Tag: shear stress

Quantifying Bioluminescence
Some single-celled organisms, like dinoflagellates, light up when disturbed. This bioluminescence is considered a defense mechanism, triggered by threats to the organism. Now researchers are quantifying just what it takes to light up a single dinoflagellate.
Dinoflagellates respond both to stress caused by the fluid flow around them and to mechanical deformation — in other words, getting poked. Both methods involve bending and stretching the dinoflagellate’s cell wall, which stretches calcium-ion channels connected to bioluminescence. The researchers found that the intensity of the light produced depended both on the amount and speed of cell wall deformation.
The model built from their observations should help scientists better understand what forces cause a specific response. That means dinoflagellates could be used as a non-invasive means of understanding fluid flow around swimmers like dolphins or sea lions! (Image and research credit: M. Jalaal et al.; via APS Physics)
July 30, 2020
Using Bubbles to Keep Clean
Keeping produce clean of foodborne pathogens is a serious issue, and delicate fruits and vegetables like tomatoes cannot withstand intense procedures like cavitation-based cleaning. But a new study suggests that simple air bubbles may have the power to keep our produce free of germs.
In particular, researchers studied air bubbles injected into water as they bounced and slid along an inclined solid surface. They found that as a bubble approaches a tilted surface, it squeezes a thin film of liquid between itself and the surface. That flow creates a shear stress that pushes contaminants like E. coli away from the point of impact. When the bubble bounces away, fluid gets sucked back into the void left behind, creating more shear stress. In their experiments and simulations, the team measured shear stresses greater than 300 Pa, more than double what’s needed to remove foodborne bacteria like Listeria. (Image credit: Pixabay; research credit: E. Esmaili et al.)
June 17, 2019
Amber Waves
When I was a teenager, I liked riding my bike along the river boardwalk near my house. There were fields there, like those in the image above and video below, with tall grass that would bend and sway in the wind. The long stalks undulated almost like a fluid, and they were mesmerizing. This video gives you a higher vantage point, where you can see the larger patterns of motion. What you’re seeing, I think, are some of the large-scale turbulent variations in the wind. Rather than being uniform and laminar, the wind contains pockets of turbulent gusts, which the sway of the long grass reveals to the naked eye. In terms of physical mechanism, I suspect it’s similar to how wind imprints its patterns on water. (Video and image credit: N. Moore)
February 6, 2019
Bioluminescent Plankton
In nutrient-rich marine waters, dinoflagellates, a type of plankton, can flourish. At night, these tiny organisms are responsible for incredible blue light displays in the water. The dinoflagellates produce two chemicals – luciferase and luciferin – that, when combined, produce a distinctive blue glow. The plankton use this as a defense against predators, creating a flash of blue light when triggered by the shear stress of something swimming nearby. The dinoflagellates respond to any sudden application of shear stress this way, so they glow not only for predators, but for any disturbance – mobula rays (above), sea lions, boats, or even just a hand splashing in the water. In person, the experience feels downright magical. I had the opportunity to experience bioluminescence in the Galapagos last year. The light from the dinoflagellates is incredibly difficult to film because it can be so dim, but as the BBC demonstrates, it’s well worth the effort it takes to capture. (Image credit: BBC from Blue Planet II and Attenborough’s Life That Glows; video credit: BBC Earth)
November 3, 2017
Unboiling an Egg
Cooking is something we think of as a one-way process. You add heat to food, it changes forms, and there’s undoing that. But that process is less one-directional than we thought, at least in some cases. Take boiling an egg. When you add heat to egg whites, it breaks down bonds between the folded proteins and lets those proteins build more bonds with other sections of proteins, eventually solidifying into a seemingly unbreakable mess. You can’t break those bonds by adding or removing thermal energy, but you can shake the proteins apart and refold them into their original shapes.
Researchers accomplish this by putting the boiled egg whites in a solution of water and urea and spinning them. When they spin the fluid mixture, the fluid near the wall spins faster than the fluid in the center of the vial, which creates shear stress. That shear stress helps untangle the proteins and reform them into their original shape–thereby unboiling the egg white. Now you definitely don’t want to eat the results – urea is, of course, a component of urine – but it does demonstrate that fluid dynamics can be used to reverse chemical processes we thought were irreversible. And that surprising discovery nabbed the researchers an Ig Nobel Prize in 2015. (Video credit: TedEd/E. Nelson; research credit: T. Yuan et al.)
February 7, 2017
Bioluminescent Plankton
The blue-outlined dolphins you see above get their glow from microorganisms called dinoflagellates. They are a type of bioluminescent plankton, shown in the lower image, that can be found in oceans around the world. Their glow comes from combining two chemicals: luciferase and luciferin. The dinoflagellates suspended in the ocean do this when they are disturbed–specifically, when the water around them transmits a shear stress above a certain threshold. Typically, this is caused by something larger–a potential predator–moving past, although it can also be stimulated by breaking waves. The higher the shear stress, the more intense the glow, but the dinoflagellates only use their bioluminescence sparingly. If you apply shear stress and keep applying it, their glow fades away without reactivating. After all, they can only produce so much chemical fuel. (Image credit: BBC from Attenborough’s Life That Glows; h/t to Gizmodo; research credit: E. Maldonado and M. Latz)
May 12, 2016
How Erosion Shapes a Flow
Erosion creates all manner of strange shapes as wind and water cut away at solids. But why does the interaction of the fluid and solid result in the geometries we observe? Above is a collage from an experiment in which a soft clay sphere was immersed in a water tunnel. After 70 minutes, the sphere had worn into a roughly conical body (Image A) reminiscent of a re-entry capsule. Images B and C show instantaneous streaklines around the clay at 10 minutes and 70 minutes, respectively. Images D and E show diagrams of the flowfield seen in B and C. Fast-moving flow above and below the stagnation point (SP) wears the front of the body into a conical shape, whereas the recirculating vortices aft of the separation point (SL) create a sloped shoulder and flattened back in the clay. The results are consistent with a model in which erosion tries to create uniform shear stress at the solid surface – essentially the process is keeping the frictional force between the fluid and air constant along the surface. This makes sense. If a region’s shear stress is higher, it will be worn more quickly than the surrounding solid, causing it to recede and experience decreased shear stress (relative to the surrounding area) as a result. (Image credit: L. Ristroph et al.)
October 30, 2013
Mixing While Laminar
Although turbulent flows are known for their mixing efficiency, in manufacturing there can often be a need to mix laminar fluid streams without the increased shear stress of a turbulent flow. This can be particularly important for polymeric liquids, where too much shear stress could damage the polymer chains. One possibility is using a static mixer, such as the one demonstrated in this video, which, when placed in pipe flow, will deflect the pipe’s contents in such a way as to produce efficient mixing over a short distance. Here two streams of high-viscosity epoxy are mixed through such a static mixer, hardened, and then ground to show the mixing at each level of the static mixer. (Video credit: Sulzer)
March 21, 2013
Inside a Blender
The fluid dynamics of a commercial-quality blender amount to a lot more than just stirring. Here high-speed video shows how the blender’s moving blades create a suction effect that pulls contents down through the middle of the blender, then flings them outward. This motion creates large shear stresses, which help break up the food, as well as turbulence that can mix it. But if you watch carefully, you’ll also see tiny bubbles spinning off the blades. These bubbles, formed by the pressure drop of fluid accelerated over the arms of the blades, are cavitation bubbles. When they collapse, or implode, they create localized shock waves that further break up the blender’s contents. This same effect is responsible for damage to boat propellers and lets you destroy glass bottles. (Video credit: ChefSteps; via Wired; submitted by jshoer)
March 14, 2013
When Fluids Behave Like Solids
Many common fluids–like air and water–are Newtonian fluids, meaning that stress in the fluid is linearly proportional to the rate at which the fluid is deformed. Viscosity is the constant that relates the stress and rate of strain, or deformation. The term non-Newtonian is used to describe any fluid whose properties do not follow this relationship; instead their viscosity is dependent on the rate of strain, viscoelasticity, or even changes with time. A neat common example of a non-Newtonian fluid is oobleck, a mixture of cornstarch and water that is shear-thickening, meaning that it is resistant to fast deformations. Like the cornstarch-based custard in the video above, these fluids react similarly to a solid when struck, resisting changing their shape, but if deformed slowly, they will flow in the manner of any liquid.
September 4, 2012