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Search results for: “droplet”

  • Why Sharper Knives Mean Fewer Onion Tears

    Why Sharper Knives Mean Fewer Onion Tears

    Onions are a well-known source of tears for many a cook. And while the chemical source of their power–onions release a chemical that reacts in our eyes to produce tears–has been known for years, no one has looked at the fluid dynamics in the process until now.

    Video of droplets sprayed as a knife cuts into an onion.

    As seen above, a knife piercing the onion’s surface releases a mist of high-speed droplets, followed by a slower spray. Much like a citrus fruit’s microsprays, the onion’s fountain depends on both solid and fluid mechanics. As the knife presses into the onion’s stiffer outer layer, pressure builds in the softer layer underneath, which contains pores of fluid. Once the knife breaks the epidermis, that pressurized fluid sprays out.

    The good news is that the team also confirmed a common culinary wisdom: using a sharper knife and a slower, gentler cut will reduce the spray and its speed, resulting in fewer tears. (Image credit: M. Stone; research credit: Z. Wu et al.)

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  • Ice Discs Surf on Herringbones

    Ice Discs Surf on Herringbones

    Inspired by the roaming rocks of Death Valley, researchers went looking for ways to make ice discs self-propel. Leidenfrost droplets can self-propel on herringbone-etched surfaces, so the team used them here, as well. On hydrophilic herringbones, they found that meltwater from the ice disc would fill the channels and drag the ice along with it.

    But on hydrophobic herringbone surfaces, the ice disc instead attached to the crest of the ridges and stayed in place–until enough of the ice melted. Then the disc would detach and slingshot (as shown above) along the herringbones. This self-propulsion, they discovered, came from the asymmetry of the meltwater; because different parts of the puddle had different curvature, it changed the amount of force surface tension exerted on the ice. Thus, when freed, the ice disc tried to re-center itself on the puddle.

    The team is especially interested in how effects like this could make ice remove itself from a surface. After all, it requires much less energy to partially melt some ice than it does to completely melt it. (Image and research credit: J. Tapochik et al.; via Ars Technica)

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  • Dusty Clouds Make More Ice

    Dusty Clouds Make More Ice

    Even when colder than its freezing point, water droplets have trouble freezing–unless there’s an impurity like dust that they can cling to. It’s been long understood in the lab that adding dust allows water to freeze at warmer temperatures, but proving that at atmospheric scales has been harder. But a new analysis of decades’ worth of satellite imagery has done just that. The team showed that a tenfold increase in dust doubled the likelihood of cloud tops freezing.

    Since ice-topped clouds reflect sunlight and trap heat differently than water-topped ones, this connection between dust and icy clouds has important climate implications. (Image and research credit: D. Villanueva et al.; via Eos)

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  • Falling From the Sky

    Falling From the Sky

    Artist Sho Shibuya paints daily meditations on a copy of The New York Times. These particular examples are part of a recent collection, Falling From the Sky, that features realistic trompe l’oeil droplets that celebrate rain and rainy days. Having spent many an hour contemplating water droplets on my window, I love these. (Image credits: S. Shibuya; via Colossal)

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  • Oil-Slicked Bubble Bursts

    Oil-Slicked Bubble Bursts

    When bubbles at the surface of the ocean pop, they can send up a spray of tiny droplets that carry salt, biomass, microplastics, and other contaminants into the atmosphere. Teratons of such materials enter the atmosphere from the ocean each year. To better understand how contaminants can cross from the ocean to the atmosphere, researchers studied what happens when a oil-coated water bubble pops.

    The team looked at bubbles about 2 millimeters across, coated in varying amounts of oil, and observed their demise via high-speed video. When the bubble pops, capillary waves ripple down into its crater-like cavity and meet at the bottom. That collision creates a rebounding Worthington jet, like the one above, which can eject droplets from its tip.

    The team found that the oil layer’s thickness affected the capillary waves and changed the width of the resulting jet. They were able to build a mathematical model that predicts how wide a jet will be, though a prediction of the jet’s velocity is still a work-in-progress. (Image credit: Р. Морозов; research credit: Z. Yang et al.; via APS)

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    “Sensations”

    Beautiful colors, subtle flows, and sudden fractals animate Thomas Blanchard’s “Sensations,” which, like his other short films, is entirely CGI-free. It’s a lovely exploration of droplets, liquid lenses, Marangoni effects, and fingering instabilities. (Video and image credit: T. Blanchard)

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  • Forming Vesicles on Titan

    Forming Vesicles on Titan

    Scientists are still debating exactly what shifts nature from chemical and physical reactions to living cells. But vesicles — small membrane-bound pockets of fluid carrying critical molecules — are a commonly cited ingredient. Vesicles help cluster important organic molecules together, increasing their chances of combining in the ways needed for life. Now scientists are suggesting that Titan, Saturn’s moon, could form vesicles of its own.

    On Earth, molecules known as amphiphiles feature a hydrophilic (water-loving) end and a hydrophobic (water-fearing) one. When dispersed in water, amphiphiles crowd at the surface, placing their hydrophilic end in the water and their hydrophobic end outward toward the air. On Titan, the Cassini mission revealed organic nitrile molecules that behave similarly with methane rather than water.

    Their two-sided structure means that these molecules — like Earth’s amphiphiles — will gather at the surface of Titan’s liquids. When methane rain falls on the Titan’s seas, the impact creates aerosol droplets that slowly settle back to the liquid surface. When that happens, the droplet’s molecular monolayer and the lake’s monolayer meet, enclosing the droplet’s contents in a double-layer of molecules that prevent contact between the droplet and the lake.

    Within that newly-formed vesicle, all kinds of molecules can bump shoulders, creating new opportunities for complex chemistry. (Image credit: Titan – ESA/NASA/JPL/University of Arizona, illustration – C. Mayer and C. Nixon; research credit: C. Mayer and C. Nixon; via Gizmodo)

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  • Crown Splash

    Crown Splash

    When a falling drop hits a thin layer of water, the impact sends up a thin, crown-shaped splash. This research poster shows a numerical simulation of such a splash in the throes of various instabilities. The crown’s thick edges are undergoing a Rayleigh-Plateau instability, breaking into droplets much the way a dripping faucet does. On the far side, the crown has rapidly expanding holes that pull back and collide. The still-intact liquid sheet at the base of the crown shows some waviness, as well, hinting at a growing instability there. (Image credit: L. Kahouadji et al.)

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    Evaporating Off Butterfly Scales

    This award-winning macro video shows scattered water droplets evaporating off a butterfly‘s wing. At first glance, it’s hard to see any motion outside of the camera’s sweep, but if you focus on one drop at a time, you’ll see them shrinking. For most of their lifetime, these tiny drops are nearly spherical; that’s due to the hydrophobic, water-shedding nature of the wing. But as the drops get smaller and less spherical, you may notice how the drop distorts the scales it adheres to. Wherever the drop touches, the wing scales are pulled up, and, when the drop is gone, the scales settle back down. This is a subtle but neat demonstration of the water’s adhesive power. (Video and image credit: J. McClellan; via Nikon Small World in Motion)

    Water droplets evaporate from the wing of a peacock butterfly.
    Water droplets evaporate from the wing of a peacock butterfly.
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  • Penguin Poo Seeds Antarctic Clouds

    Penguin Poo Seeds Antarctic Clouds

    Forming clouds requires more than just water vapor; every droplet in a cloud forms around a tiny aerosol particle that serves as a seed that vapor can condense onto. Without these aerosols, there are no clouds. In most regions of the world, aerosols are plentiful — produced by vegetation, dust, sea salt, and other sources. But in the Antarctic, aerosol sources are few. But a new study shows that penguins help create aerosols with their feces.

    Penguin feces is ammonia-rich, and that ammonia, when combined with sulfur compounds from marine phytoplankton, triggers chemistry that releases new aerosol particles. The researchers measured ammonia carried on the wind from nearby penguin colonies and found that the birds are a large ammonia source, producing 100 to 1000 times the region’s baseline ammonia levels. In combination with another ingredient in penguin guano, the researchers found the penguins boosted aerosol production 10,000-fold. That means penguins can actually influence their environment, helping to create clouds that keep Antarctica cooler. (Image credit: H. Neufeld; research credit: M. Boyer et al.; via Eos)

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