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  • Forming Zigzags

    Forming Zigzags

    Scientists are fascinated by the organized patterns that can emerge from non-living systems. Here, researchers study micron-sized magnetic particles, immersed in a viscoelastic fluid and subjected to an oscillating magnetic field. The peanut-shaped particles roll around their long axis and assemble to form millimeter-sized bands of zigzags. These patterns, the researchers found, do not depend on the particles’ specific shape or on the details of the applied magnetic field. Instead, the zigzags depend only on the symmetry of the flow generated around each particle. In their system, illustrated above, each particle pushed fluid away along their long axis and drew in fluid toward their waist; as a result, particle pairs would attract or repel, depending on their relative orientation. That interparticle force ultimately caused the particles to self-organize into zigzags. (Image, video, and research credit: G. Junot et al.; via APS Physics)

    This sped-up animation shows the zig-zag pattern that the particles self-organization into.
    This sped-up animation shows the zigzag pattern that the particles self-organization into.
  • Painting in Sediment

    Painting in Sediment

    Pale plumes of sediment flow off these islands in the Gulf of Mannar between India and Sri Lanka. As waves erode the land, currents and tides carry the sediment outward, shaping it into swirls and eddies. I rarely tire of satellite images like these because there are always subtle new details of flow to notice. The photos are much like paintings, with layer after layer to decipher the closer you look. (Image credit: A. Nussbaum; via NASA Earth Observatory)

  • Uranus’s Polar Cyclone

    Uranus’s Polar Cyclone

    Uranus is an oddity among the planets of our solar system. Where other planets spin around an axis roughly in line with their orbital axis, Uranus spins on its side, placing its poles in line with the sun. On Earth, the polar regions are naturally colder the equator, but that doesn’t hold true for Uranus. Yet new observations of the ice giant show that it, like the other planets with atmospheres in our solar system, has a polar cyclone.

    Those observations are thanks to improvements in radio astronomy over the past couple decades. Uranus’s odd orbital geometry means that each of its poles are hidden from Earth for 42 years at a time; the current northern-hemisphere spring marks our first view of Uranus’s northern pole since 1965. In the recent observations, researchers saw a bright spot on the pole, surrounded by a faint darker ring. The team modeled the temperature and gas composition necessary to match their observations and found that those patterns were consistent with a cyclone sitting at the northern pole. (Image credit: NASA/JPL-Caltech/VLA; research credit: A. Akins et al.; via Physics Today)

  • Fish Fins Work Together

    Fish Fins Work Together

    Researchers studying how fish swim have long focused on their tail fins and the flows created there. But a fish’s other fins have important effects, too, as seen in this recent study. Researchers built a CFD simulation based on observations of a swimming rainbow trout, focusing on the flow from its back and tail fins. They found that the vortex created by the back fin stabilizes and strengthens the one generated by the tail. It also played a role in reducing drag on the fish by maintaining the pressure difference across the body. When they tried changing the size and geometry of the fins, the fish’s efficiency suffered, indicating that evolution has already optimized the trout’s fins for swimming efficiency. (Image credits: top – J. Sailer, simulation – J. Guo et al.; research credit: J. Guo et al.; via APS Physics)

    Visualization of flow around a digitized rainbow trout.
    Visualization of flow around a digitized rainbow trout.
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    Relax With Hummingbirds

    Quick, agile, and fierce, the hummingbird is an amazing creature. Small for a bird but much larger than an insect, it’s able to hover in place and eat nectar directly from flowers. Many species use a forked tongue with curled edges that help it capture the sweet, viscous fluid. Even their distinctive sounds are fluid-influenced, coming from their wingstrokes and the fluttering of tail and wing feathers. (Image and video credit: BBC Earth)

  • Gravity Changes Droplet Shapes

    Gravity Changes Droplet Shapes

    With small droplets, gravity usually has little effect compared to surface tension. An evaporating water droplet holds its spherical shape as it evaporates. But the story is different when you add proteins to the droplet, as seen in this recent study.

    The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens.
    The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens.

    As a protein-doped droplet sitting on a surface evaporates, it starts out spherical, like its protein-free cousin. But, as the water evaporates, it leaves proteins behind, gradually increasing their concentration. Eventually, they form an elastic skin covering the drop. As water continues to evaporate, the droplet flattens.

    For a hanging droplet, the shape again starts out spherical. But as the drop's water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles.
    For a hanging droplet, the shape again starts out spherical. But as the drop’s water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles.

    In contrast, a hanging droplet with proteins takes on a wrinkled appearance once its elastic skin forms. The key difference, according to the model constructed by the authors, is the direction that gravity points. Despite these droplets’ small size, gravity makes a difference! (Image, video, and research credit: D. Riccobelli et al.; via APS Physics)

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    Fresh Fissures

    North of Iceland’s Fagradalsfjall, a new volcanic fissure opened in July 2023. This drone footage from Isak Finnbogason captures that fissure on its first night. Lava fountains jet from the earth, forming a complex, slow-moving river. The similarities between flowing lava and more common liquids like water never ceases to fascinate me. Even with the vast differences in temperature and viscosity, so much of their physics remains recognizably the same. (Image and video credit: I. Finnbogason; via Colossal)

  • Testing Turbulence’s Limits

    Testing Turbulence’s Limits

    Understanding chaotic, turbulent flows has long challenged scientists and engineers due to their sheer complexity. In turbulent flows, energy cascades from the largest scales — like the kilometer-size cross-section of a cloud — to the very smallest scales, less than a millimeter in size, where viscosity transforms the flow’s motion to heat. For nearly a century, our theoretical understanding of turbulence has posited that there are certain universal behaviors in the statistics of a turbulent flow — essentially that, due to this energy cascade, some aspects of every turbulent flow are the same from clouds to ocean currents to your coffee cup.

    Accordingly, experimentalists have tried for decades to measure this expected universality. Often, there are some signs of agreement, and any deviation was attributed to the finite difference between the large and small scales of the flow. (The theory assumes the difference in these scales’ size is effectively infinite.) But now researchers have achieved the largest range of scales yet — comparable to those found in the atmosphere — and the gaps between theory and experiment remain. The new study does show signs of universality but in a different way than existing theory predicts. As the authors point out, we’ll need new theories to explain these findings. (Image credit: D. Páscoa; research credit: C. Küchler et al.; via APS Physics)

  • Atlantic Blooms

    Atlantic Blooms

    In April 2023, swirls of green and turquoise burst into vivid color in the Atlantic. Much of the color comes from a phytoplankton bloom. Although phytoplankton are individually microscopic, they form eddies a hundred kilometers across that are visible from space. In detailed images like the one above (available here in full resolution) these swirls have amazing turbulent details. Some of the brightest sections almost look like a field of sea ice! (Image credit: L. Dauphin; via NASA Earth Observatory)

    This wider view shows the bloom's location off of the northeastern U.S.
  • Sliding on Sand

    Sliding on Sand

    Getting around on sandy slopes is no easy feat. On steep inclines, even small disturbances will cause an avalanche. The predatory antlion takes advantage of this fact by building a conical pit that makes ants that walk in slide down into its waiting jaws. But a new study shows that it’s more than just pressure that determines when an object slides down the slope.

    To simulate hapless ants sliding into an antlion’s pit, researchers used plexiglass disks with four smaller disks that act as legs on the granular slope. By varying the distance between these points of contact, researchers found that stance also affects when a slide starts. The closer together the contacts are, the more likely the disk would slide. In contrast, spreading the points of contact increased stability, meaning that adopting a wider stance could keep an animal, human, or robot from sliding as easily. (Image credit: NEOM; research credit: M. Piñeirua et al.; via APS Physics)

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