FYFD

Tag: oobleck

  • Slow to Relax

    Slow to Relax

    Oobleck is a decidedly weird substance. Made from a dense suspension of cornstarch in water, oobleck is known for its mix of liquid-like and solid-like properties, depending on the force that’s applied. In a recent study, researchers took a look at what happens when you really push oobleck to the extreme. When the force applied to oobleck is small or slowly added, the water between cornstarch particles helps keep the particles apart and free of contact. It’s when the force is large that those particles start jamming up against each other and having friction between them, and then the oobleck suddenly acts like a solid. But what happens once that force is removed?

    When the force is gone, we expect the particles to repel and for water to squeeze back into the spaces between them, breaking up the friction and allowing the oobleck to relax back to a liquid-like form. But the team found that sometimes the oobleck doesn’t relax as easily as expected; instead, it seems to retain some memory of its solid-like state, due to persisting friction between particles. (Image credit: T. Cox; research credit: J. Cho et al.)

  • Solid, Liquid, Both?

    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)

  • CU Flow Vis 2019

    CU Flow Vis 2019

    I love when science and art come together, which is why I’ve long been a fan of the Flow Vis course at CU Boulder. Some of my earliest posts on FYFD date from previous editions of the course. Here are a few of my favorite images from the Fall 2019 class, from the top:

    •  Ferrofluid and India ink merge in this colorful photo. A magnet underneath the mixture on the left side causes the dark spikes of ferrofluid, but without magnetic influence, the ink and ferrofluid form cell-like droplets.
    • Although it looks like a shower head, this is actually fluorescent oobleck dripping through a strainer. A relatively long exposure time means that it’s impossible to tell whether the oobleck is falling in a fluid stream or broken-up chunks.
    • These colorful water droplets are sitting on a hydrophobic surface, hence their extremely rounded edges. I particularly like how this makes each one like a little lens for the light shining through them and into their shadows.
    • A thin layer of ferrofluid reacts to the magnet beneath. Gotta love those little streaks left behind the flow.

    For those in the Front Range area, the Flow Vis class will be showcasing their work on Saturday, December 14th at the Fiske Planetarium. Snacks are at 4:30 pm and the show starts at 5 pm. For those not nearby, you can peruse the art from this semester and previous ones at your leisure online. (Image credits: colorful ferrofluid – R. Drevno; falling oobleck – A. Kumar; droplets – A. Barron; macro ferrofluid – A. Zetley)

  • Avoiding Shear Thickening

    Avoiding Shear Thickening

    Many substances – like the cornstarch and water mixture above – exhibit a property called shear-thickening. In these fluids, deforming them quickly causes the viscosity to increase dramatically. That shear-thickening occurs when particles inside the fluid jam together, creating large chains able to resist the force being applied. That’s why the oobleck on this vibrating speaker can sustain these “cornstarch monsters”.

    Shear-thickening is useful in many contexts, but it’s problematic during manufacturing, when pumping these substances can become incredibly difficult due to the fluid’s innate resistance to flowing. A new study, though, finds that it’s possible to temporarily suppress shear-thickening using acoustic waves. The researchers used piezoelectric devices to generate acoustic waves at a frequency around 1 MHz while shearing the cornstarch mixture. The acoustic waves disrupt the formation of particle chains inside the mixture, keeping its viscosity 10 times lower than during regular shear-thickening. (Image credit: bendhoward, source; research credit: P. Sehgal et al.; submitted by Brian K.)

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    Experimenting with Speakers

    In her ongoing quest to explore natural resonance, Dianna has enlisted some very nice, very expensive speakers to find out just what happens when the bass drops. If you ever wondered what the natural frequency of your eyeballs is, then this one’s for you.

    If you’re more intrigued by the idea of putting out fires with sound (and/or explosions), I’ve got some posts on that including a sound-based fire extinguisher and a supersonic cannon capable of blowing out fires. (Video credit: Physics Girl)

  • Viscoelasticity and Liquid Armor

    Viscoelasticity and Liquid Armor

    One proposed method for improving bulletproof armor is adding a layer of non-Newtonian fluid that can help absorb and dissipate the kinetic energy of impact. Thus far researchers have focused on shear-thickening fluids – like cornstarch-based oobleck – filled with particles that jam together if anything tries to deform them quickly. But is it really the shear-thickening properties that matter for high-speed impacts?

    To test this, researchers studied projectile impact on three fluids: water (left), a cornstarch mixture (not shown), and a shear-thinning polymer mixture (right). Water is Newtonian, and it slows down the projectile but doesn’t stop it. Both the shear-thickening cornstarch and the shear-thinning polymer mixture do stop the projectile. And by modeling the impacts, researchers concluded that the key to that energy dissipation isn’t their shear-related behaviors: it’s the fact that both fluids are viscoelastic.

    That means that these fluids show both viscous (fluid-like) and elastic (solid-like) responses depending on the timescale of an impact. The high speed of the impact triggered a strong viscous response in both fluids, bringing the projectile to a halt. And if, as the researchers suggest, it’s a fluid’s viscoelasticity that matters most, that widens the field of candidates when it comes to developing a fluid-based armor. (Image and research credit: T. de Goede et al.)

  • Oobleck Under Impact

    Oobleck Under Impact

    Fluids like air and water are Newtonian, which means that the way they deform does not depend on how the force on them gets applied. Many other fluids, however, are non-Newtonian. How they behave depends on how force is applied to them. The Internet’s favorite non-Newtonian fluid is probably oobleck, a mixture of cornstarch and water with some fairly extreme properties. When deformed quickly, like when struck with a bat, oobleck doesn’t flow; it shatters.

    What’s happening at the microscopic level is that the cornstarch particles in the oobleck are jamming together. They simply cannot move quickly and avoid one another. When they jam together, the friction between them goes way up and so does the apparent viscosity of the oobleck. Because it doesn’t have time to flow, all that energy goes into breaking off “solid” chunks instead. Once they hit the ground, the pieces of oobleck will puddle, just like any other liquid. (Image and video credit: Beyond the Press; via Nerdist)

  • Wild Extrusions

    Wild Extrusions

    In their continuing quest to squish all the things, the Hydraulic Press channel recently debuted a tool with a series of small holes they can extrude various substances through. The video features several great extrusions, including oobleck, temperature-sensitive putty, cheese, and crayons (above). Most of these substances are non-Newtonian fluids of some kind, and the extreme forces the hydraulic press causes makes for some wild effects.

    Many of the substances, including the crayons above, display signs of the sharkskin instability in their rough edges. When non-Newtonian fluids (like the paraffin wax in crayons) get extruded quickly, the material at the edges experiences a lot of friction and shear when trying to flow along the wall of the hole. When the fluid finally breaks free, the region along the outside accelerates to match the speed of fluid at the center of the extrusion. Parts of the mixture may resist that acceleration, resulting in the uneven edges seen above. (Video credit: Hydraulic Press Channel; GIF via Colossal)

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    Crushing Oobleck

    Oobleck is probably the Internet’s favorite non-Newtonian fluid. People vibrate it, run across it, shoot it, drop it, and even use it to fix potholes. But how does oobleck hold up to a hydraulic press? Fortunately, that’s been covered, too. Oobleck is a mixture of cornstarch and water, and it’s a bit unusual in that it is a shear-thickening material. That means that the faster you try to deform it, the more it will resist that deformation. Knowing this makes the above video’s results make more sense. When they try to crush the balloon full of oobleck, the deformation happens pretty slowly, so the fluid just flows away.

    The same thing happens initially with the pot full of oobleck; it overflows much like any other liquid. But as the press pushes deeper, the oobleck gets confined by the pot’s walls and things change. Research has shown that the shear-thickening of oobleck comes from cornstarch particles jamming up in the fluid. By confining the oobleck, the pot and hydraulic press magnify this jamming effect, causing a spurt of semi-solid cornstarch fingers and leaving the press tool thoroughly trapped by the jammed particles. (Video credit: Hydraulic Press Channel)

  • Striking Oobleck

    Striking Oobleck

    Mixing cornstarch and water creates a fluid called oobleck that has some pretty bizarre properties. Oobleck is a shear-thickening, non-Newtonian fluid, which means its viscosity increases when you try to deform it with a shearing, or sliding, force. But as the Backyard Scientist demonstrates above, striking oobleck with a solid object produces some spectacular and very non-fluid-like results. The golf ball’s impact blows the oobleck into pieces that look more like solid chunks than liquid droplets. This solid-like behavior occurs because the impact jams the suspended cornstarch particles together, creating a solidification front that travels ahead of the golf ball. Imagine how a snow plow pushes a denser region of snow ahead of it as it drives; the cornstarch behaves similarly but only in a region near the impact. Once that impact force dissipates, the particles unjam and the mixture responds fluidly again. (Image credit: The Backyard Scientist, source; research credit: S. Waitukaitis and H. Jaeger, pdf)