FYFD

Tag: instability

  • Pearls On a Puddle

    Pearls On a Puddle

    Leave a drop of coffee sitting on a surface and it will leave behind a ring of particulates once the water evaporates. But what happens to a droplet made up of multiple liquids that evaporate differently? That’s the subject of this new study. Researchers mixed a volatile drop (isopropyl alcohol) with a smaller amount of a non-volatile liquid and observed how this changed the droplet’s splash rim and evaporation pattern.

    When the surface tension difference between the two liquids was large, the researchers found that the splash formed fingers along its rim (Image 1). The fingers consist almost entirely of the non-volatile component, driven to the outskirts of the drop by Marangoni forces. The dark and light bands you see in the image are interference fringes, which the researchers used to track the film’s thickness.

    When the researchers used liquids with similar surface tensions, the droplet rim instead formed pearl-like satellite droplets. Once the volatile liquid evaporated away, the remaining liquid merged into a thick film. (Image and research credit: A. Mouat et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Vortex Collisions Leave Clues to Turbulence

    Vortex Collisions Leave Clues to Turbulence

    Vortex ring collisions have long been admired for their beauty, but they’re now shedding light on the fundamental interactions that lead to turbulence. By dying just the cores of colliding vortex rings (Image 2), researchers observed anti-symmetric perturbations that develop along each core as they interact. These are indicative of what’s known as the elliptical instability.

    But the breakdown doesn’t stop there. Instead, as the elliptical instability develops, it generates a set of secondary vortex filaments that wrap around the original cores (Image 3). Just like the original vortex cores, those counter-rotating secondary filaments interact with one another, develop their own elliptical instability, and generate a set of smaller, tertiary filaments (Image 4).

    What’s exciting is that this process gives us a physical mechanism for the turbulent energy cascade. Researchers have talked for decades about energy passing from large-scale eddies to smaller and smaller ones, but this work lets us actually observe that cascade in the form of smaller and smaller pairs of vortex filaments interacting. To see more, check out some of our previous posts on this work. (Image and research credit: R. McKeown et al.; via Cosmos; submitted by Ryan M. and Kam-Yung Soh)

  • A Microfluidic Zoo

    A Microfluidic Zoo

    Microfluidic channels are excellent at creating a steady supply of droplets. But depending on the characteristics of the two viscous fluids being used, as well as factors like flow rate and channel geometry, the results can be anything from well-defined and separated drops to steady jets to wild instabilities. The image above shows a series of different outcomes, including waves that break on the edges of drops and ligaments that stretch around the central fluid. (Image and research credit: X. Hu and T. Cubaud)

  • Using Electric Fields to Avoid Dripping

    Using Electric Fields to Avoid Dripping

    Anyone who’s painted a room at home is familiar with the frustration of drips. At certain inclinations, practically every viscous liquid develops these gravity-driven instabilities. They’re troublesome in manufacturing as well, where viscous films are often used to coat components and unexpected drips can ruin the process.

    To avoid this, researchers are adding electric fields into the mix. For dielectric fluids — liquids sensitive to electric fields — this addition acts like extra surface tension, stabilizing the film and preventing drips from forming. The researchers’ mathematical models predict the electric field strength necessary for a given fluid layer depending on its inclination. (Image credit: stux; research credit: R. Tomlin et al.; via APS Physics)

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    “Otherworld, Vol. 1”

    Roman De Giuli’s “Otherworld, Volume 1” is a beautiful exploration of color and flow. Glittery particulates act as tracers in the flow, reminiscent of the way rheoscopic fluids do. In many sequences, the glitter lends a sense of texture to the flow. Without context, I cannot say whether those are true flow features, but they certainly remind me of instabilities like Tollmien-Schlichting waves. (Image and video credit: R. De Giuli)

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    “Magic Fluids”

    In his short film, “Magic Fluids,” Roman De Giuli uses cyan, magenta, and yellow paints to generate a rainbow of macro colors. All the fluid motion you see is a practical effect, painstakingly created by layering paints and flow mediums of different densities. Like in Siqueiros’ “accidental painting” technique, the less dense paints will eventually rise through the upper layers and spread. De Giuli uses the effect for its motion, but the same physics is key for many artists who use acrylic pouring to paint. (Video and image credit: R. De Giuli)

  • Wave Clouds in the Front Range

    Wave Clouds in the Front Range

    Last Sunday night metro Denver was treated to a rare sight: clouds resembling breaking waves formed near sunset. These are Kelvin-Helmholtz clouds, and the comparison to ocean waves is apt, since the same physics is behind both. Winds were unusually calm near the ground Sunday night, but strong winds blew at the altitude just above the lower cloud layer. That velocity difference created strong shear where the two air layers met. With the cloud layer in place to differentiate the slower-moving air from the faster, we can what’s normally invisible: how the two air layers mix.

    The Denver Post has several more views of the wave clouds from around the area, and you can learn lots more about the Kelvin-Helmholtz instability here. (Image credit: R. Fields; via the Denver Post)

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    Driving Instabilities with a Twist

    Imagine that you want to study how two fluids mix when a lighter fluid is pushed into a denser one. Conceptually, it’s a straightforward situation. It would be like having a layer of oil under a layer of water and watching what happens. But how do you do that experimentally? Oil won’t naturally stay under water. If you flip the container over to start the experiment, you’ve added a bunch of extra motion from the rotation. And if you use a barrier to separate the two layers and then pull it out, you’ve added extra shear where they meet.

    To deal with challenges like these, researchers at Lehigh University spent five years designing and building the rotating wheel apparatus you see in the video above. Instead of relying on gravity to force the lighter fluid into a denser one, this set-up uses centrifugal force. The test fluids start out on the loading wheel, spinning in their naturally stable configuration. Then once both sides are rotating at the desired speed, the track flips, transferring the experiment onto the other wheel, which rotates in the opposite sense. This gives the fluids a sudden change in the direction of the centrifugal force and, once the apparatus completes shake-down, should give us new insight into the sort of mixing seen in fusion. (Video credit: Lehigh University; see also Turbulent Flow Design Group)

  • Pollock Avoided Coiling

    Pollock Avoided Coiling

    Streaks of black and gray in the Jackson Pollack painting the researchers studied.

    Artists are often empirical masters of fluid dynamics, as they must be to achieve the effects they want. Jackson Pollock was particularly known for his so-called dripping technique, in which he dropped filaments of paint from brushes, cans, and even syringes as he moved around a horizontal canvas. (Scientifically speaking, this wasn’t really dripping since the paint wasn’t breaking up into droplets for the most part, but that’s another story.)

    What Pollock was doing, fluid dynamically speaking, is the subject of a new study. Researchers analyzed historical footage of Pollock painting to measure the typical heights from which he dropped paint and the speed at which he moved. Then they built their own apparatus to mimic the painting style with modern paints and study the flow regime Pollock’s technique falls into. 

    Since much of the paint falls in a steady stream, like syrup falling onto pancakes, the researchers wondered whether the paint was likely to coil the way other viscous fluids do. What they found, however, is that Pollock’s choice of height and speed when applying paint seems deliberately designed to avoid the coiling instability. That fact suggests that art historians might identify forged paintings in part from the presence of too much coiling among the paint filaments. (Image credits: photo – M. Holmes/LIFE, painting – J. Pollock; research credit: B. Palacios et al; via Ars Technica; submitted by Kam-Yung Soh)

  • Trails from a Delta Wing

    Trails from a Delta Wing

    Top-down view of green and red dyes streaming off a delta wing

    Rhodamine (red) and fluorescein (green) dyes highlight the complex flows around a delta wing. To visualize the flow, researchers painted the apex of the delta wing with rhodamine, which gets drawn into the core of the wing’s leading edge vortex. The green fluorescein dye was added to the wing’s trailing edge, where it gets pulled into the secondary structure of the vortices. A laser illuminates the flow, making even the most delicate wisps of dye shine. As the wake behind the wing develops, the dyes reveal growing instabilities along the vortices. Given time and space, these instabilities will grow large enough to destroy any order in the wake, leaving behind turbulence. (Image and research credit: S. Morris and C. Williamson; see also poster)