Search results for: “density”

Lanes in Crowds
In nature — from atoms to human crowds — two groups moving in opposite directions often spontaneously organize into interwoven lanes flowing in their respective directions. Now researchers have built a mathematical model for this behavior, building on Einstein’s observations of Brownian motion.
To test their model, the researchers performed numerical simulations and experiments with pedestrians. Intriguingly, they found that introducing rules like “always pass on the right” created unexpected results, such as tilted lanes. With their model verified — at least for low-density crowds — the group hope to uncover other hidden patterns within crowds. (Image and research credit: K. Bacik et al.; via Physics World)
In their validation experiments, the researchers filmed groups of pedestrians walking past one another under different conditions. Note the lanes that form as the two groups interleave.
July 11, 2023
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
Turning the Beach Pink
Lab experiments and numerical simulations can only take us so far; sometimes there’s no substitute for getting out into the field. That’s why a beach in San Diego turned pink this January and February, as researchers released a safe, non-toxic dye into an estuary. The goal is to understand how small freshwater sources mix with colder, saltier ocean waters when they meet in the surf zone. Differences in temperature and salinity both affect the waters’ density and, therefore, how they’ll combine, especially in the face of the turbulent surf. Using drones, distributed sensors, and a specially-outfitted jet ski, the researchers collect data about how the dye (and therefore the estuary’s water) spreads over the 24 hours following each dye release. Check out their experiment’s site to learn more. (Image credits: E. Jepsen/A. Simpson/UC San Diego; via SFGate; submitted by Emily R.)
March 1, 2023
A Starry Nursery
This mountain of interstellar gas and dust lies in the picturesque Eagle Nebula. Though it appears solid in this near-infrared image from JWST, the density of the structure is actually quite low. Jets and solar winds from the glowing, young stars inside the region sculpt the pillar’s shape. Over the next 100,000 years, the stars’ energetic jets, solar winds, and destructive supernovas will destroy the dusty nursery. (Image credit: NASA/ESA/CSA/STScI/M. Özsaraç)
February 15, 2023
Mixing in a Winter Lake
A frozen winter lake can hide surprisingly complex flows beneath its placid surface. Since water is densest at 4 degrees Celsius — just above the freezing point — mixing two water sources can lead to counterintuitive effects. A cold lake, for example, may contain water below 4 degrees Celsius, while a stream running into the lake is a bit warmer than 4 degrees Celsius. When the two parcels of water meet, they mix to form water at an intermediate temperature. But because of water’s density anomaly, that mixed water can wind up denser than the average of its parents. This is known as cabbeling.
Mixing patterns within a cold lake with a slightly warmer inflow. Image from A. Grace et al.
As shown in a recent study, this newly mixed water sinks to the bottom of the lake, forming a warm current that heats the lake from below. The researchers were able to model this current and its behavior over a range of conditions. Understanding these winter circulation patterns is key to tracking both nutrient transport and how pollutants spread in the ecosystem. (Image credit: lake – G. Murry, simulation – A. Grace et al.; research credit: A. Grace et al.; via APS Physics)
February 2, 2023
Listen to a Martian Dust Devil
A lucky encounter led the Perseverance rover to record the first-ever sound of a dust devil on Mars. The rover happened to have its microphone on (something that only happens a few minutes every month) just as a dust devil swept directly over the rover. Check out the video above to see and hear what Perseverance captured.
Using the rover’s instrumentation, researchers worked out that the dust devil was at least 118 meters tall and about 25 meters wide. The team was even able to determine the density of dust in the vortex from the sound of individual grain impacts captured in the acoustic signal! Serendipitous as the experience was, planetary scientists may now look to include microphones on more missions, since we now know how to get useful meteorological data from them. (Video credit: JPL-Caltech/NASA; image credit: LPL/NASA; research credit: N. Murdoch et al.; via AGU Eos; submitted by Kam-Yung Soh)
January 26, 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
Flowers Through a Hazy Veil
A smoke-like haze obscures colorful bouquets in these photographs from artist Robert Peek. To achieve the effect, Peek submerges his subjects underwater with white dye that sinks due to its greater density. The wakes traced by the dye are impressively laminar, so the dye must drift rather slowly past each petal. The overall effect is beautifully dream-like. You can find more of Peek’s work on Behance and Instagram. (Image credit: R. Peek; via Colossal)
October 14, 2022
Encapsulating Drops
Sometimes a droplet needs a little protection while it’s traveling to its destination. When that’s the case, we often try to encapsulate it in a layer of material that won’t be affected by whatever environment the drop is traveling through. In this study, researchers aimed to give their drops not one but two layers of protection — in as simple a way as possible.
The team began with three layers of liquid. The lowest layer was water, the middle layer was an oil, and the top layer was a mixture of water and isopropyl alcohol. Next, they added glass particles that were denser than the alcohol, but less dense than the oil. This caused the particles to form a clump — a granular raft — along the interface between the alcohol and the oil (not shown). When the layer of particles became heavy enough, it began to sink into the oil, carrying some of the alcohol with them. This conglomeration formed the initial droplet of alcohol mixture encased in an armor of glass beads.
As this armored droplet sank, it approached the second interface: the oil-water interface. At this juncture, the team observed three different outcomes. When the glass particles were small or light, the armored drop would come to a rest at the oil-water interface. As the drop deformed, water would pierce the armor, causing the whole drop to rupture (Image 1).
In the second case, heavier particles caused the armored drop to sink through the oil-water interface, but a low oil viscosity meant that the oil film drained from the bottom of the drop before the drop was fully encapsulated. Once again, this let the water through and ruptured the droplet (Image 2).
In the final case, armored drops with just the right bead density and oil viscosity would sink through the oil-water interface until the oil pinched off behind the drop. This pinch-off allowed the oil to redistribute around the drop, encapsulating it in layers of both oil and particles, thereby protecting it as it continued its journey (Image 3). (Image credits: top – Girl with red hat, experiment – A. Hooshanginejad et al.; research credit: A. Hooshanginejad et al.)
August 16, 2022
Bubbles in Turbulence
In nature and industry, swarms of bubbles* often encounter turbulence in their surrounding fluid. To study this situation, researchers used numerical simulation to observe bubbles across a range of density, viscosity, and surface tension values relative to their surroundings. They found that density differences between the two fluids made negligible changes to the way bubbles broke or coalesced.
In contrast, viscosity played a much larger role. More viscous bubbles were less likely to deform and break, thanks to their increased rigidity. When looking at small deformations along the bubble interface, both density and viscosity had noticeable effects. With increasing bubble density, they observed more dimples on the interface; increasing the viscosity had the opposite effect, making the bubbles smoother. (Image credit: Z. Borojevic; research credit: F. Mangani et al.)
*We usually think of bubbles as air or another gas contained within a liquid. But this study’s authors use the term “bubble” more broadly to mean any coherent bits of fluid in a different surrounding fluid. Colloquially, this means their results apply to both bubbles and drops.
August 9, 2022

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