Category: Research

Water Builds Static Charge
The ancient Greeks first recognized static electricity, but the mechanisms behind it remain somewhat mysterious. In particular, it’s unclear how two pieces of the same material can build a charge between them simply by touching. Yet we regularly see examples of this when volcanic ash creates enough charge to discharge lightning. A new study sheds light on the question by studying the impact of a single grain of silica on a silica disk.
The researchers used acoustic levitation to hold their silica particle in place. By turning the acoustic waves off, they could bounce the grain off the disk, then catch the particle again with the acoustic field. After a bounce, they swept an electrical field across the particle and observed its oscillations to determine how much charge the particle held. When necessary, they could also discharge the particle.
This animation demonstrates the three phases of an experiment. In the first (left), the acoustic field is shut off, allowing the silica grain to fall and strike the disk. Then the field is turned back on to “catch” the particle. In the second phase (middle), the researchers use a sweeping electrical field to determine the charge built up on the grain. In the third phase (right), they periodically discharge the built-up charge on the particle.
What they found was that charge on the particle grew with the number of impacts. They also saw that they could reverse the polarity of the charge with careful cleaning and baking of their objects. Their conclusion is that adsorption of water from the surrounding air is what enables the build-up of static charge on identical materials. (Image credit: volcano – M. Szeglat, experiment – G. Grosjean and S. Waitukaitis; research credit: G. Grosjean and S. Waitukaitis; via APS Physics)
April 20, 2023
Explaining Salt Polygons
Around the world, salt playas are criss-crossed with meter-sized polygons formed by ridges of salt. The origins of these structures — and the reason for their consistency across different regions of the world — have been unclear, but a new study shows that salt polygons form due to convection happening in the soil underground.
Through a combination of numerical modeling, simulation, lab-scale experiment, and field work, the team revealed the mechanism underlying salt polygons. Areas that form polygons have much greater rates of evaporation than precipitation, and, as water evaporates, these areas draw groundwater from nearby. Salt gets carried with this groundwater.
With strong evaporation, the lake bed forms a highly-concentrated layer of salty water near the surface. Convection cells form, with some regions drawing less saline water upward, while denser, saltier water sinks in other areas. The subsurface convection lines up exactly with the surface structures. The interior regions of polygons are areas where less salty water rises, and salt instead concentrates along the edges of polygons, where saltier water sinks below the surface while evaporation draws solid salt to the surface.
This snapshot shows a numerical simulation of the subsurface convection and surface evaporation that lead to salt polygon formation. Low salinity areas are yellow, while high salinity ones are black. At the surface, blue regions have the maximum upward flow and red regions have the maximum downward flow. The dark, highly saline fingers under the surface align to the red areas on the surface, indicating areas where salty water is sinking.
It’s a beautiful result that matches the size, shape, and development time observed for salt polygons in the real world. The team even excavated below salt polygons in Death Valley to confirm that the salt content below ground matched their model’s patterns. Since salt playas are a major source for dust and aerosols that affect climate, their work will be an important factor in future climate modelling. (Image credit: feature – T. Nevidoma, simulation – J. Lasser et al.; research credit: J. Lasser et al.; via APS Physics; submitted by Kam-Yung Soh)
April 18, 2023
Soap Film Ruptures
Soap film ruptures are well understood for your typical bubble solution, but what happens when tiny particles get added to the soap film? That’s the question in this recent study. Researchers added 660-nanometer particles, in varying amounts, to their soap films to see how it affected rupture. When they broke the films just after formation (top image), they found results that were quite similar to the usual, particle-free case. But when the films sat for awhile before breaking spontaneously (bottom image), the rupture caused wrinkling and folding similar to a piece of fabric. The researchers hypothesize that aging allowed the soap film to thin until the film and the particles were similar in size. Then, when the film ruptured, the particles affected how it broke up. (Image and research credit: P. Shah et al.)
After aging and thinning, a colloidal film ruptures spontaneously, forming fabric-like wrinkles.
April 11, 2023
Curved Cracks
When mixtures of particles and fluids dry, they typically leave a pattern of straight cracks. Here researchers explore what happens when the drying film contains bacteria from the family E. coli. Instead of straight cracks, the films form curved ones. With bacteria that rotate or tumble, the crack pattern is spiral-like. With bacteria that swim, the remaining pattern consists of circular cracks. Thus, the motility of the bacteria affects how cracks form and spread. (Image and research credit: Z. Liu et al.)
April 6, 2023
Superradiance in Fluids
A group of excited atoms can collectively emit more photons than they could individually in a phenomenon known as superradiance. Now researchers have shown that vibrating fluids can produce superradiance as well.
Two different wave fields used in the experiment, each with a different distance between the circular cavities.
Similar to other hydrodynamic quantum analogs, the researchers vertically vibrated a pool of liquid at a frequency that produced Faraday waves. Beneath the pool, they placed two circular wells, varying the distance between them to observe how their wave fields interacted. With a large enough vibration, the two circular wells emitted droplets (top image), and the number of droplets they produced was higher than expected for two independent wells, indicating superradiance. The results suggest that it may be possible to build even more hydrodynamic analogs of quantum systems than previously thought! (Image and research credit: V. Frumkin et al.; via APS Physics)
April 4, 2023
A Game of Toss
Over the past few years, we’ve seen lots of droplets bouncing and walking on waves. But today’s example is a little different. In this set-up, the wave is a large standing wave that sloshes from side-to-side in a narrow container. As it does, the wave catches and tosses a large ~3mm water droplet. The system is surprisingly stable, with this game of catch lasting for tens of thousands of cycles and up to 90 minutes before the droplet coalesces. The researchers found that, if the droplet tries to wander from its spot, the oscillating surface wave corrects it, guiding the droplet back to the optimal position. (Image and research credit: C. Sandivari et al.; via APS Physics; submitted by Kam-Yung Soh)
March 30, 2023
Breaking Clots With Sound
Clots that block blood flow away from the brain are one of the most common causes of strokes for younger people. If caught early, anticoagulants can sometimes resolve the issue, but the treatment fails in 20-40% of cases. Now researchers are investigating a new ultrasound technique capable of quickly and effectively removing these clots.
An illustration of the vortex ultrasound technique breaking up a blood clot.
The group attached a 2 x 2 array of ultrasound transducers to the tip of a catheter like those doctors feed through blood vessels in other interventions. The offset between each ultrasound transducer creates a vortex-like flow when the array is activated. The helical wavefront creates shear stress along the clot’s face, helping to break it up faster. In one test, the new technique broke up a clot and completely restored flow in just 8 minutes. Pharmaceutical treatments take at least 15 hours and average closer to 29 hours.
The team is moving forward to animal trials next and hope, with success there, to bring the technique to clinical studies. (Image credit: abstract – C. Josh, illustration – X. Jiang and C. Shi; research credit: B. Zhang et al.; via Physics World)
March 28, 2023
Rocket-Like Supercooled Drops
Many droplets can self-propel, often through the Leidenfrost effect and evaporation. But now researchers have observed freezing droplets that self-propel, too. The discovery came when observing the freezing of supercooled water drops inside a vacuum chamber. The researchers kept losing track of drops that seemingly disappeared. Upon closer inspection, though, they found that the drops weren’t shattering; they were flying away as they froze.
Inside a drop, freezing starts at a point, the nucleation point, and spreads from there. But the nucleation point isn’t always at the center of the drop. This asymmetry, the researchers found, is at the heart of the drop’s propulsion. When ice nucleates, the phase change releases heat that increases the drop’s evaporation rate, which can impart momentum to the drop. For an off-center nucleation, that momentum is enough to send the drop shooting off at nearly 1 meter per second. (Image credit: SpaceX; research credit: C. Stan et al.; via APS Physics)
March 23, 2023
How Large Particles Get in Sea Spray
When bubbles burst at the ocean’s surface, they eject droplets that can carry high concentrations of contaminants like pollutants, viruses, and microplastics. Previous theories posited that only particles smaller than the microlayer surrounding the bubble could make their way into these drops, but new work shows otherwise.
As bubbles rise to the surface, they carry particles on their surface, collecting them to a concentration that’s even higher than the surrounding seawater. But which particles make it into the air depend on the details of what happens when the bubble pops. Previously, researchers assumed that the thin microlayer of fluid surrounding the bubble was uniform, but that turns out not to be the case. As the bubble pops, some regions of the microlayer stretch and thin, while others grow thicker. The thicker the microlayer, the larger the particles it can pull along. In their single-bubble experiments, the researchers found that 15- and 30-micrometer plastic beads — representing oceanic microplastics — appeared in high concentrations in ejected droplets.
This animated simulation shows how fluid along the edge of a bubble makes its way into ejected droplets. Green particles indicate fluid from the left half of the bubble; blue shows fluid from the right side.
Environmental scientists are keen to understand these mechanisms because they link our oceans and atmosphere, potentially affecting rainfall, pollution spread, and epidemiology. (Image, video, and research credit: L. Dubitsky et al.; via APS Physics)
March 21, 2023
Instabilities on Instabilities
The world of fluid instabilities is a rich one. Combine fluids with differing viscosities, densities, or flow speeds and they’ll often break down in picturesque and predictable manners. Here, researchers explore the Rayleigh-Taylor instability (RTI), which occurs when a denser fluid sits above a less dense one (in a gravitational field). It’s an extremely common instability, showing up in both the cream in your ice coffee and the shape of a supernova’s explosion. It’s very difficult to set up and observe, though, which is where the real cleverness of this experiment stands out.
To study the RTI, these researchers first created another instability, the Saffman-Taylor instability. They filled the space between two glass plates with a viscous fluid, then injected a less viscous one. That created the distinctive viscous fingering pattern seen in the top image. In addition to being less viscous, the injected fluid was also less dense. As it pushed into the original fluid, it displaced some of it, creating a three-layer structure with dense fluid over less-dense fluid over dense fluid. That laid the groundwork for the Rayleigh-Taylor instability form.
A side-view through the fluid mixture shows the characteristic mushroom-like plume of the Rayleigh-Taylor instability.
Check out the cell-like pattern distributed across the fluid in the top image. These are plumes formed in the RTI as dense fluid sinks into the less-dense fluid below it. From the side (see second image), each plume takes on the distinctive mushroom-like shape of a Rayleigh-Taylor instability. Given time, the two fluids mix and the cellular pattern disappears. But until then, this set-up uses one instability to study a second one. How cool is that?! (Image and research credit: S. Alqatari et al., see also)
March 16, 2023