FYFD

Tag: instability

  • Marangoni Bursting

    Marangoni Bursting

    Placing a mixture of alcohol and water atop a pool of oil creates a stunning effect that pulls droplets apart. The action is driven by the Marangoni effect, where variations in surface tension (caused in this case by the relative evaporation rates of alcohol and water) create flow. David Naylor captures some great stills of the flow, including the only example of a double burst I’ve seen so far. For more on the science behind the effect, check out this previous post or the original research paper. (Image credit: D. Naylor; see also this previous post)

  • New Signs of Turbulence in Blood Flow

    New Signs of Turbulence in Blood Flow

    Our bodies are filled with a network of blood vessels responsible for keeping our cells oxygenated and carrying away waste products. In many ways, our blood vessels are tiny pipes, but there’s a crucial difference in the flow they carry: it’s pulsatile. Because the flow is driven by our hearts, rather than a continuous pump, every heartbeat creates a distinct cycle of acceleration and deceleration in the flow. And new research has found that this cycle, when combined with curvature or flow restrictions like plaque build-up, can create turbulence in unexpected places.

    Specifically, the researchers found that decelerating pipe flows can develop a helical instability that breaks down into turbulence, even in vessels where purely laminar flow would be expected. In the animations above, you can see the flow slow, develop swirls and then break into turbulence. The flow becomes laminar again as it accelerates, but during that brief bout of turbulence there’s much higher forces on the walls of a blood vessel. Over time, that extra force could contribute to inflammation or even hardening of the arteries. (Image and research credit: D. Xu et al.; via phys.org)

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    “Dendrite Fractals”

    In this short film from the Chemical Bouillon team, dark ink drops spread in dendritic fractal patterns after being deposited on an unknown transparent liquid. Although the patterns look similar to those of the Saffman-Taylor instability, I suspect what we see here is actually driven by surface tension and not viscosity.

    The authors describe the ink they used as a “special old” “tree ink,” which — putting on my fountain pen aficionado hat — probably means some variety of iron gall ink. These inks draw on chemicals extracted from trees and other plants to create a permanent, waterproof ink. They tend to be highly acidic, which could play a role in the pattern formation seen here. (Video and image credit: Chemical Bouillon)

  • Spin Cycle

    Spin Cycle

    Rotational motion is a great way to break up liquids, as anyone who’s watched a dog shake itself dry can attest. That same centrifugal force is what allows this rotary atomizer to break liquids into droplets. Relative to the photos above, the atomizer spins in a counter-clockwise direction. This motion stretches the fluid flowing off it into skinny, equally-spaced ligaments, which eventually break down into droplets.

    Just how and when that break-up occurs depends on the fluid, as well as the characteristics of the spin. For Newtonian fluids like silicone oil — shown in the first two pictures — the break-up is driven by surface tension and happens relatively quickly. But with a viscoelastic fluid — shown in the last image — the elasticity of polymers in the fluid allow it to resist break-up for much longer. Instead, the ligaments form the beads-on-a-string instability. See more flows in action in the video below. (Video, image, and research credit: B. Keshavarz et al., video)

  • Nitro Bubble Cascades

    Nitro Bubble Cascades

    Animation of nitrogen bubbles cascading in Guinness

    Fans of nitro beers — particularly Guinness’ stout — have probably noticed the fascinating cascade of bubbles that form as the beer settles. It’s a non-intuitive behavior — bubbles rise since they’re lighter than the surrounding fluid. So why do the bubbles appear to sink in these beers?

    There are several effects at play here. Firstly, overall the bubbles in the beer are rising; even mixing nitrogen gas into a beer in place of carbon dioxide doesn’t change that. But pint glasses typically flare so that they’re wider at the top than at the bottom. Since the bubbles rise essentially straight up, this causes a bubble-less film to form near the upper walls. And as that heavier fluid sinks, it pulls some of the tiny nitrogen bubbles with it. (You don’t see this effect in typical beers because the bubbles there are larger and thus too buoyant to get pulled down by the falling fluid.)

    As for the cascading waves we see in the bubbles, this, too, comes from the shape of the glass. Hydrodynamically speaking, what’s happens as the fluid film slides down the pint glass is similar to what happens when rain runs downhill. Beyond a certain angle, the flow becomes unstable and will form rolls and waves of varying thickness instead of sinking in a thin, uniform layer. As the film goes, so go the bubbles being dragged along, giving everyone at the bar a brief but entertaining fluid dynamical show. (Image credits: pints – M. d’Itri; bubble cascade – T. Watamura et al.; research credit: T. Watamura et al.)

  • 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)