Acoustic levitation uses the radiation forces generated by sound waves to trap small, lightweight particles at the nodes of standing waves. We’ve seen this a number of times previously, both with solid objects and liquid droplets. What makes this example particularly impressive, though, is that these researchers use an array of speakers to manipulate multiple objects at once. Check out the video above for a whole series of clips from the research. (Video credit: Science; research credit: A. Marzo and B. Drinkwater)
Category: Research
Finding New Shapes in Foam
In the summer of 2018, a group of researchers announced they’d discovered a new geometrical shape, the scutoid. They found the scutoid, a sort of twisted prism, in the shape of epithelial cells packed between curved surfaces. Having heard of this new geometry, a different group of physicists wondered if they could find scutoids elsewhere, specifically, in the cells of a foam. As shown in the picture above, they did.
To visualize a scutoid, first image a prism. Take two polygons with an equal number of sides and connect them. But if you imagine packing such prisms between two curved surfaces, you’ll quickly see that it won’t work. They just don’t fit together. Instead, one face may adopt, say, six sides, while the other takes on five. To join those two end faces, one of the sides will have to have a Y-shaped junction and a triangular face. This is a scutoid.
You can see two such shapes in the image above. In the left bubble, the far side forms a pentagon, while the near face is a hexagon. On the right, the bubble has six faces in the background and eight in the foreground. And between them, you can just see the triangular face that connects the two scutoids.
It’s not only exciting to find scutoids in a new, non-biological medium; it suggests a physical mechanism behind their formation. Foams are a well-known example of energy minimization. The fact that scutoids are found in a curved foam suggests that the shape itself is connected to energy minimization, something that could help us understand how biological scutoids grow and form. (Image and research credit: A. Mughal et al.; via Physics World; submitted by Kam-Yung Soh)
Sniffing Underwater
Star-nosed moles – tiny mammals native to the northeastern United States – have an underwater superpower: sniffing. To seek prey underwater, the moles blow bubbles and suck them back into their nostrils in about a tenth of a second. Their eponymous noses seem to be key to this, as seen in newly published research. Researchers built model star noses from plastic (lower right) to explore how well different shapes could hold the bubble in place, a necessary ingredient for the mole to sniff them back up.
With a perfectly flat plate, any small tilt makes the bubble slide toward the edge and float away. Star-shaped ones, on the other hand, can hold a bubble even up to a 7-degree tilt angle, a 40% improvement. The spacing of the gaps is also important. If they’re too wide, buoyancy can pull the bubble up through them. But if they’re too narrow for the bubble to deform upward through them, they make poor anchors.
Understanding the mechanics of underwater sniffing is good for more than just appreciating this funny-looking mammal, though. The researchers hope their findings will help develop underwater chemical sensors that use bubble sniffing instead of exposing their components directly to sea water, which would significantly extend their usable life. For more, check out the paper and my interview with the lead author in the video below. (Image credits: top and lower left – K. Catania; lower right – A. Lee; research credit: A. Lee and D. Hu; video credit: N. Sharp and T. Crawford)
Stress Between Grains
Granular materials like sand and beads can shift and flow in fluid-like ways, but they’re much harder to predict. Part of this is due to the way friction between individual grains transmits force through the network. Here, we see photoelastic beads responding to the intrusion of a narrow rod. The lightning-like flashes show how stress is traveling between neighboring grains. Notice how the lower grains are essentially frozen into a state of high stress, but the movable upper grains shift and readjust themselves to try and relieve stress.
This experiment took place under lunar gravitational conditions. Lower gravity means that it takes a larger pile of grains on top to create a given stress. But it also means it’s easier for those movable top grains to shift or even get thrown up by a hastily applied force. The purpose of experiments like this is to better understand how rovers and probes should dig in low-gravity environments without kicking up a cloud of regolith and dust. (Image credit: K. Daniels et al., source)
Entrained
When an object hits water whether it draws air in with it depends on its shape, impact speed, and surface characteristics. In this experiment, though, there’s a bit of a twist. Here the sphere is passing through an interface with surfactants added. On the left, the sphere passes through smoothly without entraining air or creating a cavity. On the right, the same sphere impacts at the same speed but this time the interface is covered in a layer of bubbles. As a result, the sphere pulls a large air cavity into the water with it. Why the big difference?
As the sphere passes through the bubbles, they burst, spraying the sphere with droplets. On impact, those droplets disrupt the layer of water traveling up the sides of the sphere, causing it to pull away from the surface and form a splash. Instead of smoothly coating the sphere in water, air can now stick to the sphere and get pulled in with it. (Image and research credit: N. Speirs et al., source)
A Groovy Hovercraft
Not long ago, researchers discovered that droplets hovering over a hot grooved surface would self-propel. The extension to this was to investigate a hovercraft on a grooved, porous surface (top half of animation)–think an air hockey table with grooves. In that case, air inside the grooves flows from the point toward the edges, and it drags the hovercraft with it, thanks to viscosity. So the hovercraft travels in the direction opposite the points. This raised an obvious question: what happens if the hovercraft is grooved instead of the surface?
That’s the situation we see in the bottom half of the animation. Air flows from the table and interacts with the grooves on the bottom of the hovercraft. And this time, the hovercraft propels in the direction of the points. That means there’s a completely different mechanism driving this levitation. When the grooves are onboard the hovercraft, pressure dominates over viscous effects. The air still gets directed down the grooves, but now, like a rocket, the exhaust pushes the hovercraft in the other direction – toward the points. For more on this work, check out the mathematical model of the problem and our interview with one of the researchers in the video below. (Research credit: H. de Maleprade et al.; image and video credit: N. Sharp and T. Crawford)
Inside a Heart
You may not give it much thought, but there is important fluid dynamics happening inside you every moment of every day, especially inside your heart. Of the four chambers of the heart, the left ventricle has the thickest walls, reflecting its important task: pumping oxygenated blood throughout the body. In a healthy heart (top of poster; click here for the full-size version), a vortex ring and trailing jet fill the ventricle when the mitral valve opens. Then the ventricle contracts and pumps blood out the aortic valve and into the rest of the body.
But for individuals with a leaking aortic valve (bottom of poster), things look different. Blood leaks back through the aortic valve at the same time that the mitral valve opens to allow freshly oxygenated blood back in. The two inflows disrupt mixing in the chamber, and, instead of pumping fully-oxygenated blood into the body, the left ventricle has to struggle to pump a less-oxygenated mixture into the body. (Image credit: G. Di Labbio et al.)
ETA: (Research paper: G. Di Labbio et al., arXiv)
Fighting a Viscous World
Vaucheria is a genus of yellow-green algae (think pond scum), and some species within this genus reproduce asexually by releasing zoospores. Once mature, the zoospore has to squeeze out of a narrow, hollow filament in order to escape into the surrounding fluid (top). To do so, it uses tiny hair-like flagella on its surface. Despite the minuscule size of these micron-length flagella, they generate some major flows around the zoospore (middle and bottom). Even several body lengths away, the flow field shows significant vorticity. All this active entrainment of fluid from the surroundings helps the zoospore escape its confinement and swim away to start a new plant. (Image and research credit: J. Urzay et al., source)
Water-Walking Geckos
Many animals can run on water. The tiniest creatures, like water striders, use surface tension to keep themselves atop the water. Larger creatures like the basilisk lizard or the grebe slap the water’s surface to generate a vertical impulse that keeps them aloft. Geckos, it turns out, can run on water, too, but they’re too big to stay up with surface tension and too small to support their weight by slapping. So they’ve developed their own method.
As you see in the top image, geckos use the slapping method for part of their support. Their slaps generate a little less than half of the force needed to keep them out of the water.
Surface tension is an important component, too. Geckos are extremely water repellent, which helps boost the lift they get from surface tension. In the bottom image, you see a gecko attempting to run on soapy water, which has a lower surface tension. The gecko is mostly submerged and more swimming than running – a clear demonstration that surface tension is important to its water-walking.
Finally, the gecko undulates its body as it runs, much the way an alligator swims. The researchers suspect this helps the gecko generate forward thrust. Altogether, it creates a water-walking gait that, for now, is unique among observed mechanisms. (Image and research credit: J. Nirody et al.; via Ars Technica; submitted by Kam-Yung Soh)
Rattling Feathers for Attention
Peacocks are known for their colorful mating displays, but it turns out there’s more to them than meets the eye. To help them gain a penhen’s attention, peacocks will sometimes rattle their train-feathers. The sound this makes is mostly below the range of human hearing, but the rustle creates subtle vortices in the air that cause the feathers atop a peahen’s head to vibrate. Playing back the sound at peahens from typical train-rattling distances also gets the females’ attention. Researchers found the playback makes peahens’ crests vibrate at a resonant frequency, suggesting that these feathers are for more than display; they’re used for communication as well! (Image and research credit: S. A. Kane et al.; video credit: Science)
