FYFD

Tag: non-Newtonian fluids

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    “One”

    A 4-minute, unedited one-shot video of colorful paint sliding down a sheet? Yes, please.

    Beautiful visuals aside, there are some really interesting physics involved here. It’s unclear whether the there’s any change in the speed at which paint gets deposited at the top of the incline over the course of the video, yet we see huge changes in the visual patterns. This happens, in part, because the layer of paint is getting thicker and heavier over time, changing the dynamics of its slide under gravity. There may even be some shear-thinning going on, given that paint is usually non-Newtonian. I can imagine some connections to landslides, avalanches, and other gravity currents with non-Newtonian fluids. (Video and image credit: R. De Giuli)

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    Non-Newtonian Raindrops

    Fluids like air and water are called Newtonian because their viscosity does not vary with the force that’s applied to them. But many common fluids — almost everything in your fridge or bathroom drawer, for example — are non-Newtonian, meaning that their viscosity changes depending on how they’re deformed.

    Non-Newtonian droplets can behave very differently than Newtonian ones, as this video demonstrates. Here, their fluid of choice is water with varying amounts of silica particles added. Depending on how many silica particles are in the water, the behavior of an impacting drop varies from liquid-like to completely solid and everything in between. Why such a great variation? It all has to do with how quickly the droplet tries to deform and whether the particles within it can move in that amount of time. Whenever they can’t, they jam together and behave like a solid. (Image, video, and research credit: S. Arora and M. Driscoll)

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    Hand-Making Artist-Grade Pastels

    I’m constantly fascinated by the intersections of art and fluid mechanics. In this video, we get an inside look at a French atelier making artist-grade pastels using centuries-old methods. And although the final product doesn’t appear to have much to do with fluids — compared to, say, paint — the process behind each pastel involves a lot of fluid mechanics: mixing, pressing, drying, and rolling. It’s a neat look at how a niche product gets made. (Video and image credit: Business Insider)

    P.S. – Next week we’ll kick off our Paris Olympics coverage, but if you’d like a head start on the celebration, you can find our coverage of previous Olympics here. – Nicole

  • Saving Screens with Shear-Thinning Fluids

    Saving Screens with Shear-Thinning Fluids

    These days glass screens travel with us everywhere, and they can take some big hits on the way. Manufacturers have made tougher glass, but they continue to look for ways to protect our screens. Recently, a study suggested that non-Newtonian fluids are well-suited to the task.

    The team explored the physics of sandwiching a layer of fluid between a glass top layer and an LCD screen bottom layer, mimicking structures found in electronic devices. Through simulation, they searched for the fluid characteristics that would best minimize the forces felt by the solid layers during an impact. They found that shear-thinning fluids — fluids that, like paint or shampoo, get runnier when they’re deformed — provided the best protection. Having the impact energy go into reducing the local viscosity of the fluid stretches the length of time the impact affects the glass, which lowers the bending forces on it and helps avoid breakage. (Image credit: G. Rosenke; research credit: J. Richards et al.; via Physics World)

  • Evolving Fingers

    Evolving Fingers

    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)

  • Dendritic Painting Physics

    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)

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    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.)

  • Beijing 2022: Why Are Ice and Snow Slippery?

    Beijing 2022: Why Are Ice and Snow Slippery?

    Although every Olympic winter sport relies on the slippery nature of snow and ice, exactly why those substances are so slippery has been an enduring mystery. Michael Faraday hypothesized in the nineteenth century that ice may have a thin, liquid-like layer at its surface, something that modern studies have repeatedly found.

    One recent study used an entirely new instrument to probe the characteristics of this lubrication layer and found that it is only a few hundred nanometers thick. But the fluid in this layer is nothing like the water we’re used to. Instead it has a viscosity more akin to oil and its response to deformation is shear-thinning and viscoelastic, more like the complex fluids in our kitchens and bodies than pure, simple water. They found that using a hydrophobic probe modified the interfacial viscosity even further, which finally provides a hint at the mechanism behind waxing skis and skates. 

    Fortunately for us, we’ve found plenty of ways to employ and enjoy water’s slipperiness, even as the mystery of it slowly gives way to understanding. (Image credit: M. Fournier; research credit: L. Canale et al.; via Physics World; submitted by Kam-Yung Soh)

  • 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)

  • Microjets and Needle-Free Injection

    Microjets and Needle-Free Injection

    Some people don’t mind needles, and others absolutely detest them. But to replace needles with needle-free injections, we have to understand how high-speed microjets pass through skin. Given skin’s opacity, that’s tough, so researchers are instead using droplets as a model. If we can understand the dynamics of a microjet passing through different kinds of droplets, getting jets of medicine into arms becomes easier.

    Researchers found that jets passed completely through a droplet if they impacted above a critical velocity. For Newtonian droplets, the jet creates a cavity and shoots straight through because the inertia of the impact outweighs the countering force of surface tension. But with viscoelastic drops, the jet goes through, slows down, and gets sucked back into the droplet. In this case, the combination of surface tension and viscoelasticity can, eventually, overpower the jet’s inertia. (Image, research, and submission credit: M. Quetzeri-Santiago et al.)