Category: Research

Viscoelasticity and Bubbles
Bursting bubbles enhance our drinks, seed our clouds, and affect our health. Because these bubbles are so small, they’re easily affected by changes at the interface, like surfactants, Marangoni effects, or, as a recent study shows, viscoelasticity.
A bubble released in pure water pops at the surface, creating a rebounding jet and a daughter droplet.
In clean water, a bubble’s burst generates a rebounding jet that shoots off one or more daughter droplets, as seen in the animation above. But when researchers added proteins that modify only the water’s surface, they found something very different. As seen below, the bursting bubble no longer generated a jet, and, instead of forming droplets, it made a single, tiny daughter bubble. The difference, they found, comes from the added viscoelasticity of the surface. The long protein molecules resist getting stretched, which damps out the tiny waves that surface tension usually produces on the collapsing bubble cavity. (Image and research credit: B. Ji et al.; submission by Jie F.)
When the surface of water is viscoelastic, a bursting bubble creates no jet and a daughter bubble instead of a drop.
October 26, 2023
Bravo!
Applauding is a familiar activity, but, as you stand for an encore in the concert hall, do you think about how you hold your hands and how that affects your clap? That question prompted two scientists to embark on an acoustical exploration of clapping. By testing 11 different ways to hold their hands during clapping, the duo found some interesting results.
The loudest clap — achieving an average of 85 decibels — held the hands at 45 degrees to one another, with palms partially overlapping (A2 in the figure). But the clap that most pleased the ear was a little different (A1+). It kept the 45 degree orientation, but the palms overlapped fully with a domed shape between them. In that configuration, the palms form a little resonance chamber that makes the clap sound deeper and richer. (Image credits: top – G. Latorre, others – N. Papadakis and G. Stavroulakis; research credit: N. Papadakis and G. Stavroulakis; via Physics World)
Scientists studied the sounds made from clapping in 11 different hand configurations.
October 24, 2023
Dancing Peanuts
Bartenders in Argentina sometimes entertain patrons by tossing a few peanuts into their beer. Initially, the peanuts sink, but after a few seconds they rise, wreathed in bubbles. Once on the surface, they roll, causing the bubbles to pop, and the peanut sinks once again. The cycle repeats, sometimes for as long as a couple hours.
There are a couple physical processes governing this dance. The first is bubble nucleation. Most beers are carbonated; they contain dissolved carbon dioxide gas that remains in solution while the beer is under pressure. Once poured, that storage pressure is gone and bubbles start to form in the liquid. The shape of the peanut means that bubbles form more easily on it than on the glass walls or in the liquid. And once the peanut is covered in bubbles, buoyancy comes into play. The bubbles attached to the peanut reduce its density relative to the surrounding fluid, enabling the peanut to rise up and float.
This same process is seen with other objects in carbonated fluids, too, such as blueberries in beer and lemon seeds in carbonated water. But it’s also reflected elsewhere in nature. For example, magnetite crystals are thought to float in magma due to a similar nucleation of dissolved gases on their surface. (Image and research credit: L. Pereira et al.; via APS Physics)
October 19, 2023
Why Sea Foams
Seawater froths and foams in ways that freshwater rarely does. A new study pinpoints the ocean’s electrolytes as the reason bubbles resist merging there. By studying the final moments before bubbles coalesce in both pure and salt water, researchers found that dissolved salts slow down the drainage of the thin film of liquid between two bubbles. Once the film reaches a 30-50 nanometer thickness, its electrolyte concentration causes a difference in surface tension that slows the outward flow of liquid in the film. That keeps the film in place longer and makes bubbles form foams instead of merging or popping. (Image credit: P. Kuzovkova; research credit: B. Liu et al.; via APS Physics)
October 17, 2023
Underwater Volcanic Flows
The Hunga Tonga–Hunga Ha’apai volcanic eruption in December 2021 was the most violent in 140 years, and we are still learning from its aftermath. A recent study focuses on the eruption’s incredible underwater flows, which damaged nearly 200 kilometers of underwater cables. From the cables’ locations and the time of service loss, the team calculated that gravity currents hit the cables at speeds as high as 122 kilometers per hour and with run-outs that lasted over 100 kilometers. These fast flows were triggered by material from the volcanic plume falling into the ocean, causing dense flows that swept down the submerged slopes of the volcano and seafloor.
Illustration of volcanic plume material falling into the ocean and triggering underwater flows.
Previously, a landslide broke underwater telegraph cables off Newfoundland and a coastal construction accident severed a cable in the Mediterranean. But neither of those incidents revealed the same level of speed, distance, and destructive capacity as the Tongan eruption. It seems that these underwater gravity currents pose an ongoing threat to submerged infrastructure. As more cables are laid in volcanically-active regions of the Pacific, we will need more extensive mapping and monitoring of the seafloor to protect against future disruptions. (Image credit: eruption – Tonga Geological Services, illustration – APS/C. Cain; research credit: M. Clare et al.; via APS Physics)
October 12, 2023
Packing Disks
Liquid crystals, bottles of pills, and hoppers of grains can all involve disk-shaped particles. To better understand how disks pack together, researchers studied how disks in a box orient themselves after shaking. They used MRI to observe the disks’ interior packing.
These reconstructions show the packing found in the experiment. The disks are color-coded by orientation; horizontal disks are redder and vertical ones are bluer. Initially, the packing has many horizontal disks (left), but after shaking, the disks get more compacted (right). The disks form short stacks that are randomly oriented. This increases the overall density but the random orientations reduce the total alignment of disks.
The team found that shaking increases the disks’ density, but that increase does not come from disks orienting in the same direction. Instead, the disks form short stacks of similarly-oriented disks. The stacks themselves took on many different orientations, which reduced the system’s overall alignment in orientation. (Image credit: coins – M. Blan, packing – Y. Ding et al.; research credit: Y. Ding et al.; via APS Physics)
October 10, 2023
Forests Slow Avalanches
In snowy mountainous regions, avalanches are a dangerous and destructive problem. Researchers studying the mechanisms of these flows have a suggestion: plant more trees. A group of researchers found that a “forest” of regularly spaced pillars slowed avalanches by as much as two-thirds. On an empty slope, the avalanche picked up speed as its thickness grew. But with regularly-spaced pillars the slower flow rate became almost completely independent of avalanche thickness.
The researchers with their avalanche set-up, which releases glass beads through a forest of pillars.
For now, the researchers suggest placing trees every 3 meters on steep, avalanche-prone slopes — a technique that, admittedly, only works for slopes below the treeline. In their next round of experiments, the researchers plan to see how a randomly arranged forest affects an avalanche. (Image credit: top – N. Cool, apparatus – Université Paris-Saclay/FAST; research credit: B. Texier et al.; via Physics World)
October 5, 2023
Scooting Droplets
As a child, I always loved watching rain on the windows as I rode in the car. Hemispherical droplets got stretched by the wind flowing over them. But they never stretched smoothly; instead they seemed to shiver and shake unevenly. A recent study looks at a similar situation: drops of glycerin forced to slide along a horizontal surface under the force of the wind. Like the drops on my parents’ car, the glycerin gets stretched out into an elongated oval. Surface waves develop atop the drop and move downstream. The drops, the authors observe, move a bit like a crawling caterpillar, pilling up and smoothing out as they move. (Image credit: rain – A. Alves, experiment – A. Chahine et al.; research credit: A. Chahine et al.; via APS Physics)
This series of images shows an elongated droplet subjected to airflow moving from left to right. Waves form on the drop and move downstream in a fashion similar to a caterpillar crawling.
October 3, 2023
Stopping a Bottle’s Bounce
A few years ago, the Internet was abuzz with water bottle flips. Experimentalists are still looking at how they can arrest a partially fluid-filled container’s bounce, but now they’re rotating the bottles vertically rather than flipping them end-over-end. Their work shows that faster rotating bottles have little to no bounce after impacting a surface.
This image sequence shows how water in a rotating bottle moves during its fall (top row) and after impact (bottom row). Water climbs the walls during the fall, creating a shell of fluid that, after impact, forms a central jet that arrests the bottle’s momentum.
The reason for this is visible in the image sequence above, which shows a falling bottle (top row) and the aftermath of its impact (bottom row). When the bottle rotates and falls, water climbs up the sides of the bottle, forming a shell. On impact, the water collapses, forming a central jet that shoots up the middle of the bottle, expending momentum that would otherwise go into a bounce. It’s a bit like the water is stomping the landing.
The authors hope their observations will be useful in fluid transport, but they also note that this bit of physics is easily recreated at home with a partially-filled water bottle. (Image and research credit: K. Andrade et al.; via APS Physics)
September 28, 2023
Weathering Spilled Oil
As long as we continue to extract and transport oil, marine oil spills will continue to be a problem. Recent work shows that spilled oil weathers differently depending on both sunlight and water temperature. When exposed to sunlight, crude oil undergoes chemical reactions that can change its makeup. Researchers studied the mechanical properties of crude oil samples kept at different temperatures in both sunlight and the dark.
They discovered that sunlight-exposed crude oil kept at a high temperature had twice the viscosity of a sample kept in the dark at the same temperature. In contrast, the high-temperature sunlit sample’s viscosity was 8 times lower than a sunlit sample kept at a lower temperature. That’s quite a large difference, and it implies that tropical oil spills may behave quite differently than Arctic ones. Cold-water spills will entrain and dissolve less than warm-water ones, so there may be more surface oil to collect at high-latitude spills. The differences in viscosity may also necessitate different spill mitigation techniques. (Image credit: NOAA; research credit: D. Freeman et al.; via APS Physics)
September 26, 2023