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Search results for: “droplet”

  • A Groovy Hovercraft

    A Groovy Hovercraft

    Not long ago, researchers discovered that droplets hovering over a hot grooved surface would self-propel. The extension to this was to investigate a hovercraft on a grooved, porous surface (top half of animation)–think an air hockey table with grooves. In that case, air inside the grooves flows from the point toward the edges, and it drags the hovercraft with it, thanks to viscosity. So the hovercraft travels in the direction opposite the points. This raised an obvious question: what happens if the hovercraft is grooved instead of the surface?

    That’s the situation we see in the bottom half of the animation. Air flows from the table and interacts with the grooves on the bottom of the hovercraft. And this time, the hovercraft propels in the direction of the points. That means there’s a completely different mechanism driving this levitation. When the grooves are onboard the hovercraft, pressure dominates over viscous effects. The air still gets directed down the grooves, but now, like a rocket, the exhaust pushes the hovercraft in the other direction – toward the points. For more on this work, check out the mathematical model of the problem and our interview with one of the researchers in the video below. (Research credit: H. de Maleprade et al.; image and video credit: N. Sharp and T. Crawford)

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    A Musical Splatter

    High-speed video is wonderful for appreciating fluid motion in ways we can’t on our own. In this video from Warped Perception, we see what happens when a vibrating tuning fork is lowered into water. The tines of the tuning fork create a spray of tiny droplets, reminiscent of what happens in ultrasonic atomization or when blowing through an immersed straw. The ejected droplets fall slowly back onto the disturbed surface; many of them bounce rather than coalescing. This is because the surface’s vibration pushes the drops aloft again before the air layer separating the drop from the surface has the time to drain away. (Video credit: Warped Perception)

  • Using Instabilities for Manufacturing

    Using Instabilities for Manufacturing

    Manufacturing textured, flexible surfaces can be difficult, but researchers are exploring ways to use fluid dynamical instabilities to make the process easier. They begin with a pourable polymer mixture that cures and solidifies over time. By putting the mixture on a cylinder and rotating it, engineers trigger the Rayleigh-Taylor instability – the same instability that makes dense fluids sink into lighter ones. Here, the instability is driven not only by gravity but by the added acceleration caused by centrifugal force. It causes the fluid film to drain and form arrays of droplets, which then cure into dimples. The researchers can control the size, shape, and spacing of the droplets by changing parameters like the spin rate. And by repeating the process multiple times on the same piece, they can build up spikier shapes, like the ones shown on the poster below. (Image and research credit: J. Marthelot et al., poster)

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    Reminder for those at the APS DFD meeting! My talk is tonight at 5:10PM in Room B206. You’ll probably want to come early if you want a seat!

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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)

  • Supernumerary Bows

    Supernumerary Bows

    After the rain of Hurricane Florence came the rainbow, or rainbows, in this case. Photographer John Entwistle captured this image of a rainbow with several additional supernumerary bows. The inner fringes seen here form when light passes through water droplets that are all close to the same size; given the spread seen here, the droplets are likely smaller than a millimeter in diameter. Supernumerary rainbows cannot be explained with a purely geometric theory of optics; instead, they require acknowledging the wave nature of light. (Image credit: J. Entwistle; via APOD; submitted by Kam-Yung Soh)

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    The Actual Shape of Raindrops

    If you imagine the shape of a raindrop, you probably think of a tear drop shape, but the reality of rain is much more complicated. It’s Okay to Be Smart has a great primer on the subject that takes a look at the forces on a raindrop and shows you the actual shape they take, which depends largely on their size.

    Small raindrops tend to coalesce together over time and get larger and progressively flatter. When the drop’s volume gets too large (below), it balloons up like a parachute. Researchers call this a bag. Stretched into a film, the drop’s surface tension is no longer able to win its fight against aerodynamic forces, and the drop shreds into smaller droplets. (Video and image credit: It’s Okay to Be Smart)

  • Levitating with Sound

    Levitating with Sound

    Sound can manipulate fluids in fascinating ways, from levitation to vibration. Here researchers use sound to levitate and manipulate droplets and turn them into bubbles. Increasing the acoustic pressure on the levitating droplet flattens it, then slowly causes the drop to buckle. When the buckled film encloses a critical volume, the sound waves resonate inside it. That causes a big jump in acoustic pressure, which makes the drop snap closed into a bubble. (Image and research credit: D. Zang et al.; via Science News; submitted by Kam-Yung Soh)

  • 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)

  • Breaking Up Drops

    Breaking Up Drops

    Lots of applications – from rocket engines to ink jet printing – require breaking large droplets into smaller ones, so there are many methods to do this. Some techniques rely on fluid instabilities, others use ultrasonic vibration. But one of the most effective methods may also be the simplest: placing a mesh between large drops and their target.

    That’s the idea at the heart of this new study, which uses a wire mesh to break large droplets into a spray of finer ones 1000 times smaller. The target application is agricultural spraying, and the researchers argue that their method would allow farmers to treat their crops effectively with fewer chemicals and less run-off. Drops impacting the mesh form a narrow cone over the plant, and the smaller, slower droplets are better at sticking to the plant instead of bouncing away. They’re also less likely to injure crops, since they don’t disturb the leaves the way larger drops do. (Image and research credit: D. Soto et al.; via MIT News; submitted by Omar M.)

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