Since Harold Edgerton’s experiments with stroboscopic photographs in the 1930s, we’ve been fascinated by the shape of splashes. These days students and artists can take advantage of programmable external flashes to capture this split-second moment of impact. Here, a pink-dyed drop of ethanol strikes a jet rising from a pool of glycerin, milk, and food coloring. The resulting splash is umbrella-like, with a thickened rim that shows tiny ligaments of fluid — an early sign of the instability that will ultimately detach droplets from the splash. This image was taken by students in a course that connects art and fluid mechanics. (Image credit: L. Sharpe et al.; via Physics Today)
Tag: instability
A Shallow Origin for the Sun’s Magnetic Field
The Sun‘s complex magnetic field drives its 11-year solar activity cycle in ways we have yet to understand. During active periods, more sunspots appear, along with roiling flows within the Sun that scientists track through helioseismology. Longstanding theories posit that the Sun’s magnetic field has a deep origin, about 210,000 kilometers below the surface. But new measurements have prompted an alternate theory: that the Sun’s magnetic field originates in its outer 5-10% due to a magnetorotational instability.
Magnetorotational instabilities are usually associated with the accretion disks around black holes and other massive objects. When an electrically-conductive fluid — like the Sun’s plasma — is rotating, even a small deviation in its path can get magnified by a magnetic field. In accretion disks, these little disruptions grow until the disk becomes turbulent.
By applying this idea to the sun, researchers found they were better able to match measurements of the plasma flows beneath the Sun’s surface. With measurements from future heliophysics missions, they believe they can work out the mechanisms driving sunspot formation, which would help us better predict solar storms that can damage electronics here on Earth. (Image credit: NASA/SDO/AIA/LMSAL; research credit: G. Vasil et al.; via Physics World)
Viscous Fireworks
Inject a less viscous fluid into a gap filled with a more viscous fluid, and you’ll get finger-like patterns spreading radially. Here, researchers put a twist on this viscous fingering by taking turns injecting different liquids. Each injection cycle disrupts what came before, layering fingering patterns on fingering patterns. The results resemble fireworks. Happy 4th of July! (Image credit: C. Chou et al.)
Dripping Viscoelastics
An ultrasoft viscoelastic fluid drips in this research poster from the Gallery of Soft Matter. Complex materials like this one have stretchy, elastic behaviors typical of a solid along with the flowing, viscous properties of a fluid. Here, gravity overcomes the material’s elasticity, leaving it to sag and flow. As that happens, the fluid must slide past air, and the density difference between the two fluids creates the small distortions seen on the liquid sheet. This is an example of a Rayleigh-Taylor instability. (Image credit: J. Hwang et al.)
Searching for Stability in Cleaner Flames
Spiking natural gas power plants with hydrogen could help them burn cleaner as we transition away from carbon power. But burners in power plants and jet engines can be extremely finicky, thanks to thermoacoustic instabilities. As a flame burns, it can sputter and fluctuate in its heat output. That creates pressure oscillations (which we sometimes hear as sound waves) that reflect off the burner’s walls and return toward the flame, causing further fluctuations. This feedback loop can be destructive enough to explode combustion chambers.
Adding hydrogen to a burner designed purely for natural gas can trigger these instabilities (above image), but researchers hope that by exploring fuel-mixtures and their effect at lab-scale, they can help designers find safe ways to adapt industrial burners for the cleaner fuel mixture. (Image and research credit: B. Ahn et al.; via APS Physics)
Exciting a Flame in a Trough
A viewer sent Steve Mould his accidental discovery of this odd flame behavior. In these 3D-printed troughs, a flame lit in lighter fluid will rocket around the track repeatedly as it burns the local supply of gaseous lighter fluid. As Steve shows in his video, this system is an excitable medium and the trick works for a whole array of 3D-printed shapes. Check out the full video above. (Video and image credit: S. Mould)
Kelvin-Helmholtz and the Sun
Kelvin-Helmholtz instabilities (KHI) are a favorite among fluid dynamicists. They resemble the curls of a breaking ocean wave — not a coincidence, since KHI create those ocean waves to begin with — and show up in picturesque clouds, Martian lava coils, and Jovian cloud bands. The instability occurs when two layers of fluid move at different speeds and the friction between them causes wrinkles that grow into waves.
Scientists have long suspected that KHI could occur in solar phenomena, too, like the coronal mass ejections that drive space weather. The Parker Solar Probe, a spacecraft designed to explore the sun, caught evidence of a series of turbulent eddies during a 2021 coronal mass ejection, and a recent study of those observations shows that the series of vortices are consistent with KHI. Put simply, the team found that the features are spaced and aligned as we’d expect for KHI and, during the probe’s measurements, the features grew at the rate Kelvin-Helmholtz eddies would. Although the instability itself may be common in the sun’s corona, it’s unlikely that we’ll see it often, simply because conditions need to be just right for them to be visible. (Image credit: NASA/Johns Hopkins APL/NRL/Guillermo Stenborg and Evangelos Paouris; research credit: E. Paouris et al.; via Gizmodo)
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“Bulging Balloons”
This planet-like balloon started out as two elastomer sheets, heat-sealed together into a spiraling tube. As the balloon was inflated, it changed from flat to a saddle-like shape. With more air, the pressure inside increased, triggering an instability that caused the middle of the balloon to bulge. As inflation continued, the central bulge expanded, unbonding layer after layer of the seal. Even late in inflation, the balloon maintains hints of its original shape in the form of a ring around the Jovian bulge in the middle. (Image credit: N. Vani et al.)
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Supernova Rings
Some 20,000 years ago, a massive star blew off a ring of dust and gas that expanded into the surrounding interstellar medium. Later, in 1987, the star exploded as supernova 1987A. That explosion lit the surrounding area, revealing a clumpy ring astronomers have struggled to explain. But a new team believes they have a fluid dynamical answer: the Crow instability.
Closer to home, we see the Crow instability when an airplane’s contrails break up. It happens when two vortices that rotate in opposite directions are close to one another. Any wobble in one vortex is enhanced by the influence of its neighbor. Eventually, this breaks the original vortices apart and causes them to reform as a series of smaller vortex rings.
A comparison between an image of SN 1987A and an illustration of the vortex ring interaction thought to create that shape. In the case of supernova 1987A, the researchers propose that the star originally blew off two vortex rings that, due to their mutual influence, broke down into a clumpy ring of vortices. (Image credits: NASA/ESA/CSA/M. Matsuura/R. Arendt/C. Fransson and NASA/ESA/A. Angelich + M. Wadas et al.; research credit: M. Wadas et al.; via APS Physics)
Evolving Fingers
If you sandwich a viscous fluid between two plates and inject a less viscous fluid, you’ll get viscous fingers that spread and split as they grow. This research poster depicts that situation with a slight twist: the viscous fluid (transparent in the image) is shear-thinning. That means its viscosity drops when it’s deformed. In this situation, the fingers formed by the injected (blue) fluid start out the way we’d expect: splitting as they grow (inner portion of the composite image). But then, the tip-splitting stops and the fingers instead elongate into spikes (middle ring). Eventually, as the outer fluid’s viscosity drops further, the fingers round out and spread without splitting (outer arc of the image). (Image credit: E. Dakov et al.; via GoSM)
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