Coffee-making continues to be a rich source for physics insight. The roasting and brewing processes are fertile ground for chemistry, physics, and engineering. Recently, one research group has focused on the phenomenon of channeling, where water follows a preferred path through the coffee grounds rather than seeping uniformly through the grounds. Channeling reduces the amount of coffee extracted in the brew, which is both wasteful and results in a less flavorful cup. By uncovering what mechanics go into channeling, the group hopes to help baristas mitigate the undesirable process, creating a repeatable, efficient, and tasty espresso every time. (Image credit: E. Yavuz; via Ars Technica)
Category: Phenomena
Salt Fingers
Any time a fluid under gravity has areas of differing density, it convects. We’re used to thinking of this in terms of temperature — “hot air rises” — but temperature isn’t the only source of convection. Differences in concentration — like salinity in water — cause convection, too. This video shows a special, more complex case: what happens when there are two sources of density gradient, each of which diffuses at a different rate.
The classic example of this occurs in the ocean, where colder fresher water meets warmer, saltier water (and vice versa). Cold water tends to sink. So does saltier water. But since temperature and salinity move at different speeds, their competing convection takes on a shape that resembles dancing, finger-like plumes as seen here. (Video and image credit: M. Mohaghar et al.)
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Winter in Chicago
Fresh winter snow blankets Chicago in this satellite image. Over on Lake Michigan, ice dots the coastline out to about 20 kilometers from shore. Darker regions near land mark thinner ice being pushed outward by the wind. Further out, the ice appears white and may be thicker thanks to wind-driven ice piling up. (Image credit: M. Garrison; via NASA Earth Observatory)
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Chaotic Hose Instability
Steve Mould is back with another video looking at wild fluid behaviors. This time he’s considering hose instabilities like the one that makes a water-carrying hose beyond a certain length to whip wildly back and forth. He tries to track down the reasoning for these flexible hoses snapping and whipping. In truth, both the hoses and the wind dancers do their thing due to interactions between the elasticity of the hose and the fluid dynamics of the flows within. These applications are ripe for a few control volume thought experiments. (Video and image credit: S. Mould)
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Twisting in the Flow
What happens to liquid crystals in a flow? In this video, researchers look at liquid crystals flowing through the narrow gap of a microfluidic device. Initially, all the crystals are oriented the same way, as if they are logs rolling down a river. But as the flow rate increases, narrow lines appear in the flow, followed by disordered regions, and, eventually, a new configuration: vertical bands streaking the left-to-right flow. The colors, in this case, indicate the orientation of the liquid crystals. As the researchers show, the crystals collectively twist to form the spontaneous bands. (Video and image credit: D. Jia and I. Bischofberger)
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Growing Ice
While much attention is given to the summer loss of sea ice, the birth of new ice in the fall is also critical. Ice loss in the summer leaves oceans warmer and waves larger since wind can blow across longer open stretches. Those warmer waters and more dynamic waves affect how ice forms once autumn sets in. Higher waves mean that ice tends to form in “pancakes” like those seen here. Pancake ice is small — typically under 1 meter wide — and can only be observed from nearby, since they’re smaller than typical satellite resolution. Only once there’s enough pancake ice to dampen the waves will the pieces begin to cement together to form larger pieces that will form the basis of the year’s new ice. (Image credit: M. Smith; see also Eos)
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A Stellar Look at NGC 602
The young star cluster NGC 602 sits some 200,000 light years away in the Small Magellanic Cloud. Seen here in near- and mid-infrared, the cluster is a glowing cradle of star forming conditions similar to the early universe. A large nebula, made up of multicolored dust and gas, surrounds the star cluster. Its dusty finger-like pillars could be an example of Rayleigh-Taylor instabilities or plumes shaped by energetic stellar jets. (Image credit: NASA/ESA/CSA/JWST; via Colossal)
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Imaging a New Era of Supersonic Travel
Supersonic commercial travel was briefly possible in the twentieth century when the Concorde flew. But the window-rattling sonic boom of that aircraft made governments restrict supersonic travel over land. Now a new generation of aviation companies are revisiting the concept of supersonic commercial travel with technologies that help dampen the irritating effects of a plane’s shock waves.
One such company, Boom Supersonic, partnered with NASA to capture the above schlieren image of their experimental XB-1 aircraft in flight. The diagonal lines spreading from the nose, wings, and tail of the aircraft mark shock waves. It’s those shock waves’ interactions with people and buildings on the ground that causes problems. But the XB-1 is testing out scalable methods for producing weaker shock waves that dissipate before reaching people down below, thus reducing the biggest source of complaints about supersonic flight over land. (Image credit: Boom Supersonic/NASA; via Quartz)
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Salt Affects Particle Spreading
Microplastics are proliferating in our oceans (and everywhere else). This video takes a look at how salt and salinity gradients could affect the way plastics move. The researchers begin with a liquid bath sandwiched between a bed of magnets and electrodes. Using Lorentz forcing, they create an essentially 2D flow field that is ordered or chaotic, depending on the magnets’ configuration. Although it’s driven very differently, the flow field resembles the way the upper layer of the ocean moves and mixes.
The researchers then introduce colloids (particles that act as an analog for microplastics) and a bit of salt. Depending on the salinity gradient in the bath, the colloids can be attracted to one another or repelled. As the team shows, the resulting spread of colloids depends strongly on these salinity conditions, suggesting that microplastics, too, could see stronger dispersion or trapping depending on salinity changes. (Video and image credit: M. Alipour et al.)
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Seeing Sound
Sound, vibration, and motion are all inextricably linked. In this BBC video, physicist Helen Czerski shows how an object’s sound and vibrations relate through the classic Chladni experiment. She vibrates a metal plate scattered with sand. At most vibration frequencies, the particles of sand bounce all over the place with no distinctive pattern. But at an object’s natural frequencies, there are standing waves and the sand gathers in spots where the standing wave has no vertical motion. The higher the vibration frequency, the more complex the pattern the sand makes. All of this plays into the sounds we hear, too. When struck, an object vibrates at many of its natural frequencies at once. That’s what gives us a rich, musical tone — all those layered frequencies. (Video and image credit: BBC)
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