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  • Beijing 2022: Monobob

    Beijing 2022: Monobob

    Bobsleigh, as a discipline, has been dominated in recent years by teams seeking every aerodynamic advantage to shave hundredths of a second off their runs. So it’s fascinating that the newest event in the discipline — the women-only monobob — cuts away that secretive part of the sport by permitting sleds from only one manufacturer. Every athlete competes in an identical sled. Not only that, they swap sleds between runs based on their times! So the fastest athlete from the first run will switch sleds with whomever had the slowest time.

    The event’s rules refocus the competition on athletic performance and skill rather than incentivizing countries who can afford to spend more money on wind tunnel testing and F1 design companies. That’s a great step toward leveling the playing field. I can’t wait to watch! (Image credit: OIS)

  • 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)

  • Luminous Fruits

    Luminous Fruits

    Light shines through citrus and melon in this photographic photorealistic series of paintings from artist Dennis Wojtkiewicz. The strong illumination reveals the underlying structure of pith, pulp, and juice. The deformable pockets of fluid in the peel of citrus fruits are the source of some incredible microjets. When the peel bends, it compresses these tiny fluid-filled pockets, creating incredibly high pressures that eventually drive a burst of oil at g-forces comparable to those felt by a bullet fired from a gun. Learn more about citrus jets here and see more of Wojtkiewicz’s work and purchase prints here on his site. (Image credit: D. Wojtkiewicz; via Colossal)

    ETA: Thanks to A.J. for pointing out that Wojtkiewicz is, in fact, a painter (and not a photographer), making his work all the more astounding! We regret the error.

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    Acrylic Paint Fractals

    Here’s a simple fluids experiment you can try at home using acrylic paints, ink, isopropyl alcohol and a few other ingredients. When dropped onto diluted acrylic paint, a mixture of black ink and alcohol spreads in a fractal fingering pattern. The radial (outward) flow is driven by the alcohol’s evaporation, which increases the local surface tension and draws fluid outward. The shape and density of the fingers depends, at least in part, on the viscosity of the underlying paint layer; more viscous paint layers grow smaller and denser fractal patterns. (Image and video credit: S. Chan et al.)

  • Antarctic Meltwaters

    Antarctic Meltwaters

    Cerulean blue meltwater glints in this satellite image of the George VI Ice Shelf. Wedged between the Antarctic Peninsula on the right and Alexander Island on the left, the ice shelf itself floats on the ocean. When ice shelves collapse, they do not directly raise sea levels since their weight has already displaced water; but a collapsed ice shelf lets glaciers flow and break up faster, thereby raising water levels.

    In past ice shelf collapses, scientists have noted major buildup and sudden drainage of surface lakes like the ones seen here. Meltwater penetrating through snow and ice can destabilize the shelf and hasten collapse, but the exact mechanisms are hard to track. This Physics Today article summarizes our understanding of the process and some of the methods scientists use to study it. (Image credit: L. Dauphin/NASA Earth Observatory; see also Physics Today)

  • Swept Along

    Swept Along

    When a car drives over a leaf-strewn autumn road, it pulls leaves up with its passage. This tendency to drag fluid along when an object passes is called entrainment, and it may be a key to transporting loads like medicine in microfluidic applications.

    As shown above, a self-propelled microswimmer — in this case, an oil droplet — pulls the surrounding fluid and tracer particles with it (Image 1). Researchers modeled this single-swimmer entrainment (Image 2) to quantify just how much fluid the droplet pulls with it. Then they studied what happens when many swimmers pass through an area (Image 3). They found that the droplet swarm entrained ten times the volume of fluid compared to the fluid entrained by the same number of isolated droplets. The fluid volume pulled along was also far larger than any payload the droplets themselves could carry. So future microswimmer swarms may simply sweep their cargo along in their wake. (Image and research credit: C. Jin et al.; via APS Physics)

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    “Halo”

    Fluids create mesmerizing practical effects in this new experimental film from the Julia Set Lab. I love how the visuals mess with your sense of scale. Some of the sequences look like they could be a solar firestorm or disintegrating sea ice, though in reality the camera’s field of view is probably smaller than your palm. The filmmakers provide no information on the fluids they use, but I spy some hints of partially miscible ingredients, some chemical reactions, and plenty of Marangoni action. (Video and submission credit: S. Bocci/Julia Set Lab)

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    The Yarning Droplet

    Marangoni bursting takes place in alcohol-water droplets; as the alcohol evaporates, surface tension changes across the liquid surface, generating a flow that tears the original drop into smaller droplets. Here researchers add a twist to the experiment using PMMA, an additive that dissolves well in alcohol but poorly in water. As the alcohol evaporates, the PMMA precipitates back out of the water-rich droplet, forming yarn-like strands. (Image and video credit: C. Seyfert and A. Marin)

  • Laser-Induced Jet Break-Up

    Laser-Induced Jet Break-Up

    A falling stream of water will naturally break up into droplets via the Plateau-Rayleigh instability. Those droplets are random, unless something like vibration of the nozzle sets their size. In this study, though, researchers found that shining a laser beam on the stream can trigger an orderly break-up with droplets that are consistent in size and spacing.

    The optofluidic phenomenon depends on a few different effects. The changing curvature of the liquid stream reflects the laser light, some of which undergoes total internal reflection and travels up the jet as if it were a fiber optic cable. Look closely in the right side of the second image, and you’ll see a periodic flicker of green light at the mouth of the nozzle. Those flashes of green reveal that the liquid jet is guiding the light upstream in bursts, each of which exerts an optical pressure that triggers the Plateau-Rayleigh instability.

    When the laser first turns on, there’s a transition period before the orderly break-up begins, and, likewise, turning the laser off triggers a transition from orderly to random (top image). (Image and research credit: H. Liu et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Volcanic Shocks

    Volcanic Shocks

    A violent underwater eruption at the Hunga Tonga-Hunga Ha’apai caldera on January 15th sent literal shock waves around the world. This animation, based on satellite images from Japan’s Himawari 8, shows the fast-moving shock waves and the growing ash plume coming from the uninhabited island. Although most recent eruptions from this volcano have been small, experts suspect that this latest eruption is part of a major event, similar to the volcano’s last big eruption about 1,000 years ago.

    The explosiveness of the eruption comes from the interaction of seawater and fresh magma. When the magma erupts quickly underwater, the hot liquid contacts seawater directly rather than forming a protective layer of vapor (as in the Leidenfrost effect). The resulting explosion tears the magma apart, exposing more hot surfaces to the cold water and further driving the chain reaction. (Image credit: S. Doran/Himawari 8; submitted by jpshoer; see also S. Cronin)

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