FYFD

Tag: surface tension

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    Self-Propelled Droplets

    Drops of ethanol on a heated surface contract and self-propel as they evaporate. My first thought upon seeing this was of Leidenfrost drops, but the surface is not nearly hot enough for that effect. Instead, it’s significantly below ethanol’s boiling point. Looking at the drops in infrared reveals beautiful, shifting patterns of convection cells on the drop. The patterns are driven by the temperature difference along the drop; at the bottom, the drop is warmest, and at its apex, it is coldest. Those differences in temperature create differences in surface tension, which drives a surface flow that breaks the drop’s symmetry. The asymmetry, the authors suggest, is responsible for the drop’s propulsion. (Image and video credit: N. Kim et al.)

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    “A Sense of Scale – Reminiscence”

    In so much of fluid dynamics, size does not matter. We see the same patterns mirrored across nature from a fuel injection nozzle to galactic clusters. And no one plays with that sense of scale better than artist Roman De Giuli, whose microscale practical effects give the impression of flying above glittering alien coastlines. Ink and paint squeeze around craggy islands, leaving perfect streamlines to mark their passage. Fractal fingers expand like river deltas seeking the path to the sea. Enjoy more of De Giuli’s work on his website and Instagram. (Image and video credit: R. De Giuli; via Colossal)

  • Soapy Solutions

    Soapy Solutions

    When a drop of soap falls into a pool of water, its surface-loving molecules spread out on the water’s surface. Exactly how the soap spreads depends on the local concentration of its surfactant molecules, which create areas with different surface tensions that cause flow. All in all, it’s a tough process to predict because it varies in time at every point on the pool. But a recent paper offers a new class of exact solutions for the problem.

    The paper considers a surfactant-laden droplet spreading over a (relatively speaking) deep pool. Other researchers showed recently that this situation can be described with a complex version of the Burgers’s equation, which was originally developed to describe turbulent flows. The authors solved the equation for a variety of initial conditions and found that the time-dependent spread of the surfactants was sensitive to the initial surface distribution. The higher the initial surface concentration, the faster the surfactants spread. (Image credit: T. Despeyroux; research credit: T. Bickel and F. Detcheverry; via APS Physics; submitted by Kam-Yung Soh)

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    A Fractal Raft From a Spinning Top

    File this one under Cool Things I Would Have Never Thought Of. In this video, researchers play around with the flow around a spinning top and end up creating a fractal, granular raft. By immersing a top in dyed fluid, they show the toroidal vortices that form around the spinning toy. Then, instead of dye, they add a stretchy elastomer compound that cures over time. The elastomer stretches into thin ligaments in the swirling flow around the top. Eventually, it breaks apart into spherical drops of all different sizes.

    Once the top is removed, the elastomer drops slowly float to the surface. Surface tension and the Cheerios effect draw the drops together, and because of their many sizes, the rafts that form are fractal. (Image and video credit: B. Keshavarz and M. Geri)

  • Free Contact Lines

    Free Contact Lines

    How a simple drop of water sits on a surface is a strangely complicated question. The answer depends on the droplet’s size, its chemistry, the roughness of the surface, and what kind of material it’s sitting on. Vetting the mathematical models that describe these behaviors is especially difficult since droplets often get stuck, or “pinned,” along their contact line where water, air, and surface meet.

    To get around this issue, researchers sent their experiment to the International Space Station, asking astronauts to run the tests for them. Without gravity‘s influence squishing drops, the astronauts could use much larger droplets than they could on Earth. Larger drops are less likely to get pinned by a stray surface defect, so on the space station, astronauts could place droplets on a vibrating platform and observe their contact line freely moving as the drop changed shape. Under these conditions, the experiment tested many surfaces with different wetting characteristics, thereby gathering data to test models we cannot easily confirm on Earth. (Image and research credit: J. McCraney et al.; via APS Physics)

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    Backswimmers

    Backswimmers rule the surface of ponds, streams, and other bodies of water. These insects spend much of their time clinging just beneath the air-water interface, where they hunt larvae and other insects. They use oversized, oar-shaped back legs to row, and they breathe using an air bubble that clings to their abdomen like a personal scuba tank. Oxygen from the water diffuses into the bubble, keeping the insect’s air supply fresh. When the time comes to move to greener pastures, they flip to the other side of the water’s surface, unfurl their wings, and take off. (Image and video credit: Deep Look)

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    Escaping the Flood

    Fire ants clump together into giant rafts to stay alive during floods. But these rafts won’t form with just any number of ants. Researchers found that individual ants will actually kick one another away. It’s not until there are about ten ants that the raft formation becomes stable. In this video, the team lays out their experiments and models for fire ant rafting, showing that capillary action helps draw the raft together and individual ants’ activity can destabilize rafts if they’re too small. (Image and video credit: H. Ko and D. Hu)

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    “Life and Chaos”

    In “Life and Chaos,” artists Roman Hill and Paul Mignot shot fluid flows live in a 1 cm x 1 cm square, then projected those images across 3,300 square meters. There’s something incredible about art on this immersive scale. It is literally impossible for any one visitor — or even the artists themselves — to experience the full piece; each person, by definition, can only take in a small part of the whole. That makes it all the more incredible to derive such a piece from a tiny, tiny canvas. As venues for this sort of immersive art spread, I can only imagine the amazing art we’ll see! (Image and video credit: R. Hill and P. Mignot)

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    “I See You”

    In “I See You,” filmmaker Rus Khasanov captures fluid flows that give the screen an eye with which to gaze back at us. The textures visible in the flows are incredible at mimicking the details of a human iris. These are some seriously neat Marangoni flows. For a similar effect, check out this film of his. (Image and video credit: R. Khasanov)

  • When Seeing a Flow Changes It

    When Seeing a Flow Changes It

    Adding dye to a flow is a common technique for visualization. After all, many flows in fluids like air and water are invisible to our bare eyes. But for some classes of flows — especially those driven by variations in surface tension — adding dye can have unforeseen effects. A recent study shows how true this is for bursting Marangoni droplets, where evaporation and alcohol concentration can pull a water-alcohol droplet apart.

    Composite series of photos showing the effect of increased dye concentration on Marangoni bursting.
    As more dye is added to the experiment, the daughter droplets grow larger and more ligaments form. In the first three images, a dashed black line has been added to show the location of the droplet rim.

    Without dye, it’s nearly impossible to see the phenomenon since the refractive indices of the two component liquids are so close. But the researchers found that, as they added more methyl blue dye, it did more than increase the contrast in the flow. It changed the flow, making the droplets larger and creating ligaments between them. They believe that the dye’s own surface tension creates local gradients that alter the flow. It’s a reminder that experimentalists have to be careful to consider how our efforts to measure and observe a flow can change it. (Image credit: top – The Lutetium Project, bottom – C. Seyfert and A. Marin with modification; research credit: C. Seyfert and A. Marin)