We’ve all experienced the frustration of ketchup refusing to leave the bottle or toothpaste that shoots out suddenly. These materials are yield stress fluids, which transition from solid-like behavior to liquid flow once the right amount of force is applied. A new study suggests that — despite their wide range of characteristics — these fluids share a universal relation: their yield transition (when they start to flow) depends on their characteristics when at rest. Interestingly, this relationship seems to hold not only for polymeric fluids like the one in the study but also nonpolymeric ones. (Image credit: haideyy; research credit: D. Keane et al.; via APS Physics)
Search results for: “art”
“Droplet on a Plucked Wire”
What happens to a droplet hanging on a wire when the wire gets plucked? That’s the fundamental question behind this video, which shows the effects of wire speed, viscosity, and viscoelasticity on a drop’s detachment. With lovely high-speed video and close-up views, you get to appreciate even subtle differences between each drop. Capillary waves, viscoelastic waves, and Plateau-Rayleigh instabilities abound! (Video and image credit: D. Maity et al.)
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Ponding on the Ice Shelf
Glaciers flow together and march out to sea along the Amery Ice Shelf in this satellite image of Antarctica. Three glaciers — flowing from the top, left, and bottom of the image — meet just to the right of center and pass from the continental bedrock onto the ice-covered ocean. The ice shelf is recognizable by its plethora of meltwater ponds, which appear as bright blue areas. Each austral summer, meltwater gathers in low-lying regions on the ice, potentially destabilizing the ice shelf through fracture and drainage. This region near the ice shelf’s grounding line is particularly prone to ponding. Regions further afield (right, beyond the image) are colder and drier, often allowing meltwater to refreeze. (Image credit: W. Liang; via NASA Earth Observatory)
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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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Proving Superdiffusion
Turbulence is very good at spreading things out. Drop dye into a turbulent flow and it will quickly disperse. Add in particles — like rubber ducks — and they can spread apart, often at speeds quicker than one would expect, based on the background flow. This is (roughly speaking) a phenomenon known as “superdiffusion,” where turbulence makes particles that start out as neighbors part ways.
Physicists conjectured that turbulence — including simplified and idealized versions of it that are simpler to deal with — had this superdiffusion property, but no one was able to show that in a mathematically rigorous way. But now a group of mathematicians has done so, using a technique known as homogenization. There’s a lot more on the story over at Quanta, or you can check out the original papers on arXiv. (Image credit: J. Richard; research credit: S. Armstrong et al. and S. Armstrong and T. Kuusi; see also Quanta)
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“Architecture in Music”
Inside musical instruments gapes an emptiness that, to the eye of photographer Charles Brooks, resembles the vast architecture of music halls and cathedrals. In his series “Architecture in Music,” Brooks takes us into these empty spaces, revealing where the resonance at the heart of the instrument’s sound lies. In a stringed instrument like a violin, the vibration of the strings makes a relatively quiet sound on its own; it’s only in making the violin’s entire hollow body vibrate that resonance amplifies the strings. Similarly, wind instruments rely on air resonating within them to produce their sound. (Image credit: C. Brooks; via Colossal)
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How Insects Fly in the Rain
Getting caught in the rain is annoying for us but has the potential to be deadly for smaller creatures like insects. So how do they survive a deluge? First, they don’t resist a raindrop, and second, they have the kinds of surfaces water likes to roll or bounce off. The key to this second ability is micro- and nanoscale roughness. Surfaces like butterfly wings, water strider feet, and leaf surfaces contain lots of tiny gaps where air gets caught. Water’s cohesion — its attraction to itself — is large enough that water drops won’t squeeze into these tiny spaces. Instead, like the ball it resembles, a water drop slides or bounces away. (Video and image credit: Be Smart)
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“Soap Bubble Bonanza
This video offers an artistic look at a soap bubble bursting. The process is captured with high-speed video combined with schlieren photography, a technique that makes visible subtle density variations in the air. The bubbles all pop spontaneously, once enough of their cap drains or evaporates away for a hole to form. That hole retracts quickly; the acceleration of the liquid around the bubble’s spherical shape makes the retracting film break into droplets, seen as falling streaks near the bottom of the bubble. The retraction also affects air inside the bubble, making the air that touched the film curl up on itself, creating turbulence. Then, as the film completes its retraction, it pushes a plume of the once-interior air upward, as if the interior of the bubble is turning itself inside out. (Video and image credit: D. van Gils)
Non-Newtonian Effects in Magma Flows
As magma approaches the surface, it forces its way through new and existing fractures in the crust, forming dikes. When a volcano finally erupts, the magma’s viscosity is a major factor in just how explosive and dangerous the eruption will be, but a new study shows that what we see from the surface is a poor predictor of how magma actually flows within the dike.
Researchers built their own artificial dike using a clear elastic gelatin, which they injected water and shear-thinning magma-mimics into. By tracking particles in the liquids, they could observe how each liquid followed on its way to the surface. All of the liquids formed similar-looking dikes at a similar speed, but within the dike, the liquids flowed very differently. Water cut a central jet through the gelatin, then showed areas of recirculation along the outer edges. In contrast, the shear-thinning liquids — which are likely more representative of actual magma — showed no recirculation. Instead, they flowed through the dike in a smooth, fan-like shape.
The team cautions that surface-level observations of developing magma dikes provide little information on the flow going on underneath. Instead, their results suggest that volcanologists modeling magma underground should take care to include the magma’s shear-thinning to properly capture the flow. (Image credit: T. Grypachevska; research credit: J. Kavanagh et al.; via Eos)
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The Hidden Beauty in the Mundane
Physicist Sidney Nagel has spent his career on topics that are somewhat unexpected: how coffee stains form, how droplets splash — or don’t, and how fluid flows into viscous fingers. Often this means looking at the mechanics of everyday occurrences that we otherwise take for granted. Instead, Nagel probes carefully at things like a coffee stain, asking why it’s darker at the edges and what he could do to keep that from happening — all to ultimately uncover the forces and mechanisms at play. Quanta has a great little interview with him on this and other topics. Check it out here. (Image credit: S. Nagel and K. Norman; via Quanta)
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