Tag: physics

Tricking a Kettle
Electric kettles are designed to shut off when the water inside them boils. But what does that mean exactly? In this video, Steve Mould explores that question by trying to trick his kettles into boiling off ethanol, a liquid with a lower boiling temperature than water. Steve figures that, if a kettle is designed to shut off at 100 degrees Celsius (water’s boiling point), then it will overboil ethanol since its temperature will never get that high.
I’ll let you watch the video and see what happens…
(more…)
March 22, 2022
Liquid Bridges
In 1893, Baron Armstrong demonstrated a peculiar phenomenon — a liquid bridge of water suspended between two beakers with a strong electric charge between them (Image 1). More than a century later, the details of the mechanism remain challenging to pin down thanks to the setup’s combination of electohydrodynamics, heat transfer (Image 2), evaporation, and chemistry (the electrodes can split water).
Researchers have pinned down a few details, though, like that the break-up of the liquid bridge (Image 3) depends on its effective length and that the effective length grows as applied voltage increases. Researchers also found that inducing an external flow can extend the bridge’s lifetime, though it does not affect the length at which it breaks up. Interestingly, the phenomenon is not limited to water (and its odd chemistry); ethanol and glycerol have been used for liquid bridges, too! (Image and research credit: X. Pan et al.)
March 21, 2022
“Heaven”
Wispy white cirrus clouds cover dark skies glittering with stars in Roman De Giuli’s “Heaven”. Or so it appears. In reality, these skyscapes are made with watercolors, ink, and acrylic paint. The vistas are gorgeous regardless of whether they’re driven by turbulent convection (as in the atmosphere) or the Marangoni effect (as in this video)! (Video and image credit: R. De Giuli)
March 18, 2022
When Bubbles Don’t Die
In a pure liquid, most bubbles pop almost immediately. But with a simple ingredient — a little heat — bubbles can live almost indefinitely. The mechanism is revealed in this video when the researchers use an infrared camera to watch a bubble on a heated pool. The top of the bubble is cooler than the rest of the liquid, forming colder, denser droplets that slide down. But the cooler liquid also has a higher surface tension, which draws warm liquid up the bubble, replenishing it. The result is a stable bubble that simply carries on. (Image and video credit: S. Nath et al.)
March 17, 2022
Schooling Relies on Vision
For fish, collective motions like schooling rely on a few mechanisms, including flow sensing and — as beautifully demonstrated in this experiment — vision. Researchers used an infrared camera to track fish motions both in light and dark conditions and compared how orderly the school of fish was in each. As expected, the school’s motion was much more orderly when the fish could see one another clearly. Interestingly, the researchers then ran an experiment in which the illumination rose continuously from dark to fully bright. The fish school’s organization grew continuously with the light! The better they could see one another, the more organized their schooling. (Video and research credit: L. Baptiste et al.)
March 16, 2022
Morphing Particle Rafts
A layer of tiny glass beads sitting atop a pool of castor oil becomes a morphing surface in this video. Applying an electric field creates enough electrostatic force to draw the interface upward against the power of both gravity and surface tension. Moving the electric field — either by shifting the electrode or simply moving a finger over the surface — is enough to pull columns of fluid along! I could imagine this making some very cool human-machine interfaces one day. (Image and video credit: K. Sun et al.)
March 15, 2022
Turbulence in Flight
Eagles and other birds spend much of their lives in the turbulence of our atmospheric boundary layer. Some of their interactions with turbulence — like using topographical effects to aid their flight — are well-known, but much remains uncertain. One team of researchers looked at a tagged golden eagle’s flight data, compared with known wind conditions, and looked for evidence of turbulence’s influence. To do this, they drew on years of research into how turbulence interacts with inertial particles — particles that are heavier than the surrounding fluid and thus unable to follow the flow exactly.
What they found is that turbulence seems to be baked into many aspects of the eagle’s flight. Even the basic accelerations of the eagle’s body during flight showed characteristics that match those of turbulent flows. The findings suggest that turbulence — rather than something to be avoided — is an integral part of flight for birds, an energy source they’ve learned to exploit. (Image credit: J. Wang; research credit: K. Laurent et al.; submission by G. Bewley)
March 14, 2022
Portraits of Flight
During lockdown, photographer Doris Mitsch turned her eyes to the sky and began capturing these mesmerizing composite images of animals in flight. Vultures, crows, starlings, gulls, and bats all feature in her series. Some images, like “Lockdown Vulture (Signature)”, feature a single bird’s movement over a minute. Others show entire flocks over extended periods.
I love how the images capture a sense of speed. Given equal timing between images, the lines with more space between each snapshot of a bird indicate a faster speed. It’s a bit like having particle image velociometry frames stacked atop one another! (Image credit: D. Mitsch; via Colossal)
March 11, 2022
Mushy Layers
In many geophysical and metallurgical processes, there is a stage with a porous layer of liquid-infused solid known as a mushy layer. Such layers form in sea ice, in cooling metals, and even in the depths of our mantle. Within the mushy layer, temperature, density, and concentration can vary dramatically from one location to another.
The image above shows a mushy layer made from a mixture of water and ammonium chloride. Above the mushy layer, green plumes drift upward, carrying lighter fluid. Look closely within the mushy layer and you’ll see narrow channels feeding up to the surface. These are known as chimneys. In sea ice, chimneys like these carry salty brine out of the ice and into the seawater, increasing its salinity. See this Physics Today article for more details on the dynamics of mushy layers. (Image credit: J. Kyselica; via Physics Today)
March 10, 2022
Splashing on Spheres
The splash of a droplet is a surprisingly complex phenomenon, depending not only on the droplet’s characteristics but also the surrounding air pressure, the roughness and temperature of the impact surface, and the surface’s curvature. In this study, researchers investigated the effects of surface curvature on splashing, finding that it’s harder for a drop to splash on spheres of smaller radius than ones with a larger radius of curvature.
In Image 1, the falling droplet coats the 2-mm sphere with no sign of splashing. But as the radius gets larger (Images 2 and 3), splashing becomes more and more pronounced. They found that the splash suppression is due to a modification of the lift force on the leading edge of the lamella, the thin liquid layer created as the drop impacts and spread. (Image, research, and submission credit: T. Sykes et al.; also available here)
March 9, 2022