Filmmaker Thomas Blanchard creates a slow and dreamy fluid landscape in “Le Temps et l’Espace”. Made with ink and paints, the visuals are beautiful and serene. For similar works, be sure to check out the “fluids as art” tag! (Image and video credit: T. Blanchard)
Snails and other gastropods move using their single muscular foot and a viscoelastic fluid they secrete. Muscular waves in the foot run from tail to head and are transmitted to the ground through the thin, sticky mucus layer without the snail ever fully detaching from the surface. The characteristics of this mucus layer are critical to the snail’s locomotion. As a movement cycle begins, the mucus behaves like an elastic solid. As the muscular wave approaches, it shears the fluid, increasing its stress and ultimately reaching the yield point, where the gel begins to flow. Once the wave passes, the mucus quickly transitions back to its elastic solid behavior. The net result of each cycle is an asymmetric force that propels the snail forward while keeping it adhered to whatever surface it’s crawling on.
Many animals rely on similarly complex fluids to move, attack prey, defend against predators, or enable their reproduction. Check out this review article for more examples. (Image credit: A. Perry; see also P. Rühs et al.; submitted by Pascal B.)
Like Earth, Jupiter is home to polar auroras that light the sky as charged particles interact with the planet’s magnetosphere. A recent paper identifies interesting features in the aurora that appear similar to expanding vortex rings (see inset below). Although the researchers cannot yet identify the origin of the rings, they hypothesize that the process begins at the far edges of Jupiter’s magnetosphere where it interacts with the incoming solar wind. One theory posits that shear flows and Kelvin-Helmholtz instabilities where the magnetosphere and solar wind meet drive the phenomenon. (Image credit: Jupiter – NASA, ESA, and J. Nichols, aurora features – NASA/SWRI/JPL-Caltech/SwRI/V. Hue/G. R. Gladstone/B. Bonfond; research credit: V. Hue et al.; via Gizmodo)
In March 2021, the world watched as the Ever Given container ship got stuck in the Suez Canal, disrupting global shipping for more than a week. In this Practical Engineering video, Grady delves into some of the phenomena that may have played a role in the incident of the ship that launched a thousand memes.
Heavy container ships displace a lot of water, and in a narrow, shallow canal, there isn’t much space left for that water to go. To squeeze by, the water must speed up, which (per Bernoulli’s law) creates a pressure drop and suction force on the ship. For a ship too close to a canal bank, that suction will pull the ship further to the side, increasing its chances of lodging in the bank. (Video and image credit: Practical Engineering)
Many species of spider fly with a technique calling ballooning. We’ve touched on spider flight before, but more recent research adds a new dimension to the phenomenon. Researchers showed that spiders can actually use electrical fields in their flight. When isolated from flow or outside electrical fields, researchers found that spiders would still begin ballooning behaviors when subjected to electrical fields similar to those found in nature. The spiders were even able to take off in the artificial environment, using the electrostatic force between the surrounding fields and their negatively charged silk strands. While electrical fields alone were enough to get spiders aloft, the team thinks spiders in nature likely still use a combination of electrostatic force and aerodynamic drag in order to travel the vast distances spiders have been known to cover. (Video and image credit: BBC; research credit: E. Morley and D. Robert)
“The Golden Sutra” is an homage to the colors of Buddhism, specifically the Longzangjing scripture illustrated in yellow, red, green, blue, and white with letters of gold. Artist Roman De Giuli captures some incredible fluid eddies and streaks with ink, paint, and glitter on paper. (Image and video credit: R. De Giuli)
Often researchers are interested in flows around and between objects, but seeing those flows is a challenge in a crowded field of view. One useful trick for this problem is matching the refractive index of your objects and the fluid they’re immersed in. Here we see the glass beads in a container seemingly disappear when a mixture of water and ammonium thiocyanate is poured in. Now the researchers can use many different visual diagnostic techniques to observe the interior flow! (Image credit: Datta Lab, Princeton University, source)
Early in the COVID-19 pandemic health officials resisted the idea that the novel coronavirus was transmissible through tiny aerosol droplets rather than larger, non-buoyant droplets. One case that made headlines and helped shift opinion was that of an outbreak among patrons of a Guangzhou restaurant traced to a single, pre-symptomatic patient zero. The pattern of who became sick at the carrier’s table and those nearby made little sense unless the restaurant’s air flow played a role in spreading the virus.
This paper studies the incident in detail, using an in-house computational fluid dynamics (CFD) code to simulate both airflow in the restaurant and the paths aerosol droplets would follow in that environment. It takes into account flow from the air conditioner and the warm air rising from customers. The study’s predictions of which areas would have the highest concentrations of virus-laden aerosols matches well with the actual pattern of the outbreak. The authors hope that tools like theirs can help prevent future outbreaks by indicating the most dangerous paths for transmission and measures that can block those. (Image credit: Center for Disease Control; video, research, and submission credit: H. Liu et al.)
An inquiring reader wants to know:
How does kinetic sand work to make it flow like a liquid? Thanks!
– 3 Year Olds Everywhere
I confess I don’t have any firsthand experience with Kinetic Sand, but it certainly looks fun. It’s a colorful, moldable sand toy that holds together far better than your typical pile of sand. From what I’ve been able to find, the secret ingredients are a little bit of polydimethylsiloxane (PDMS) — a type of silicon-based polymer — and olive oil, which coats the sand and keeps it from drying out.
PDMS is viscoelastic, which is what gives the Kinetic Sand its unique properties. When a force is applied quickly, the material reacts like a solid, which is why you can mold or cut the sand and have it maintain its shape. But when left alone for awhile under gravity’s influence, the sand will flow like a liquid. This combination of behaviors usually comes down to the polymers in the material. When forces try to stretch these long molecules quickly, they resist; that’s what creates the elasticity of the material. On the other hand, when a force is gradual, the complex molecules have the time to untangle and relax, allowing the material to flow. (Image credit: Kinetic Sand, source)
Wind turbines face a paradoxical challenge: they must extract the wind’s kinetic energy while still allowing the air to pass. In this Minute Physics video, Henry gives a crash course on wind turbine efficiency, based on the restrictions of conservation of mass and conservation of energy. When the two are combined, they show that an ideal wind turbine reduces the wind speed by 2/3rds to achieve ~59% efficiency.
Of course, actual wind turbines are far from ideal. They’re typically placed in staggered configurations in which upstream turbines can disrupt the flow seen by those downstream. And real wind turbines have to contend with dust, bugs, and other grime that builds up on the blades and disrupts air flow and their efficiency. But calculations like this one are still important for engineers seeking to make these machines as efficient as they can be. (Image and video credit: H. Reich/Minute Physics)