Drying fluids can leave behind all kinds of fascinating patterns, as we’ve seen before with whiskey, coffee, and even blood. Here researchers study patterns left behind by lipids, dyes, and other fluids. They place their mixture in a rotating flask kept in a warm bath. For a few hours, the fluids mix, chemically react, and evaporate. The complex interactions that take place in that time leave behind fascinating, rune-like patterns, seen here under a microscope. It’s a bit like looking at photos of Martian landscapes! (Image credit: M. Murali and L. Shen)
Tag: fluid dynamics
A Toad’s Sticky Saliva
Frogs and toads shoot out their tongues to capture and envelop their prey in a fraction of a second. They owe their success in this area to two features: the squishiness of their tongues and the stickiness of their saliva. The super squishy toad tongue deforms to touch as much of the insect as possible. That shape-changing helps deliver the saliva, which is an impressively fast-acting, shear-thinning fluid. Under normal circumstances, the saliva is sticky and about as viscous as honey. But the shear from the tongue’s impact makes the saliva flow like water, spreading across the insect’s body. Then it morphs back into its viscous, sticky self, providing enough adhesive power that the insect can’t escape the toad pulling its tongue back in. (Video credit: Deep Look/KQED; research credit: A. Noel et al.)
Long-Lived Bubbles
Without surfactants to stabilize them, bubbles don’t last long at room temperature. But adding a little heat changes the picture. When heated, the bubbles get stabilized by a thermal gradient that lifts fluid toward the bubble’s peak, where it cools and gathers. Eventually, the cold fluid grows heavy enough to sink down the side of the bubble (in either a constant stream or occasional drips); with warm fluid getting pulled up to replace it (via the Marangoni effect), the process repeats and the bubble lives on. (Video credit: S. Nath et al.; see also)
Swimming With Corkscrews
For many microswimmers, like bacteria or spermatozoa, swimming through common fluids is like moving through mud. Unless they can produce enough thrust to overcome a fluid’s yield-stress, they are effectively stuck in a solid. A recent study breaks down exactly what a microswimmer has to manage, assuming they use a helical, corkscrew-like tail for propulsion.
The first barrier is creating enough force to be able to rotate in the fluid, but that alone is not enough to ensure forward motion. Once rotating, the swimmer’s thrust has to be large enough to deform the fluid around it. Without that, the swimmer is stuck. And, finally, once they’re moving, the swimmer’s tail pitch determines how fast they can move and whether the fluid’s characteristics slow it down.
The researchers hope their work can shed light on propulsion for bacteria in the body, as well as larger creatures like burrowing earthworms and fruit-invading parasites. (Image credit: SwedishStockPhotos; research credit: F. Nazari et al.; via APS Physics)
Abel Prize Winner Luis Caffarelli
Tomorrow mathematician Luis Caffarelli will receive the Abel Prize — one of the highest honors in mathematics — in part for his work in fluid dynamics. Caffarelli is one of the authors of a partial proof of regularity for the Navier-Stokes equations, the equations governing fluid motion. A full proof of regularity and smoothness — essentially showing that the equations never break down or blow up to infinity — is one of the open Millennium Problems. Caffarelli is the first mathematician born and educated in South America to receive the Abel Prize. Congratulations to Professor Caffarelli! (Image credit: N. Zunk/University of Texas at Austin; via Nature; submitted by Kam-Yung Soh)
“Fusion of Helios”
Built from approximately 90,000 individual images, “Fusion of Helios” reveals the wisp-like corona of our Sun. Astrophotographers Andrew McCarthy and Jason Guenzel joined forces to combine eclipse images with data from NASA to build this fusion of art and science. Jets of plasma, known as spicules, dot the sun’s surface, and a towering tornado of plasma shoots off one side. For scale, that vortex stretches as far as 14 Earths stacked atop one another. (Image credit: A. McCarthy and J. Guenzel; via Colossal)
Oil-Covered Bubbles Popping
When bubbles burst, they release smaller droplets from the jet that rebounds upward. Depending on their size, these droplets can fall back down or get lofted upward on air currents that spread them far and wide. Thus, knowing what kind of bubbles produce small, fast droplets is important for understanding air pollution, climate, and even disease transmission.
The jet from a bubble of clean water is broad and slow, releasing fewer and larger drops. In a recent study, researchers compared droplets made by clean, water-only bubbles, and the ones generated from water bubbles with a thin layer of oil coating them. The clean bubbles created jets that were broad and relatively slow moving; this motion produced a few large drops that quickly fell back down.
The jet from an oil-covered bubble is skinny and fast-moving. It produces many small droplets. In contrast, the oil-slicked bubbles made a narrow, fast-moving jet that broke into many small droplets. These droplets could stay aloft for longer periods, indicating that contaminated water can produce more aerosols than clean. (Image credit: top – J. Graj, bursting – Z. Yang et al.; research credit: Z. Yang et al.; submitted by Jie F.)
Acoustic Cameras
Acoustic cameras use arrays of microphones to isolate where sounds are coming from. As Steve Mould shows in this video, they have some incredibly cool properties. They can show engineers which part of a device is producing particular sound frequencies, which is handy, for example, when trying to quiet a vacuum cleaner or learn which wheels on a train need maintenance. They can also show how sound moves around a room; near the end of the video, you can see the echo from a clap flashing around a room. Check out the full video for more! (Video credit: S. Mould)
Drying Cracks
Droplets with particles in them can leave complex stains when they dry — just look at coffee rings and whiskey marks! Here, researchers look at the patterns left on glass by small droplets that evaporated and left behind their nanoparticles. As evaporation takes place, the droplet’s shape changes, adding stress to the growing layer of nanoparticle residue. Cracking is one way to relieve that stress. Another method is delamination — peeling up from the surface. On the leftmost drop, the outer rim of nanoparticles delaminated — as seen from the circular fringes — which released stress without cracking. The rightmost drop, which had a smaller contact angle with the surface, couldn’t delaminate and instead cracked throughout. (Image credit: M. Ibrahim et al.)
Mermaid Cereal
In the Cheerios effect, floating objects can fall into one another due to capillary attraction — just like Cheerios link up in a cereal bowl. Here researchers play with that effect by adding repulsive magnets to their “cereal” pieces. They find that their so-called mermaid cereal falls into preferential spacing, with pieces pairing up but never touching. Adding lots of these pieces in a confined space creates interesting crystalline and striped patterns, as seen later in the video. (Video credit: A. Hooshanginejad et al.)
