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)
Search results for: “shear”
Dendritic Painting Physics
In the art of Akiko Nakayama, colors branch and split in a tree-like pattern. In studying the process, researchers found the physics intersected art, soft matter mechanics, and statistical physics. In dendritic painting, the process starts with an underlying layer of acrylic paint, diluted with water. Atop this wet layer, you place a drop of acrylic ink mixed with isopropyl alcohol.
The combination of both layers is key. The alcohol-acrylic drop on a Newtonian substrate will show spreading, driven by Marangoni forces, but no branching. It’s the slightly shear-thinning nature of the diluted acrylic paint substrate that allows dendrites to form. As the overlying drop expands, it shears the underlayer, changing its viscosity and allowing the branches to form. You can see video of the process here. (Image credit: A. Nakayama; research credit: S. Chan and E. Fried; via Physics World)
Tornadoes in a Bucket
In nature, some powerful tornadoes form additional tornadoes within their shear layer. These subvortices revolve around the main tornado, causing massive destruction in their wake. In the laboratory, researchers create a similar multi-tornado system with a spinning disk at the bottom of a shallow, cylindrical layer of water. Depending on how fast the disk spins, different numbers of subvortices form around the main vortex.
In this poster, researchers show the transition from a 3-subvortex system to a 2-subvortex one. Starting at the 12 o’clock position and moving clockwise, we see 3 subvortices arranged in a triangle. A sudden change in the disk’s rotation speed destabilizes the system, causing the subvortices to break down and shift into a new 2-subvortex configuration. As this happens, material that was isolated in each subvortex (darker blue regions) is suddenly able to mix. That suggests that a real-world multiple vortex tornado might suddenly shed debris if it lost enough angular momentum. Back in the lab, though, the shift to a stable 2-subvortex system once again isolates material in individual subvortices and prevents it from mixing with the rest of the flow. (Image and research credit: G. Di Labbio et al. 1, 2)
Vortex Below
When a drop of ethanol lands on a pool of water, surface tension forces draw it into a fast-spreading film. Evenly-spaced plumes form at the edges of the film, then the film stops spreading and instead retracts. All of this takes place in about 0.6 seconds. But, as the image above shows, there’s more that goes on beneath the surface. A vortex ring forms and spreads under the film, driven by the shear layer under the edge of the plumes. Here, the vortex ring is visible in the swirling particles near the water surface. (Image and research credit: A. Pant and B. Puthenveettil)
Fire in Ice
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)
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.)
