Tag: diffusion

Proving Superdiffusion
Turbulence is very good at spreading things out. Drop dye into a turbulent flow and it will quickly disperse. Add in particles — like rubber ducks — and they can spread apart, often at speeds quicker than one would expect, based on the background flow. This is (roughly speaking) a phenomenon known as “superdiffusion,” where turbulence makes particles that start out as neighbors part ways.
Physicists conjectured that turbulence — including simplified and idealized versions of it that are simpler to deal with — had this superdiffusion property, but no one was able to show that in a mathematically rigorous way. But now a group of mathematicians has done so, using a technique known as homogenization. There’s a lot more on the story over at Quanta, or you can check out the original papers on arXiv. (Image credit: J. Richard; research credit: S. Armstrong et al. and S. Armstrong and T. Kuusi; see also Quanta)
Fediverse Reactions
June 12, 2025
Why Nature Loves Fractals
$A brightly colored arm in a Mandelbrot set, demonstrating fractal self-similarity.$
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Trees, blood vessels, and rivers all follow branching patterns that make their pieces look very similar to their whole. We call this repeating, self-similar shape a fractal, and this Be Smart video explores why these branching patterns are so common, both in living and non-living systems. For trees, packing a large, leafy surface area onto the smallest amount of wood makes sense; the tree needs plenty of solar energy (and water and carbon dioxide) to photosynthesize, and it has to be efficient about how much it grows to get that energy. Similarly, our lungs and blood vessels need to pack a lot of surface area into a small space to support the diffusion that lets us move oxygen and waste through our bodies. Non-living systems, like the branches of viscous fingers or river deltas or the branching of cracks and lightning, rely on different physics but wind up with the same patterns because they, too, have to balance forces that scale with surface area and ones that scale with volume. (Video and image credit: Be Smart)
Fediverse Reactions
February 19, 2025
“Ink In The Water. Mix One.”
In this ASMR video, black ink diffuses in water. When the video starts, the ink is so diffuse that it’s not apparent the video is playing backward. It’s only as specific structures — things like Rayleigh-Taylor instabilities, plumes, and jets — coalesce from the background that we recognize the time reversal. Though it’s probably unintentional, this makes for a neat, subtle commentary on the nature of isotropic turbulence. (Video and image credit: Wryfield Lab)
September 20, 2024
How a Storm Can Ruin Your Tea
Last November, a windstorm, known as Storm Ciarán in the U.K., blew through Europe with wind speeds as high as 130 kilometers per hour. All that wind came with a significant drop in atmospheric pressure. Researchers found that the pressure drop was large enough to lower the boiling point of water more than full 2 degrees Celsius. That difference probably wouldn’t register for anyone waiting for their kettle to boil, but it could decidedly affect the final cup of tea. Tea flavor is quite sensitive to the temperature of the boiling water used to brew it, as it affects how well the tannins get extracted. According to the researchers, Ciarán’s conditions potentially ruined millions of cups of breakfast tea in the greater London area. (Image credit: E. Akyurt; research credit: G. Harrison et al.; via Gizmodo)
August 20, 2024
How We Got Atoms From Brownian Motion
In 1827, botanist Robert Brown observed an odd jittery motion of particles as he watched grains of pollen floating in water under his microscope. He saw the random motion also with inorganic — which is to say definitely Not Alive — particles as well. But it was Einstein nearly 80 years later who figured out how to connect this observable motion to atoms. Einstein realized Brown’s particles were being constantly jostled by atomic collisions, and, with a little work, we could use those moving particles to determine Avogadro’s number. Steve Mould walks you through the whole story in this video. (Video and image credit: S. Mould)
May 27, 2024
Disease and Placental Flows
The human placenta functions as a life-support system for a growing fetus. Despite its frisbee-like appearance, the organ is packed with nearly 10 square meters of blood vessels. On the fetal side, these blood vessels form villous trees where diffusion across the placental boundary exchanges molecules with the maternal blood that fills the space between villous trees. This setup allows oxygen, glucose, carbon dioxide and other key chemicals to cross between the parent and fetus while (ideally) keeping diseases out.
Views of the placenta. Beige areas show the intervillous space where maternal blood flows while pink areas show villous trees where exchanges between the fetus and mother take place. The first three images show a) preeclamptic, b) normal, and c) diabetic placentas. The final image d) shows a 3D view of placental tissue taken with x-ray tomography.
But when diseases directly affect the structure of the placenta, flow to the fetus gets disrupted. The image above shows cross-sections of placental tissues, with villous trees marked in pink, under (a) preeclamptic, (b) normal, and (c) diabetic conditions. Preeclampsia is associated with reduced density of villous trees, which restricts the amount of nutrients a fetus receives and can lead to reduced growth or stillbirth. In contrast, with gestational diabetes villous trees can proliferate, causing a high resistance to flow that also affects exchanges.
For more on the complex physics of the placenta, check out this article from Physics Today. (Image credit: sketch – L. da Vinci, placentas – A. Clark et al.; see also A. Clark et al.)
June 21, 2023
Placental Fluid Dynamics
The placenta, critical as it is to human life and development, is likely the least-studied organ in the body. Reasons for that abound, from the ethics of studying pregnant people to the historical marginalization of female bodies in medical studies. But what little we do know shows that the placenta is quite incredible.
Shaped somewhat like a flattened cake, the placenta contains a tangle of fetal blood vessels — an estimated 550 kilometers’ worth — bathed in maternal blood. The enormous surface area — nearly 13 meters squared — enables the exchange of oxygen, glucose, and urea through diffusion. These exchanges don’t take place in still conditions, though; blood is always flowing through the vessel network. This means that each exchange depends on both the speed of diffusion and the speed of the flow, a relationship that’s captured with the dimensionless Damköhler number.
Illustration of the intertwined blood vessels of the placenta.
Some exchanges, like carbon monoxide and glucose, are diffusion-limited, meaning that increased blood flow cannot speed up the process (though additional blood vessel surface area could). In contrast, carbon dioxide and urea are flow-limited exchanges. Fascinatingly, oxygen is close to being both diffusion- and flow-limited, suggesting that the organ has optimized for this exchange. Since pregnancy also involves a large increase in maternal blood volume and changes in lung capacity to help provide oxygen, it seems like the pregnant body heavily emphasizes delivering oxygen to the developing fetus. (Image credit: newborn – J. Borba, placenta – iStock/Sakurra; via Physics World ; submitted by Kam-Yung Soh)
April 3, 2023
Chemical Flowers
These “flowers” blossom as two injected chemicals react in the narrow space between two transparent plates. The chemical reaction produces a darker ring that develops a streaky outer edge due to competition between convection and chemical diffusion.
To show how gravity affects the instability, the researchers repeated the experiment on a parabolic flight. In microgravity conditions, no instability formed. That’s exactly what we’d expect if convection (i.e. flow due to density differences) is a major cause. No gravity = no convection. In contrast, under hypergravity conditions, the instability was initially spotty before developing streaks. (Image and video credit: Y. Stergiou et al.)
November 7, 2022
Anoles Revisited
Longtime readers may recall seeing this little bubble-crowned anole previously. This species dives underwater to escape predators and will breathe and rebreathe a bubble of air for as much as 18 minutes before resurfacing. At the time of my original post, I speculated that the reptile’s hydrophobic skin might provide a large enough bubble surface area to provide some diffusion of fresh oxygen from the surrounding water.
Since then, there’s been at least one study of this anole rebreathing process. Researchers found that many anole species share this behavior, but aquatic species use it more regularly. They noted that the plastron — that flat, silvery bubble that’s spread over the lizard’s skin — helps hold the bigger, exhaled bubble in place and might facilitate a little of the diffusion I speculated about but the results are unclear on that last point. The authors note that it’s unlikely that the anoles could support their full metabolism through rebreathing and diffusion but that the plastron may yet support some rejuvenation of oxygen, which would help prolong anoles’ dives. (Image and research credit: C. Boccia et al.)
October 6, 2022
Backswimmers
Backswimmers rule the surface of ponds, streams, and other bodies of water. These insects spend much of their time clinging just beneath the air-water interface, where they hunt larvae and other insects. They use oversized, oar-shaped back legs to row, and they breathe using an air bubble that clings to their abdomen like a personal scuba tank. Oxygen from the water diffuses into the bubble, keeping the insect’s air supply fresh. When the time comes to move to greener pastures, they flip to the other side of the water’s surface, unfurl their wings, and take off. (Image and video credit: Deep Look)
October 3, 2022