FYFD

Tag: viscous flow

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    The Art of Paper Marbling

    Known as ebru in Turkey and suminagashi in Japan, the art of paper marbling has flourished in cultures around the world since medieval times. The details of methods vary, but in general, the technique uses a base of oily water to float various dyes and pigments. Artists then use brushes, wires, and other tools to manipulate the dyes into the desired pattern. Paper is spread over the top to soak up the color pattern before being hung to dry. Every print made in this manner is a unique result of buoyancy, surface tension variation, and viscous manipulation. Check out the video above to watch a timelapse video showing the technique in action. (Video and image credit: Royal Hali)

  • Liquid Antispiral

    Liquid Antispiral

    Spiral formations are common in nature, from galaxies to chemical reactions. But most examples in nature rotate such that their arms trail the direction of rotation. Viewed side-on, this makes the arms appear to spiral outward from the the center. The opposite – an antispiral, where the arms appear to be drawn in toward the center – also exists, but there are far fewer examples. Which is why it’s notable that physicists have described a new one, seen above.

    You’re watching silicone oil draining through a plate with an array of holes in it. There’s a reservoir of oil on top supplying a constant flow rate. The patterns that form in this system vary widely – they can form between one and six arms – but the results are always antispirals. The driving mechanism seems to be the periodic nature of the discharge from individual holes, which is caused by a Rayleigh-Taylor instability. Hopefully systems like this can shed some light on why spirals are often preferred over antispirals. (Image and research credit: H. Yoshikawa et al.; via APS Physics)

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    Dripping Down the Rivulet

    If you’ve ever watched water running down the side of the street, you’ve probably noticed that it doesn’t flow smoothly. Instead, you’ll see waves, rivulets, and disturbances that form. That’s because the simple action of flowing down an incline is unstable. Water and other viscous liquids can’t flow downhill smoothly. Any disturbances – an uneven surface, the rumble of passing cars, a pebble in the way – will create a disruption that grows, often until the entire flow is affected. This video shows some of the complex and beautiful patterns you get then. (Video and image credit: G. Lerisson et al.)

  • Wheeling Drops

    Wheeling Drops

    Leidenfrost drops – which skitter almost frictionlessly across extremely hot surfaces on a thin layer of their own vapor – are notoriously mobile. We’ve seen numerous methods of controlling their propulsion, often using specially-shaped surfaces. But it turns out that some Leidenfrost drops can self-propel even on a smooth, flat surface (top image). 

    Internally, large Leidenfrost drops have complicated, but symmetric flows that are driven by temperature and surface tension variations across the drop. But as the drop evaporates, that symmetry eventually gets broken, leaving behind a single large circulating flow. 

    Beneath the drop, that internal circulation affects the vapor layer. It causes the layer to take on an overall tilt, and the rotation, along with that slight angle in the vapor layer, causes the Leidenfrost drop to roll away like a wheel. (Image and research credit: A. Bouillant et al.; via NYTimes)

  • Using Sound to Print

    Using Sound to Print

    Inkjet printing and other methods for directing and depositing tiny droplets rely on the force of gravity to overcome the internal forces that hold a liquid together. But that requires using a liquid with finely tuned surface tension and viscosity properties. If your fluid is too viscous, gravity simply cannot provide consistent, small droplets. So researchers are turning instead to sound waves. 

    Using an acoustic resonator, scientists are able to generate forces up to 100 times stronger than gravity, allowing them to precisely and repeatably form and deposit micro- and nano-sized droplets of a variety of liquids. In the images above, they’re printing tiny drops of honey, some of which they’ve placed on an Oreo cookie for scale. The researchers hope the technique will be especially useful in pharmaceutical manufacturing, where it could precisely dispense even highly viscous and non-Newtonian fluids. (Image and research credit: D. Foresti et al.; via Smithsonian Mag; submitted by Kam-Yung Soh)

  • The Jumping Flea

    The Jumping Flea

    Nearly every lab has a magnetic stirrer for mixing fluids, but this ubiquitous tool still holds some surprises, like its ability to unexpectedly levitate. Magnetic stirrers consist of two main parts, a driving magnet that creates a rotating magnetic field, and a bar magnet – commonly referred to as the flea – that is submerged in the fluid to be stirred. When the driver’s rotating field is active, the flea will spin at the bottom of its container, keeping its magnetic field in sync with the driver.

    But if you place the flea in a viscous enough fluid, the drag forces on the flea can pull it out of sync with the driver’s field. Above a certain speed, the flea will jump so that its field repulses the driver’s. That makes the flea levitate as it spins. Depending on the interplay of viscous and magnetic forces, that spin can be unstable (left) or stable (right). The researchers suggest that this peculiar behavior could help artificial swimmers propel themselves or lead to new methods for measuring fluid viscosity. (Image and research credit: K. Baldwin et al.; via APS; submitted by Kam-Yung Soh)

  • Folding Fluids

    Folding Fluids

    Highly viscous liquids – like cake batter, lava, or the spider silk above – fold as they fall. Several factors impact this instability including the fluid’s density, viscosity, surface tension, and how thin the falling sheet is. As with the coiling of falling honey, this behavior is actually a form of buckling. It’s also fascinating to watch how persistent the layers are. Even out near the edge of the puddle, you can still see individual folds. This is a sign of just how incredibly viscous the spider silk is. Imagine if this were cake batter instead: we’d see folding just like we do with the spider silk proteins, but the individual folds would quickly fade as the batter flowed to fill its container. The spider silk is more viscous, so it’s more resistant to flowing. (Image credit and submission: D. Breslauer, source)

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    The Fluid Dynamical Sewing Machine

    If you’ve drizzled viscous liquids like honey or syrup, you’ve no doubt witnessed their ability to coil. Combine that coiling with a moving platform and you form a system known as the fluid dynamical sewing machine, which creates different consistent patterns of loops and curves depending on the speed at which the liquid falls and the velocity of the moving platform. The predictability of these patterns makes them especially useful for 3D printing. Previously a group at MIT developed a glass printer that could use the instability, and here a group from Montreal demonstrates how they can build solid coils at various scales. Their video also explores what the structural properties of such coils are after they solidify. (Image, video, and research credit: R. Passieux et al.)

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    Bees, Squid, and Oil Plumes

    It’s time for another JFM/FYFD collab video! April’s video brings us a taste of spring with research on how bees carry pollen, squid-inspired robotics, and understanding the physics of underwater plumes like the one that occurred in the Deepwater Horizons spill eight years ago. Check it all out in the video below. (Image and video credit: T. Crawford and N. Sharp)

  • Sunset Flow

    Sunset Flow

    Day and night mix in this flow visualization of watercolor pigments and ferrofluid. The former, as suggested by their name, are water-based, whereas ferrofluids typically contain an oil base. This means the two fluids are immiscible. Like oil and vinegar in salad dressing, the only way to mix them is to break one into tiny droplets floating in the other. This is what happens near their boundary, where brightly-colored paint droplets float in a network of dark channels. To the right, the paint and ferrofluid have been swirled around to create viscous mixing patterns among the paint colors with occasional intrusions of thin ferrofluid fingers. (Image credit: G. Elbert)