Icy crystals burst forth against a dark background in Thomas Blanchard’s short film “Belletrix.” The process is one of chemical crystallization. Blanchard supersaturates a chemical in a dish of hot water, then cools the fluid, which then spontaneously crystallizes when disturbed. Depending on the solution’s temperature, the crystals vary from feather-like to radial stars, each reflecting, expanding, and overlapping to cover the full surface. (Image and video credit: T. Blanchard)
With the low sun angle of dawn, the details of this cloudscape stand out. Captured by an external camera on the International Space Station, this image shows cloud formations over the northwest Atlantic. In the foreground, towering cumuli mark rising plumes of warm, moist air evaporating from the ocean. Beyond those clouds, a flat anvil cloud spreads horizontally after a temperature inversion prevented it from rising any further. (Image credit: NASA; via NASA Earth Observatory)
Compared to its interior, the surface of our sun is a cool 6,000 degrees Celsius. But beyond the surface, the sun’s corona heats up dramatically through interactions between plasma and strong magnetic fields. The exact mechanisms of this interaction have been mostly theoretical thus far, but a recent laboratory experiment has validated a part of that theory.
One explanation for coronal heating posits that the strong magnetic fields can accelerate magnetohydrodynamic waves called Alfvén waves to speeds faster than sound, and that at this crossover point, changes occur in the waves’ behavior. Using liquid rubidium, researchers were able to observe this crossover under laboratory conditions, confirming that the Alfvén waves change at the speed of sound in exactly the manner predicted by theory. (Image credit: NASA SDO; research credit: F. Stefani et al.; via Physics World)
Most cooks know that their frying oil isn’t hot enough if dropping the food in doesn’t create a furious burst of bubbles. But the canniest cooks know they can check the temperature just by listening to the sound made when inserting a utensil, like a wooden chopstick. When oil nears the right temperature, a cloud of bubbles forms around the utensil, leading to a flurry of sound as those bubbles break.
In this video, researchers explore the sound and bubble dynamics together as a function of temperature. They show how the final sound carries the signature of the its bursting bubble, too. So next time you’re getting ready to fry and you can’t find your thermometer, don’t panic. Just listen! (Image and video credit: A. Kiyama et al.)
Years ago, physicists discovered that water flows with surprisingly little friction through narrow carbon nanotubes. At our scale, flow behavior is typically the opposite: there’s greater friction (and, thus, slower flow) in a narrower pipe. To unravel the mystery, researchers had to delve into quantum mechanics and model the interactions between the atoms of a water molecule and the electrons of the carbon atom. Essentially, this meant building a quantum picture of the liquid-solid interface inside the nanotube.
The team found that the electrons of the nanotube exert a drag-like force on the water molecules, creating friction that slows the flow. Since narrow nanotubes have fewer electrons than larger tubes, there is less friction on the flow and the water flows faster! (Image credit: cintersimone; research credit: N. Kavokine et al.; via SciAm; submitted by Kam-Yung Soh)
Unusually high rainfall in Bolivia’s Salar de Uyuni turned the world’s largest salt flat into a shallow salt lake. These natural-color satellite images show the area in late January 2022. If you zoom in on the full resolution image, there are incredible detailed swirls in the water. It’s like peering at an abstract or Impressionist painting. The many colors are attributable to several sources, including volcanic sediments, runoff, and a variety of microbes and algae thriving in the mineral-filled waters. (Image credit: L. Dauphin; via NASA Earth Observatory)
At first glance, Steve Mould’s video on parametric resonance has nothing whatsoever to do with fluid dynamics. He uses a pendulum suspended on a spring to demonstrate how driving a system at a frequency that’s a multiple of the system’s natural frequency can add energy through resonance. Although his examples don’t use fluids, this phenomenon happens there, too, especially in vibrated fluid systems. Take, for example, this droplet bouncing on a vibrating pool. Depending on the amplitude of the vibrations driving the system, the droplet may bounce in time with the vibration, in time with the waves, or at a frequency twice that of the vibration. (Image and video credit: S. Mould)
We’ve seen many forms of Leidenfrost effect — that wild, near-frictionless glide that liquid droplets make on a very hot surface — over the years, but here’s a new one: the three-phase Leidenfrost effect. Researchers found that they could generate a Leidenfrost effect using an ice disk placed on an extremely hot surface. During the effect, the ice and its melting layer of water glide on vapor, hence the name.
The team found that getting a three-phase Leidenfrost effect requires a much, much higher temperature than the regular Leidenfrost effect. Water will get its glide on at 150 degrees Celsius. Getting ice to glide on the same surface required a stunning 550 degrees Celsius! Why the big difference? The challenge is that water layer, which, by definition, has a 100-degree difference between its boiling side and its frozen boundary. It takes so much heat to maintain that layer that there’s little energy left over for evaporation; that’s why it takes so much more energy to get the three-phase Leidenfrost effect. (Image and research credit: M. Edalatpour et al.; via Ars Technica; submitted by Kam-Yung Soh)
What does the flow look like around a spinning top? Here, researchers used dye to visualize what happens in a Newtonian fluid (like air or water) as well as a viscoelastic fluid. The Newtonian fluid (upper images) divides into two circulating zones, one below the top and one above. They both take the shape of a toroidal, or donut-shaped, vortex, visible here in cross-section.
The long molecules of the viscoelastic fluid lend it elasticity to resist stretching. The result is a very different flow field. Beneath the top, there’s still a toroidal vortex, though it appears tighter. But around the upper part of the top, there’s a butterfly-like region of recirculation! (Image credit: B. Keshavarz and M. Geri)
Using air or bubbles to reduce drag on boats is a popular idea, whether using supercavitation, the Leidenfrost effect, or superhydrophobic coatings. But most of the experiments done thus far use spheres rather than realisitic boat shapes. In this study, the researchers used two model boats — one with a hydrofoil and the other in a conventional motorboat shape — and applied superhydrophobic coatings to different parts of the model to see how superhydrophobicity affected the overall drag.
Perhaps surprisingly, they found that superhydrophobic coatings can actually increase the drag! The effect was particularly stark for the hydrofoil boat (Image 2), where the surface jets (lower half) caused by the superhydrophobic coating slowed the boat by 30% compared to its unmodified speed (upper half).
For the speedboat, a superhydrophobic hull made no overall difference in its drag, though it changed how water splashed in its wake. And coating the boat’s propeller was particularly detrimental, resulting in a speed up to three times slower. Overall, the study suggests that superhydrophobic coatings may be useful in some circumstances, but they have to be applied carefully, as they can have negative impacts, too. (Image credits: top – S. Anghan, others and research credit: I. Vakarelski et al.)
