concentrations of organic particulates in the air and more ice-producing clouds during periods of high wave action. So, oddly enough, the loss of sea ice may lead to more cloud cover and precipitation in the Arctic (though the effect is likely not strong enough to entirely mitigate the effects of ice loss). It’s another example of the intricate and complex connections between ice, ocean, and atmosphere in the Arctic climate. (Image credit: A. Antas-Bergkvist; research credit: J. Inoue et al.; via Gizmodo)

  • Hagfish Slime

    Hagfish Slime

    The eel-like hagfish is a superpowered escape artist, thanks to its slime. When threatened, the hagfish releases long protein-rich threads that, when combined with turbulent sea water, unravel to form large volumes of viscoelastic slime that clog the gills of its predators. A new study shows that larger hagfish produce longer and thicker threads in their slime, enabling them to escape larger predators than their smaller brethren can.

    The properties of hagfish slime are tuned for defense. When stretched, the long protein threads resist, making the slime more viscous. Since most fish use suction methods to catch prey, that means a predator attacking a hagfish will quickly exacerbate its slimy problems. But the hagfish itself can easily escape its slime by tying itself in a knot. The threads inside the slime collapse when sheared, so the knot-tying of the hagfish slips the slime right off. (Image credit: T. Winegard; research credit: Y. Zeng et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • Turbulent Puffs

    Turbulent Puffs

    When a burst of air gets expelled into still surroundings — like when a person coughs — it forms a turbulent puff like the one seen here. Puffs can be surprisingly long-lasting, though these miniature clouds slow down and expand over time. How they behave is critical to understanding the spread of pollution as well as how respiratory illnesses like COVID-19 travel. In this study, researchers found that buoyancy is also a critical factor. When the puff is warmer than its surroundings, it rises higher, lasts longer, and travels further. That might help explain why respiratory illnesses like the flu spread more readily in the winter than in warmer months. (Image and research credit: A. Mazzino and M. Rosti; via Physics World; submitted by Kam-Yung Soh)

  • Modelling Volcanic Bombs

    Modelling Volcanic Bombs

    When magma meets water on its journey to the surface, the two form a large, partially molten chunk known as a volcanic bomb. As you would expect from their name, these bombs can often be explosive, either in the air or upon impact. But a surprising number of these bombs never explode. Since catching volcanic bombs in action is far too dangerous, researchers modeled them instead to determine what makes a dud.

    Examples of porous volcanic bombs.

    The type of volcanic bomb they were most interested in comes from Surtseyan eruptions, where the bombs travel through shallow sea or lake water, collecting moisture along the way. When the water reaches the molten interior of the volcanic bomb, it flashes into steam. That’s where the pressure to explode the bombs comes from. But the team found that the bombs are also extremely porous, thanks to bubbles created as the magma depressurizes on its trip to the surface. If the bomb is porous enough, steam escapes the rock before it can build to explosive pressures. (Image credit: top – NASA, others – E. Greenbank et al.; research credit: E. Greenbank et al.; via NYTimes; submitted by Kam-Yung Soh)

  • Fractal Frost

    Fractal Frost

    As nightly temperatures drop in the northern latitudes, many of us are beginning to wake up to frosty patterns on leaves, windows, and cars. Frost‘s spread is a complex dance between evaporation and nucleation, as seen in this recent study.

    Here, researchers watched frost grow on a surface covered in 30-micrometer-wide micropillars. The pillars serve as anchor points for droplets, making frosting easier to observe. At low humidity levels (Image 1), droplets evaporate so quickly that frost regions remain isolated and do not interact. At high humidity levels (Image 3), on the other hand, the droplets evaporate so slowly that they’re able to poach water vapor from their neighbors to form frost spikes. When a spike touches another droplet, it freezes the region almost instantly. As a result, the frost spreads quickly and covers nearly every part of the surface. At intermediate humidity levels (Image 2), though, this frost chain reaction and evaporation compete, causing the frost to grow in fractals. (Image and research credit: L. Hauer et al.; via APS Physics)

  • Solid, Liquid, Both?

    Solid, Liquid, Both?

    Materials like oobleck — a suspension of cornstarch particles in water — are tough to classify. In some circumstances, they behave like a fluid, but in others, they act like a solid. Here researchers sandwiched a thin layer of oobleck between glass plates and injected air into the mixture. For a fluid, this setup creates a classic Saffman-Taylor instability where rounded fingers of air push their way into the more viscous fluid. And, indeed, for low air pressures and low concentrations of cornstarch, the oobleck forms these viscous fingers. You can see examples in the top row’s first and third image, the second row’s middle image, and the bottom row’s third image.

    Injecting air at high pressures and high cornstarch concentrations fractures the oobleck like a solid (middle row, first and third images). At intermediate pressures and concentrations, the oobleck forms a pattern called dendritic fracturing, where new branches can grow perpendicularly to their parent branch. Examples of this pattern are in the top row’s second image and the bottom row’s first and second images. (Image and research credit: D. Ozturk et al.; via Physics Today)

  • Superfluid Instabilities

    Superfluid Instabilities

    Superfluids — like Bose-Einstein condensates — are bizarre compared to fluids from our everyday experience because they have no viscosity. Without viscosity, it’s no surprise that they behave in unusual ways. Here, researchers simulated superfluids moving past one another. In each of these images, the blue fluid is moving to the left, and the red fluid is moving to the right. In a typical fluid, such motion causes ocean-wave-like curls due to the Kelvin-Helmholtz instability.

    Instead, with a low relative velocity and high repulsion between atoms of the two layers, the superfluids form a tilted, finger-like interface (Image 1) that the authors refer to as a flutter-finger pattern. (Repulsion essentially sets the miscibility between the superfluids. With a high repulsion, the superfluids resist mixing.)

    With a higher relative velocity (Image 2), the wavelength of the ripples becomes comparable to the thickness of the interface, and the superfluids take on a very different, zipper-like pattern. Note how the tips detach and reconnect to the neighboring finger!

    With lower repulsion, the interface between the two liquids is thicker and breaks down quickly (Image 3). The authors call this a sealskin pattern. (Image credits: water – M. Blažević, simulations – H. Kokubo et al.; research credit: H. Kokubo et al.; via APS Physics)

  • Better Inhalers Through CFD

    Better Inhalers Through CFD

    As levels of air pollution rise, so does the incidence of pulmonary diseases like asthma. Treatments for these diseases largely rely on inhalers containing drug particles that need to be carried into the small bronchi of the lungs. To better understand how the process works, researchers used computational fluid dynamics to simulate how air and particles travel through the human respiratory tract.

    The team found that larger particles tended to get stuck in the mouth instead of making it down into the lungs. This problem was made worse at high inhalation rates because the particles’ inertia was too large for them to make the sharp turn down into the trachea. In contrast, smaller particles could travel down into the lungs and into the smaller branches there before settling. The authors concluded that inhalers should use fine drug particles to maximize delivery into the lungs. They also note that adjusting inhalers to deliver more medication to the lungs may also lower the overall price because less of the dosage gets wasted in the patient’s mouth.

    Of course, the study’s results also serve as a warning about the dangers of air pollution from fine particulates. Here in Colorado, our summers are punctuated with wildfire smoke, much of it in the form of tiny particles about the same size as the drug particles in this study. If fine drug particles are effective at making it into the smaller branches of our lungs, so are those pollutants. That’s a good reason to stay inside in smoky conditions or use a high-quality N-95 mask while out and about. (Image credit: coltsfan; research credit: A. Tiwari et al.; via Physics World; submitted by Kam-Yung Soh)