This false-color satellite image of Malaspina Glacier (Sít’ Tlein) is a riot of color. Composed of coastal/aerosol, near infrared, and shortwave infrared bands from Landsat 9, the colors highlight features otherwise hard to identify. Watery features appear in reds, oranges, and yellows; vegetation is green and rock appears in blue. The glacier covers more than 4000 square kilometers, an area larger than the state of Rhode Island. The dark lines atop the glacier are moraines, where rock, soil, and other debris has been scraped up along the glacier’s edge. Over time, changes in the glacier’s velocity cause the moraines to fold and shear, creating the zigzag pattern seen here. (Image credit: W. Liang; via NASA Earth Observatory)
Search results for: “shear”
Imitating a Cough
Coughing and sneezing create violent air flows in and around our bodies. As that fast air rushes over mucus layers in our lungs, throat, and sinuses, the resulting flow breaks up the mucus into droplets. To explore the details of that process, researchers built a “cough machine” that sends a rush of air over a thin film of water mixed with glycerol. The setup allows them to observe the physics in a way that’s nearly impossible in a human cough or sneeze.
Imitating a cough: high-speed video shows how a thin film made of water and glycerol breaks down in a strong airflow. Parts of the film inflate into hollow bags that form thinner weak spots. When the film breaks in those places, it forms rims and ligaments that create a spray of droplets. As seen above, air flowing past shears the viscous fluid, stretching it out. The leading edge of the film destabilizes and breaks into large drops, but it’s what comes next that really gets things going. Areas of the film inflate to form hollow bags. When sections of the bag thin to about 1 micron, the film ruptures and the bags burst. This triggers a cascade of instabilities in the film’s rim that ultimately rip the film into a spray of tiny aerosol droplets. The researchers found that, despite their tiny size, these droplets collectively carry a large volume of liquid, making them all the more important for understanding transmission of respiratory illnesses. (Image credit: top – A. Piacquadio, experiment – P. Kant et al.; research credit: P. Kant et al.)
Studying Earth’s Interior
The Earth’s interior is almost entirely inaccessible to humanity, so how do we know what it consists of? As explained in this video, our knowledge of the planet’s interior is based on measuring waves sent out by earthquakes and nuclear blasts. Both produce two kinds of waves — pressure waves (P-waves) and shear waves (S-waves) that travel through the earth and get picked up by seismometers. Scientists noticed that pressure waves travel through the center of the planet while shear waves — which get dissipated in liquids — do not. This led them to conclude that part of Earth’s interior is a liquid. The idea of a solid inner core came from observations of pressure waves scattering in a way that only made sense if they’d hit something solid. (Video and image credit: Science)
How Squall Lines Form
Summertime in the middle U.S. means thunderstorms, many of which can form long lines of storms known as squall lines. Complex convective dynamics feed such storms. Here is an illustration of one part of a squall’s lifecycle:
As rain falls and evaporates, it fuels the formation of a cold pool of air below the cloud. Incoming wind (gray arrows) blocks the cold pool from spreading. In turn, the cold pool acts as a ramp that redirects this warm, moist air upward. The vertical variation in wind speed (wind shear, shown with pink arrows) creates a positive vorticity. Together with the negative vorticity in the cold pool, this induces a vorticity dipole that lifts air and moisture, feeding the growing line of storms. As it falls, rain evaporates, cooling air near the ground and forming a cold pool. If incoming winds block the cold pool from spreading, the pool will act instead as a ramp that redirects the wind upward, carrying any warmth and moisture up into the storm cloud. Wind shear — a vertical variation in wind strength with altitude — creates positve vorticity that opposes the negative vorticity inherent to the cold pool. Together these two regions of opposing vorticity lift more air and moisture into the squall, generating more clouds and more rainfall. (Image credit: top – J. Witkowski, illustration – C. Muller and S. Abramian; see also C. Muller and S. Abramian)
Bubble Cleaning
Removing dirt and bacteria from fruits and vegetables is a delicate job; too much force can bruise the produce and hasten spoiling. That’s why fluid mechanicians want to give the job to bubbles. Placing objects in a stream of air bubbles inside a bath is a surprisingly effective method for gently cleaning surfaces. A recent study finds that 22.5 degrees is the optimal angle for sliding bubbles to scrape a surface clean.
As the bubbles slide past the surface, they exert a shear force that scrapes away debris, just as you might use a loofah in the shower. The angle the bubble makes with the surface determines how long it’s in contact and how much force the bubble exerts. Increasing the angle makes the bubble slide faster, increasing its shear force. But above 22.5 degrees, the bubble’s buoyancy means that it spends less time pressed against the surface, which decreases its cleaning ability.
The team hopes to use their results to build a “fruit Jacuzzi” device that will direct bubble streams to gently and effectively clean fruits and vegetables in a matter of minutes. (Image and research credit: A. Hooshanginejad et al.; via APS Physics)
A Toad’s Sticky Saliva
Frogs and toads shoot out their tongues to capture and envelop their prey in a fraction of a second. They owe their success in this area to two features: the squishiness of their tongues and the stickiness of their saliva. The super squishy toad tongue deforms to touch as much of the insect as possible. That shape-changing helps deliver the saliva, which is an impressively fast-acting, shear-thinning fluid. Under normal circumstances, the saliva is sticky and about as viscous as honey. But the shear from the tongue’s impact makes the saliva flow like water, spreading across the insect’s body. Then it morphs back into its viscous, sticky self, providing enough adhesive power that the insect can’t escape the toad pulling its tongue back in. (Video credit: Deep Look/KQED; research credit: A. Noel et al.)
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)
A Look at Hagfish
Hagfish are the lords of slime. Their viscoelastic protection mechanism is so effective that they’ve hardly changed up their game in the past 300 million years. Instead, at the first sign of trouble, they release a mucus that rapidly expands in salt water. When attacking fish try to pull water into their gills, they get clogged with slime instead, sometimes suffocating and becoming the hagfish’s meal instead. To get out of their slime, hagfish knot themselves and wipe it away, thanks to its shear-thinning properties. (Image and video credit: Deep Look)
Exascale Simulations
Capturing what goes on inside a combustion engine is incredibly difficult. It’s a problem that depends on turbulent flow, chemistry, heat transfer, and more. To represent all of those aspects in a numerical simulation requires enormous computational resources. It’s not simply the realm of a supercomputer; it requires some of the fastest supercomputers in existence.
Exascale computing, like that used for the simulations in this video, is defined as at least 10^18 (floating-point) operations per second. For comparison, my PC has a recent, high-end graphics card, and it’s about a million times slower than that. These are absolutely gigantic simulations. (Image and video credit: N. Wimer et al.)
Slab Avalanche Physics
Slab avalanches like the one shown here begin after weak, porous layers of snow get buried by fresher, more cohesive snow layers. On a steep slope, the weight of the new snow can be too great for friction to hold the slab in place, causing the upper layer to crack and slide at speeds up to 150 meters per second. Scientists had two competing theories for how slab avalanches began. One theory presumed that the weak layer of snow failed under shear; the other argued that the collapse of the lower, porous layer was at fault.
In a new study combining large-scale numerical simulation with real-life observations, scientists came to a new conclusion: cracks began to form in the porous layer as the weight of heavier snow crushed down, but once the cracks formed, the shear mechanism took over. Cracks formed by shear could propagate along the existing cracks in the porous layer, allowing faster crack propagation than through undamaged snow. In the end, it’s the combination of the two mechanisms that triggers the avalanche. (Image credit: R. Flück; research credit: B. Trottet et al.; via Physics World)