FYFD

Category: Research

  • Building Triboelectric Charge

    Building Triboelectric Charge

    In volcanic eruptions, collisions between ash particles can sometimes build up enough electric charge for lightning to arc through the plume. Scientists have long debated how this happens–it’s not obvious that insulating materials like oxides would build up electric charges through contact, especially when dealing with substances of the same material. It’s not like rubbing a balloon against your hair, where each material–and its tendency to hold a charge–differs.

    A 500-micron silica sphere acoustically levitated above a silica plate in the experiment.
    A 500-micron silica sphere acoustically levitated above a silica plate in the experiment.

    To test how charges build on identical materials, a team of scientists used acoustic levitation to repeatedly bounce a silica bead against an identically treated silica plate, observing their charge build-up. Then they would take one of the pieces–either the sphere or the plate–and treat it to strip away the film of molecules that naturally adsorb onto the surface over time. Then they bounced the treated and untreated surfaces off one another again.

    The result was–pardon the pun–striking. Whichever surface had been treated to remove adsorbates charged more negatively the second time around. Looking more closely at what they were removing, the team found their surfaces were mostly adsorbing carbon molecules. And if they iteratively removed the carbon from both the sphere and plate, they could no longer charge the two through collision. It seems that the key to charging two oxides off one another is actually the difference between the incidental amounts of carbon on their surfaces! (Image credit: volcano – M. Szeglat, experiment – G. Grosjean et al.; research credit: G. Grosjean et al.; via Gizmodo)

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  • Making a Star-Shaped Droplet

    Making a Star-Shaped Droplet

    We usually think of surface tension turning droplets into spheres in order to minimize their area. But spheres aren’t the only shape surface tension can enforce. Here, researchers suspend tiny droplets of oil in a soapy fluid. At the right temperature, these droplets form a crystalline surface while the fluid within remains liquid. As in the fully liquid droplet, surface tension tries to minimize the shell’s surface energy, enabling it to take on many different shapes.

    Video showing the droplet's transition from hexagon to star and back. The shape changes occur as the liquid's temperature changes, thereby affecting its surface tension.
    The droplet’s transition from hexagon to star and back. The shape changes occur as the liquid’s temperature changes, thereby affecting its surface tension.

    In this study, researchers demonstrate that the shell-enclosed droplets can even change, reversibly, from a hexagon to a six-pointed star and back. The transformation is shown above, in an experiment that gradually changes the droplet’s temperature–and, thus, its surface tension.

    Although shape changes similar to these have been described before, this experiment was the first where the shell’s defects–the vertices of the hexagon–don’t shift during the transformation. (Video, image, and research credit: C. Quilliet et al.; via APS)

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  • Explaining the Swirl of Wildfire Smoke

    Explaining the Swirl of Wildfire Smoke

    In recent years, smoke from powerful wildfires has raised questions among atmospheric scientists by always swirling in the same direction. The confounding structures were observed in the stratosphere, where smoke injected at around 15 kilometers in altitude absorbed sunlight and rose further, up to about 35 kilometers of altitude. The rising column of fluid would stretch, causing any residual rotation to get stronger and form vortices.

    None of this was a surprise. What was surprising is that all of the observed vortices were anticyclones, when theory–at least for a heat-driven vortex from a stationary heating source–called for a cyclone-anticyclone pair.

    Researchers looked at how a self-heating (and, therefore, moving) source would rotate. They concluded that this, too, would create a pair of vortices–one cyclonic and one anticyclonic–but the anticyclone would be stronger than the cyclone that trailed behind it. By further considering the vertical shear the vortex pair would encounter, the researchers found that the trailing cyclone could get stripped away, leaving behind only the anticyclone–matching our wildfire observations. (Image credit: J. Stevens/NASA Earth Observatory; research credit: K. Shah and P. Haynes 1, 2; via APS)

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  • Observing Ice Giant Atmospheres

    Observing Ice Giant Atmospheres

    Uranus is one of our solar system’s oddest inhabitants, stuck spinning on its side with a tilted and offset magnetosphere. To better understand it, a team observed the planet for 17 hours with JWST. The near-infrared measurements gave new insight into the planet’s ionosphere, where auroras form. They found that temperatures peaked between 3,000 and 4,000 kilometers, while ion densities peaked at 1,000 kilometers. They also confirmed previous observations that Uranus’s upper atmosphere is cooling down. (Image and video credit: ESA/Webb/NASA/CSA/STScI/P. Tiranti/H. Melin/M. Zamani; research credit: P. Tiranti et al.; via Gizmodo)

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  • Turbulence and Bioluminescence

    Turbulence and Bioluminescence

    If you’ve ever seen crashing waves glowing blue, you’ve been treated to bioluminescence. Although many creatures can bioluminesce, tiny dinoflagellates–a type of marine phytoplankton–are one of the easiest to spot. These microscopic organisms create a flash of light in response to viscous stresses. Their response to flow-induced stresses is so robust that they can be used to visualize stress fields.

    In a new study, researchers explored how turbulence affects the dinoflagellate’s luminescence. They mathematically modeled the dinoflagellate as an elastic dumbbell that emitted light based on its extent and rate of deformation. Then they explored how this model dinoflagellate behaved in different types of turbulent flows. They found that the fluctuations and intermittency of turbulent flows both encouraged the radiant displays. (Image credit: T. McKinnon; research credit: P. Kumar and J. Picardo)

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  • Thunderstorms Make Trees Glow

    Thunderstorms Make Trees Glow

    Scientists have long hypothesized that the high electrical charge of thunderstorms could produce an opposite charge in the ground that would discharge from the forest canopy. But this phenomenon, known as a corona, had never been observed on actual trees. A new study, however, has observed this ghostly ultraviolet (UV) glow from the tips of sweetgum leaves and loblolly pine needles during thunderstorms.

    Catching these coronae in action required a new kind of UV detector that was ultra-sensitive to the particular band of UV-light emitted by coronas, hot fires, or mercury lamps. Since the latter two weren’t present during the team’s field observations, they were able to conclude that the light they detected came from coronae.

    The group observed that corona discharges were transient, jumping from leaf to leaf and branch to branch across the forest canopy. For any creature capable of detecting that glow by eye, it must be incredible to watch the treetops lit by their own ever-shifting auroras during every thunderstorm. (Image credit: W. Brune; research credit: P. McFarland et al.; via SciAm)

    A UV corona forms on tree leaves beneath a thunderstorm.
  • Aging Salty Ice

    Aging Salty Ice

    When ice forms in salty water, it starts out mushy and porous. Salt does not freeze neatly into ice’s crystalline structure, so the forming ice has pores and gaps where salty brine gathers. As the ice ages, more brine is pushed out and gradually convects downward, due to its greater density. Over time, this makes the ice layer thinner but more solid, with fewer pores. You can see a timelapse of the process in a laboratory experiment below. (Image credit: sea ice – C. Matias, experiment – F. Wang et al.; research credit: F. Wang et al.)

    Timelapse of ice forming and aging in salt water over the course of ~16 days.
  • Richtmyer-Meshkov Instability

    Richtmyer-Meshkov Instability

    If you send a shock wave through a magnetized plasma–something that happens in both supernova explosions and inertial confinement fusion–it can trigger an instability known as the Richtmyer-Meshkov instability. The image above shows a form of this, taken from a simulation. Rather than treating the plasma as a single idealized fluid, the researchers represented it as two fluids: an ion fluid and an electron fluid. This allowed them to better capture what happens when certain components of the plasma react to changes faster than others do.

    The image itself shows the electron number density across the fluid, where darker colors represent higher electron number density. The interface between high and low-densities shows a roll-up instability that resembles the Kelvin-Helmholtz instability, but there are also regions of mushroom-like plumes that more closely resemble Rayleigh-Taylor instabilities.

    The authors note that these structures don’t appear in simulations that represent a plasma as a single fluid; you need the two-fluid representation to see them. (Image and research credit: O. Thompson et al.)

  • Improving Turbulence Models

    Improving Turbulence Models

    Calculating turbulent flows like those found in the ocean and atmosphere is extremely expensive computationally. That’s why forecasting models use techniques like Large Eddy Simulation (LES), where large physical scales are calculated according to the governing physical equations while smaller scales are approximated with mathematical models. Researchers are always looking for ways to improve these models–making them more physically accurate, easier to compute, and more computationally stable.

    In a new study, researchers used an equation-discovery tool to find new improvements to these models for the smaller turbulent scales. They started by doing a full, computationally expensive calculation of the turbulent flow. The equation-discovery tool then analyzed these results, looking to match them to a library of over 900 possible equations. When it found a form that fit the data, the researchers were then able to show analytically how to derive that equation from the underlying physics. The result is a new equation that models these smaller scales in a way that’s physically accurate and computationally stable, offering possibilities for better LES. (Image credit: CasSa Paintings; research credit: K. Jakhar et al.; via APS)

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    Mimicking Quantum Effects

    Over the last 15 years or so, researchers have been exploring pilot-wave theory–originally proposed by De Broglie in the 1920s as a way to understand quantum mechanics–using hydrodynamic quantum analogs. In these experiments, researchers vibrate pools of silicone oil, which allows oil drops to bounce–and in some conditions, walk–indefinitely on the pool. By mixing in obstacles that mimic classic quantum mechanical experiments, they reproduce effects like the double-slit experiment in a macroscopic system.

    In this video and the accompanying papers, a team recreates the Kapitsa-Dirac effect where a standing electromagnetic wave diffracts electrons. Here, the standing wave is instead a Faraday wave in the surface of the pool. Yet the droplets, too, diffract in a manner resembling the quantum version. (Video credit: B. Primkulov et al.; research credit: B. Primkulov et al. 1, 2)