FYFD

Tag: droplets

  • The Bouncing Drop

    The Bouncing Drop

    For a droplet to bounce, we expect it to hit a wall or a sharp interface of some kind. But in a new study, researchers demonstrate a droplet that bounces with neither. Shown above is an oil droplet sinking through a stratified mixture of ethanol (toward the top) and water (toward the bottom). Because the oil is heavier than ethanol, it initially sinks, dragging some of the ethanol with it as it falls. Over time, some of that ethanol rises again, forming what’s known as a buoyant jet.

    Simultaneously, the gradient of ethanol to water between the top and bottom of the drop creates an imbalance in surface tension. The ethanol near the top of the drop has a lower surface tension than the water at the bottom. This creates a downward Marangoni flow along the drop interface.

    The bounce itself happens quickly after a long, slow sinking period. As the drop’s sinking slows, the buoyant jet weakens until it disappears completely. At the same time, the downward Marangoni flow pulls fresh ethanol-rich fluid toward the top of the drop. That increases the surface tension difference and strengthens the Marangoni flow, creating a positive feedback loop. In less than a second, the Marangoni flow increases by two orders of magnitude, pulling so hard that the drop shoots upward.

    That resets the cycle by weakening the Marangoni flow and strengthening the buoyant jet. The droplet can continue bouncing for about 30 minutes until the concentration gradient is so well-mixed that the cycle can’t continue. (Image and research credit: Y. Li et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Phase-Switching to Avoid Icing

    Phase-Switching to Avoid Icing

    Preventing ice and frost from forming on surfaces – especially airplane wings – is a major engineering concern. The chemical de-icing cocktails currently used in aviation are a short-lived solution, and while superhydrophobic surfaces can be helpful, they tend to be easily damaged and therefore impractical. Another possible solution, shown here, are so-called phase-switching liquids – substances like cyclohexane that have freezing points higher than that of water. This means that they form a solid coating near the freezing temperature of water.

    Water droplets on these coatings move in a random stick-slip walk (above) but they tend not to freeze. This is because freezing requires the droplets to release heat, which melts part of the phase-switching liquid. Now, instead of solidifying to the surface, the droplet moves on a film of the phase-switching liquid. Re-freezing that liquid is tough because it’s thermodynamically unfavorable, and the smoothness of the liquid layer makes it harder for ice to find a nucleation point. In lab tests, the phase-switching liquid surfaces resisted ice and frost more than an order of magnitude longer than conventional materials. (Image and research credit: R. Chatterjee et al.; video credit: Univ. of Illinois at Chicago; submitted by Night King)

  • The Color of Droplets

    The Color of Droplets

    In nature, color comes from many sources: like the pigmentation of skin and hair, the structural iridescence of a butterfly’s wings, or the refraction of a rainbow from water droplets. Recently, scientists discovered another source of brilliant color in simple, hemispherical water droplets.

    When small droplets form on a transparent surface, they form concave shapes capable of total internal reflection. This means that two light rays entering from the same angle can follow different paths inside the droplet. After reflecting several times, the light rays exit the droplet with a phase difference and how large that phase difference is determines the color. Check out the video below for some brightly colored examples of the effect. The researchers hope the technique will eventually be suitable for creating dye-free, color-changing technologies. (Image credit: F. Frankel; video credit: MIT News; research credit: A. Goodling et al.)

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    “The Empire of C”

    Filmmaker Thomas Blanchard has once again released a beautiful, fluid-filled short to captivate us. Built from paint, oil, and liquid soap, “The Empire of C” feels like it gives viewers a birds-eye perspective over a fantastical land. I was particularly drawn to two fluid dynamical aspects of the film. The first were the dendritic sequences in the opening, which feel a bit like watching river deltas form in real time. Despite their resemblance to the Saffman-Taylor instability, I think these fingers are interfacially driven – meaning that they result from differences in surface tension between the different liquids Blanchard is using. 

    The second thing that caught my eye and made me rewind the video over and over were the glittery droplets. The glitter acts like tracer particles, allowing you to see the flow inside the droplets. Check out that counter-circulation compared to the paint flowing by outside! It’s a reminder that even inside a seemingly still droplet, there’s lots going on. (Video and image credit: T. Blanchard)

  • What Drives Droplets

    What Drives Droplets

    There’s been a lot of interest recently in what goes on inside droplets made up of more than one fluid as they evaporate. This can be entertaining with liquids like whiskey or ouzo, but it has practical applications in ink-jet printing and manufacturing as well. And a new experiment suggests that we’ve been fundamentally wrong about what drives the flow inside these drops.

    As these drops evaporate, a donut-shaped recirculating vortex forms inside them, as seem in the cutaway views above. Conventional wisdom says that vortex is driven by surface tension. Evaporation of components like alcohol is more efficient at the edges of the drop, and as the alcohol evaporates, it creates a higher surface tension at the drop’s edge than at its peak. Marangoni forces then pull fluid down toward the edges, creating the vortex. That explanation is  consistent with observations of a sessile drop sitting on top of a surface (left side of images).

    But those observations are also consistent with another explanation: evaporating ethanol makes the local density higher, so alcohol-rich parts of the drop rise toward the peak while alcohol-poor regions sink. This difference in density would also create a flow pattern consistent with observations. So which is the real driver, surface tension or gravity?

    To find out, researchers flipped the drop upside-down (right side of images). When hanging, the preferred flow direction due to surface tension doesn’t change; flow should still go from the deepest point on the drop toward the edge. But gravity is swapped; alcohol-rich areas should be found near the edge and attachment points of the drop because buoyancy drives them there. And that is exactly what’s observed. The flow direction inside the hanging droplet is consistent with the direction prescribed by buoyancy-driven flow, thereby upending conventional wisdom. It turns out that gravity, not surface tension, is the major driver of internal flow in these multi-component droplets! (Image and research credit: A. Edwards et al.; via APS Physics; submitted by Kam-Yung Soh)

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    “Float”

    In “Float” artist Susi Sie uses water and oil to create a whimsical landscape of bubbles and droplets. Coalescence is a major player in the action, though Sie uses some clever time manipulations to make her bubbles and droplets multiply as well. Watching coalescence in reverse feels like seeing mitosis happen before your eyes. (Video and image credit: S. Sie)

  • A Splat is Born

    One day calligrapher Mae Nguyen accidentally squeezed a droplet out of her waterbrush pen, and a fun, new technique was born. Nguyen sometimes uses the arrays of droplets to paint and other times blows on them to create colorful splatters, like in the video above. I’d love to see the latter technique, in particular, in slow motion! I expect there is some really cool mixing as the droplets coalesce. Check out more of Nguyen’s work on her website and Instagram account. (Video credit: M. Nguyen)

  • 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)

  • Giving Droplets a Kick

    Giving Droplets a Kick

    Giving droplets a kick by accelerating the surface they sit on creates elaborate shapes as the drops respond. As the surface accelerates upward, the droplet flattens into a pancake. When the plate slows down, the droplet continues rising, stretching into a cone as its rim flies upward and its lower surface adheres to the surface. The rim retracts with a constant acceleration while the drop detaches with a constant velocity. That velocity depends on how well it adheres to the surface. The interplay between those two variables determines how conical or cylindrical the drop appears. See more in the full video below. (Image and video credit: P. Chantelot et al.)

  • Nestling Droplets

    Nestling Droplets

    Pay attention after a rainfall, and you may notice beads of water gathering in the corners of a spider’s web or along the leaves of a cypress tree (bottom right). Look closely and you’ll notice that the largest droplets don’t form along a straight fiber. Instead they nestle into the corners of a bent fiber (top image). Researchers recently characterized this corner mechanism and found that the angle at which the largest droplets form is about 36 degrees. This angle provides the optimal conditions for capillary action and surface tension to hold large drops in place. At smaller angles, a growing droplet’s weight pulls it down until the thin film holding the droplet near the top ruptures and the droplet falls. At larger angles, a heavy droplet will slowly detach from one side of its fiber and shift toward the other side until its weight is too great for the wetted length of fiber to hold. Then it detaches completely and falls. (Research and image credit: Z. Pan et al.; via T. Truscott)