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Search results for: “art”

  • Eroding Grains

    Eroding Grains

    When a spacecraft comes in for a landing (or a tag similar to what OSIRIS-REx did), there’s a turbulent jet that points straight into a bed of particles. How those particles react — how they erode and the crater that forms — depends on many factors, including the cohesion between particles. In these experiments, researchers investigated such a jet (in air) and its impact on particles with differing amounts of cohesion.

    When there is little cohesion between particles, erosion takes place a single particle at a time (Image 1). Once there’s some cohesion, the jet’s velocity has to be higher to trigger erosion (Image 2). Once erosion does begin, it includes both singular and clumped particles. In highly cohesive beds, velocities must be even higher to create erosion, which takes place with large clusters of particles flying off together (Image 3). (Image and research credit: R. Sharma et al.)

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    Peering Into the Gap

    This video offers a glimpse into turbulence developing in a classic flow set-up, a Taylor-Couette cylinder. The apparatus consists of two upright, concentric cylinders; the outer cylinder is fixed, and the inner one rotates. This video shows the gap between the cylinders, and it’s rotated so that the inner cylinder is at the bottom of the frame. Gravity points from left to right in the video. The fluid in the 8-cm gap between the cylinders is water, seeded with rheoscopic particles to visualize the flow.

    The video begins as the inner cylinder has just begun to rotate, dragging nearby fluid with it. A thin, laminar boundary layer forms at the bottom of the frame, growing as time goes on. A few seconds in, the boundary layer transitions to turbulence; look closely and you’ll see hairpin-shaped vortices appear. Just after that, the boundary layer becomes entirely turbulent and continues to slowly move upward to take over the full gap. The video is available in a full 4K resolution if you really want to get lost in the flow. (Video credit: D. van Gils)

  • Hydrophobic Ice

    Hydrophobic Ice

    Water is an endlessly peculiar substance, eager to adopt many configurations. Each molecule can form up to four, highly-directional bonds. In this study, researchers found an unexpected configuration, a 2D type of ice known as bilayer hexagonal ice, on a corrugated gold surface. Bilayer hexagonal ice has been known since the late 1990s, but it was thought to be comparatively rare. In this form, water molecules assemble in an ice only two molecular layers thick, with hydrogen bonds between neighboring molecules taking up nearly all possible binding sites. With nowhere to bind, additional water cannot add to the ice’s thickness, making the ice as a whole hydrophobic or “water-fearing”.

    Illustration of 2D hydrophobic ice.
    This illustration shows a type of 2D ice, known as bilayer hexagonal ice, as it forms on a corrugated gold surface. From above (top half), the water molecules align to the surface with some molecules (red) in the troughs and others (pink) along the ridges. Viewed from the side (lower half), most of the molecules bind with their neighbors, leaving few H-bond sites available where more water layers of water could attach. This inability to add more vertical layers is why the ice appears hydrophobic.

    Previously, this type of ice had only been found on hydrophobic, flat surfaces. In the latest research, though, researchers found that surface corrugations allowed the ice to form, even on a surface that was only slightly hydrophobic. Observations like these help theorists modeling water and its interactions with surface. (Image credit: top – E. McKenna, illustration – APS/A. Stonebraker; research credit: P. Yang et. al.; via APS Physics; submitted by Kam-Yung Soh)

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    Aerated Faucets

    So much goes on in our daily lives that we never see. But with the power of the smartphones in our pockets, we can catch more than ever before, as illustrated in this video. Here a researcher uses the standard “slo-mo” (240 fps) video mode on a smartphone to look at the flow from a typical kitchen faucet. Household faucets often have an aerator that adds air bubbles to the flow, something that’s particularly visible in slow motion at high flow rates. What you can see depends on more than just the frame rate, though. Without strong illumination — provided in this case by sunlight — you could easily miss the cloud of droplets ejected by the faucet. (Image and video credit: M. Mungal)

  • When Rivers Jump

    When Rivers Jump

    Avulsions — sudden changes in the course of a river — are a river’s equivalent of an earthquake, and they can be similarly devastating for those in the river’s path. In a recent study, authors combed through 50 years’ worth of satellite data to catalog over 100 avulsions and categorize them into three regimes. About a quarter of the observed avulsions took place in the river delta’s fan, where the river spreads out once it exits a canyon or valley. These avulsions, they found, occur when rivers lose confinement and sediment can build up.

    This animation of satellite images shows the sudden avulsion -- a dramatic change in the river's course -- that took place on the Kosi River in 2008.
    This animation of satellite images shows the sudden avulsion — a dramatic change in the river’s course — that took place on the Kosi River in 2008.

    Among the other observations, the team linked avulsion location to the river’s flow properties. Most of these remaining avulsions took place in the river’s backwater region, where the river begins to slow down before its outlet. The last category of avulsion took place far upstream of the backwater region on rivers with high sediment flows. During flood conditions, erosion can travel far upstream on these rivers, causing avulsions in unexpected places. Changes in sediment load due to human activities, like deforestation, could even cause rivers to change from the backwater regime to the high-sediment load one. (Image credit: top – R. Simmon/USGS, bottom – S. Brooke et al.; research credit: S. Brooke et al.; via AGU Eos; submitted by Kam-Yung Soh)

  • Saffman-Taylor Instability

    Saffman-Taylor Instability

    Air and blue-dyed glycerin squeezed between two glass plates form curvy, finger-like protrusions. This is a close-up of the Saffman-Taylor instability, a pattern created when a less viscous fluid — here, air — is injected into a more viscous one. If you reverse the situation and inject glycerin into air, you’ll get no viscous fingers, just a stable, expanding circle. Although you sometimes come across this instability in daily life — like in a cracked smartphone screen — the major motivation for studying this phenomenon historically has been oil and gas extraction. (Image credit: T. Pohlman et al.)

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    Seeing the Flow

    Experimentalists often need a sense for the overall flow before they can decide where to measure in greater detail. For such situations, flow visualization techniques are a powerful tool since they provide quick ways to see and compare flows.

    Here, researchers paint a viscous oil atop their flying wing model and observe how the oil moves once the air flow starts up. This oil flow visualization shows the large-scale shifts in how air flows over the craft as the angle of attack increases. The disadvantage is that these techniques often give only a qualitative sense of the flow. But they can allow experimentalists to test many different conditions to decide which specific cases they should examine quantitatively. (Image and video credit: V. Kumar et al.)

  • Rain-Driven Prey Capture

    Rain-Driven Prey Capture

    Pitcher plants often entice their insect victims with sweet nectar before trapping them in inescapable viscoelastic goo. But some species go even further. Nepenthes gracilis, a species native to Southeast Asia uses its leafy springboard to lure its prey. Once an ant crawls to the underside of the leaf, a falling rain drop will spell its doom. When drops hit the leaf, it deflects down and jerks up, thanks to its shape and stiffness. The motion catapults insects into the pitcher, where digestive fluids await. While we’ve seen some fast-moving plants before, this is a rare example of a plant with an externally-driven speed mechanism. With it, the pitcher plant doesn’t have to wait or expend any metabolic effort to reset for the next insect. (Image credit: GFC Collection/Alamy; research credit: A. Lenz and U. Bauer; via New Scientist)

  • Reefs Along New Caledonia

    Reefs Along New Caledonia

    Brown reefs edge a turquoise lagoon in this astronaut snapshot of the New Caledonian coastline. Reefs like these form a natural barrier that protects coastlines from storms by breaking up waves (seen here as those white edges) before they reach the shore. The lagoon is streaked with lines of tan where sediment flows from the uplands into the water. Similarly, the color variations from green to blue in water hint at changes in depth, organic content, and more. (Image credit: NASA; via NASA Earth Observatory)

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    A Levitated Boil

    When acoustically levitated, objects tend to clump together and move like a single, large solid. But researchers found more fluid-like states for their levitated particles when the particles were smaller. At low acoustic power, the particles behave like a liquid and shift primarily within a plane. But as the acoustic power increases, the granular liquid begins to “boil” and transition into a gaseous state, with particles moving in all directions. It’s amazing how often these metaphors (e.g., treating a group of particles as a “liquid”) hold true when observing different physical systems! (Image and video credit: B. Wu et al.)

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