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Tag: viscous fingering

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    Fractal Fingers

    As bizarre as the branching fractal fingers of the Saffman-Taylor instability look, they’re quite a common phenomenon. In his video, Steve Mould demonstrates how to make them by sandwiching a viscous liquid like school glue between two acrylic sheets and then pulling them apart. The more formal lab-version of this is the Hele-Shaw cell, which he also demonstrates. But you may have come across the effect when pealing up a screen protector or in dealing with a cracked phone screen. In all of these cases, a less viscous fluid — specifically air — is forcing its way into a more viscous fluid, something that it cannot manage without the fluid interface fracturing. (Video and image credit: S. Mould)

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    Why Nature Loves Fractals

    Trees, blood vessels, and rivers all follow branching patterns that make their pieces look very similar to their whole. We call this repeating, self-similar shape a fractal, and this Be Smart video explores why these branching patterns are so common, both in living and non-living systems. For trees, packing a large, leafy surface area onto the smallest amount of wood makes sense; the tree needs plenty of solar energy (and water and carbon dioxide) to photosynthesize, and it has to be efficient about how much it grows to get that energy. Similarly, our lungs and blood vessels need to pack a lot of surface area into a small space to support the diffusion that lets us move oxygen and waste through our bodies. Non-living systems, like the branches of viscous fingers or river deltas or the branching of cracks and lightning, rely on different physics but wind up with the same patterns because they, too, have to balance forces that scale with surface area and ones that scale with volume. (Video and image credit: Be Smart)

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    Marangoni Blossoms

    When surface tension varies along an interface, fluids move from regions of low surface tension to higher surface tension, a behavior known as the Marangoni effect. Here, a drop of (dyed) water is placed on glycerol. The two fluids are miscible, but water has much a lower viscosity and density yet a higher surface tension. The drop’s interface quickly becomes unstable; viscous fingers form along the edge as the less viscous water pushes into the more viscous glycerol. Eventually, the surface-tension-driven Marangoni flow breaks those fingers off into lip-like daughter drops. The researchers also show how the interplay between viscosity and surface tension affects the size of fingers that form by varying the water/glycerol concentration. (Image and video credit: A. Hooshanginejad et al.)

  • Peering Inside Viscous Fingering

    Peering Inside Viscous Fingering

    Viscous fingers form when a low-viscosity fluid is pumped into a narrow, viscous-fluid-filled gap. The branching pattern that forms depends on the ratio of the two viscosities, among other factors. To better understand what goes on inside these fingers, researchers carefully alternated injecting dyed and undyed fluid. This creates a pattern of concentric rings that deform as the fingers spread.

    In this particular study, the initial fluid and injected fluids are miscible, meaning that they can mix into one another. In modeling their experiments, the team found that this mixing created stratification — i.e., layers of fluids with different densities — in the narrow gap between their plates. The stratification’s effects were large enough that the model required a correction term for them; that’s a bit surprising because we’d usually expect that the tiny third-dimension of the gap would be too small to matter! (Image and research credit: S. Gowan et al.)

  • Viscous Fireworks

    Viscous Fireworks

    Inject a less viscous fluid into a gap filled with a more viscous fluid, and you’ll get finger-like patterns spreading radially. Here, researchers put a twist on this viscous fingering by taking turns injecting different liquids. Each injection cycle disrupts what came before, layering fingering patterns on fingering patterns. The results resemble fireworks. Happy 4th of July! (Image credit: C. Chou et al.)

  • 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)

  • Controlling Finger Formation

    Controlling Finger Formation

    When gas is injected into thin, liquid-filled gaps, the liquid-gas interface can destabilize, forming distinctive finger-like shapes. In laboratories, this mechanism is typically investigated in the gap between two transparent plates, a setup known as a Hele-Shaw cell. In the past, researchers looking to control the instability have explored how surface tension, viscosity, and the elasticity of the gap itself affect the flows. But a new set of studies look at the compressibility of the gas being injected.

    The team found that viscous fingers formed later the higher the gas’s compressibility. That provides a potential control knob for people trying to exploit the mechanism, especially geologists. For geologists trying to extract oil, viscous fingering is detrimental, but, on the flip side, viscous fingers are desirable when injecting carbon dioxide for sequestration. With these results, users can tweak their injection characteristics to match their goals. (Image credit: C. Cuttle et al.; research credit: C. Cuttle et al. and L. Morrow et al.; via APS Physics)

  • The Best of FYFD 2022

    The Best of FYFD 2022

    In keeping with our annual tradition, here’s a look back at the most popular posts of 2022:

    1. The Assassin’s Teapot can pour two different liquids from the same spout
    2. The Florida Keys formed from fossilized coral reefs and sandbars
    3. Take a look inside a gas pump’s nozzle
    4. Hot chocolate hides a strange acoustic effect
    5. Under strong electric fields, liquid bridges form
    6. Growing fractal fluids
    7. A peek inside a coronavirus aerosol
    8. Wind-powered Strandbeests wander the beaches
    9. Tongan volcano sends shocks around the world
    10. Why do tea leaves swirl up in the middle of a stirred mug?

    Lots of beverage-inspired posts this time around! It’s a good reminder that there’s always interesting science around us all the time. Also, a special shout out to Steve Mould, whose videos appear in three of the top ten posts of the year – wow! Congrats, Steve!

    If you enjoy FYFD, please remember that it’s a reader-supported website. I don’t run ads and it’s been years since my last sponsored post. You can help support the site by becoming a patron, making a one-time donation, buying some merch, or simply by sharing on social media. And if you find yourself struggling to remember to check the website, remember you can get FYFD in your inbox every two weeks with our newsletter. Happy New Year!

    (Image credits: teapot – S. Mould, Florida Keys – L. Dauphin/USGS, gas pump – S. Mould, hot chocolate – C. Kalelkar, liquid bridge – X. Pan et al., fractal fluids – R. Camassa et al., coronavirus – R. Amaro et al., strandbeests – T. Jansen, shocks – S. Doran/Himawari 8, tea leaves – S. Mould)

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    Inside Viscous Fingers

    Sandwich a viscous fluid between two transparent plates and then inject a second, less viscous fluid. This is the classic set-up for the Saffman-Taylor instability, a well-studied flow in which the interface between the two fluids forms a wavy edge that develops into fingers. Despite its long history, though, there is still more to learn, as shown in this video. Here, researchers alternately injected a dyed and undyed version of the less viscous fluid. The result (Image 3) is a set of concentric dye rings that show how the fluid moves far from the fingers along the edge. Notice that the waviness of the fingers appears in the flowing fluid well before it approaches the interface. (Image and video credit: S. Gowan et al.)