As tides and waves flow back and forth over a beach, they erode the sandy shore. Here photographer Michael Shainblum captures the streaks and rivulets left by a falling tide. These “ruins” resemble an extensive river delta viewed from above. I love the complicated branches carved by the water’s retreat. (Image credit: M. Shainblum)
Year: 2021
Modelling Volcanic Bombs
When magma meets water on its journey to the surface, the two form a large, partially molten chunk known as a volcanic bomb. As you would expect from their name, these bombs can often be explosive, either in the air or upon impact. But a surprising number of these bombs never explode. Since catching volcanic bombs in action is far too dangerous, researchers modeled them instead to determine what makes a dud.
The type of volcanic bomb they were most interested in comes from Surtseyan eruptions, where the bombs travel through shallow sea or lake water, collecting moisture along the way. When the water reaches the molten interior of the volcanic bomb, it flashes into steam. That’s where the pressure to explode the bombs comes from. But the team found that the bombs are also extremely porous, thanks to bubbles created as the magma depressurizes on its trip to the surface. If the bomb is porous enough, steam escapes the rock before it can build to explosive pressures. (Image credit: top – NASA, others – E. Greenbank et al.; research credit: E. Greenbank et al.; via NYTimes; submitted by Kam-Yung Soh)
The Acoustics of Stonehenge
Stonehenge has long been an astronomical wonder, but did you know it’s an aural wonder as well? A team of acoustic engineers and an archaeologist constructed and tested a 1:12 scale model of the monument as it existed around 2200 B.C. Their model included 157 3D-printed stones (which took about nine months to print!), carefully engineered to reflect ultrasonic frequencies the way the full-size Stonehenge reflects frequencies in our auditory range. (Using the higher frequency sound at a smaller physical scale allows engineers to match the physics of the real henge.)
The team found that the stones of the henge amplified sound by about 4 decibels, enough to make a speaker’s voice easy to hear, even when facing a different direction. The structure also provided some reverberation that would enhance musical instruments or singing. Stonehenge had reverberation levels similar to a modern-day large movie theater, which is absolutely incredible for a prehistoric structure constructed in the open air.
For more interesting details on the model’s construction and testing, check out this article at Physics Today. (Image and research credit: T. Cox et al.)
Filming a Calving Glacier
The San Rafael Glacier, one of the fastest calving glaciers in the world, sits above a fjord in Patagonia. About 10 – 25 meters of the glacier is lost to calving every day. Here, filmmakers take you behind-the-scenes to show what it takes to film in such a remote, unpredictable, and dangerous environment. (Image and video credit: BBC Earth)
Fractal Frost
As nightly temperatures drop in the northern latitudes, many of us are beginning to wake up to frosty patterns on leaves, windows, and cars. Frost‘s spread is a complex dance between evaporation and nucleation, as seen in this recent study.
Here, researchers watched frost grow on a surface covered in 30-micrometer-wide micropillars. The pillars serve as anchor points for droplets, making frosting easier to observe. At low humidity levels (Image 1), droplets evaporate so quickly that frost regions remain isolated and do not interact. At high humidity levels (Image 3), on the other hand, the droplets evaporate so slowly that they’re able to poach water vapor from their neighbors to form frost spikes. When a spike touches another droplet, it freezes the region almost instantly. As a result, the frost spreads quickly and covers nearly every part of the surface. At intermediate humidity levels (Image 2), though, this frost chain reaction and evaporation compete, causing the frost to grow in fractals. (Image and research credit: L. Hauer et al.; via APS Physics)
Witch’s Broom
Known by many names — including the Witch’s Broom Nebula — NGC 6960 is part of a supernova remnant visible in the constellation Cygnus. The wisp-like filaments of the nebula are shock waves moving through the cloud of dust and ionized gas. Based on observations using the Hubble Space Telescope, the nebula is expanding at around 1.5 million kilometers per hour. When the original supernova exploded thousands of years ago, astrophysicists estimate it would have been bright enough to see during daytime! (Image credit: K. Crawford)
Solid, Liquid, Both?
Materials like oobleck — a suspension of cornstarch particles in water — are tough to classify. In some circumstances, they behave like a fluid, but in others, they act like a solid. Here researchers sandwiched a thin layer of oobleck between glass plates and injected air into the mixture. For a fluid, this setup creates a classic Saffman-Taylor instability where rounded fingers of air push their way into the more viscous fluid. And, indeed, for low air pressures and low concentrations of cornstarch, the oobleck forms these viscous fingers. You can see examples in the top row’s first and third image, the second row’s middle image, and the bottom row’s third image.
Injecting air at high pressures and high cornstarch concentrations fractures the oobleck like a solid (middle row, first and third images). At intermediate pressures and concentrations, the oobleck forms a pattern called dendritic fracturing, where new branches can grow perpendicularly to their parent branch. Examples of this pattern are in the top row’s second image and the bottom row’s first and second images. (Image and research credit: D. Ozturk et al.; via Physics Today)
Seeking Magma
In 2009, drillers seeking geothermal energy in Iceland accidentally pierced a hidden magma chamber. After a billowing pillar of steam and glass shards poured out from the hole, it created the hottest geothermal well ever, until the casing failed. Now drillers are preparing to return to the area, this time with the intention of reaching magma. Capturing a sample of magma before it rises to the surface (thereby losing its trapped gases) is something of a holy grail for geophysicists, who otherwise rely on seismic wave detections and observations of magma that’s reached the surface. Building a long-term magma observatory will be an enormous engineering challenge, but the technologies developed may help us explore other hellish environments like the surface of Venus. (Image credit: G. Fridleifsson/IDDP; via Science)
Superfluid Instabilities
Superfluids — like Bose-Einstein condensates — are bizarre compared to fluids from our everyday experience because they have no viscosity. Without viscosity, it’s no surprise that they behave in unusual ways. Here, researchers simulated superfluids moving past one another. In each of these images, the blue fluid is moving to the left, and the red fluid is moving to the right. In a typical fluid, such motion causes ocean-wave-like curls due to the Kelvin-Helmholtz instability.
Instead, with a low relative velocity and high repulsion between atoms of the two layers, the superfluids form a tilted, finger-like interface (Image 1) that the authors refer to as a flutter-finger pattern. (Repulsion essentially sets the miscibility between the superfluids. With a high repulsion, the superfluids resist mixing.)
With a higher relative velocity (Image 2), the wavelength of the ripples becomes comparable to the thickness of the interface, and the superfluids take on a very different, zipper-like pattern. Note how the tips detach and reconnect to the neighboring finger!
With lower repulsion, the interface between the two liquids is thicker and breaks down quickly (Image 3). The authors call this a sealskin pattern. (Image credits: water – M. Blažević, simulations – H. Kokubo et al.; research credit: H. Kokubo et al.; via APS Physics)
RC Ground Effect Plane
The ekranoplan was a massive, Soviet-era aircraft that relied on ground effect to stay aloft. In this video, RC pilots test out their own homemade version of the craft, including some neat flow visualization of the wingtip vortices. When an aircraft (or, for that matter, a bird) flies near the ground, it experiences less drag than at higher altitudes. This happens primarily because of the ground’s effect on wingtip vortices.
In normal flight, the vortices from an aircraft’s wingtips create a downwash that reduces the wing’s overall lift. But in ground effect, the vortices cannot drift downward as they normally do. Instead, they spread apart from one another, thereby reducing the drag caused by downwash from the aircraft. The end result is better performance, though it comes with added risk since there’s very little time to correct an error when flying at an altitude less than half the aircraft’s wingspan. (Video and image credit: rctestflight; submitted by Simplicator)
