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Search results for: “droplet”

  • Water Impacts

    Water Impacts

    In the clean and simplified world of the laboratory, a droplet’s impact on water is symmetric. From a central point of impact, it sends out a ring of ripples, or even a crown splash, if it has enough momentum. But the real world is rarely so simple.

    Here we see how droplets impact when the wind is blowing against them. The drops fall at an angle, creating an oblique cavity. Rings of ripples spread from the impact, but the ligaments of a splash crown form only on the leeward side. As the wind speed increases, so does the violence of the impact, eventually beginning to trap tiny pockets of air beneath the surface. Those miniature bubbles can spray droplets and aerosols into the air when they finally pop. (Image and video credit: A. Wang et al.)

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    Freezing Drop Impact

    At the altitudes where aircraft fly, it’s often cold enough for water drops to freeze in seconds or less. Once attached to a wing, such frozen drops disrupt the flow, reducing lift and increasing drag. To help understand how such droplets freeze, scientists study droplet impact on cold surfaces. Starting at room temperature (counter-clockwise from upper left), a drop will spread on the surface, then retract. When the temperature is colder, parts of the droplet freeze before retraction completes, leaving a thin sheet with a thicker center. At even colder temperatures, the droplet’s rim destabilizes and freezing occurs before the droplet has time to retract fully. And at the coldest temperatures, the droplet breaks apart into a frozen splash. (Image and video credits: V. Thievenaz et al.)

  • Putting a Spin on Splashes

    Putting a Spin on Splashes

    Researchers put a spin on splashing droplets with selective wetting. When a drop impacts on a water-repellent, superhydrophobic surface, it will spread circularly, then pull back together and rebound off the surface. That’s because the surface coating resists actually touching – or being wetted by – the water. But just as there are surface coatings that resist water, there are those that attract it.

    Above, researchers have coated a surface so that it’s mostly superhydrophobic, but it also has narrow pinwheel-like arms that are hydrophilic. As the drop impacts, it spreads across the surface and then retracts. But where the hydrophilic arms are, the drop lingers. This creates the four lobes we see on the droplet, and the asymmetric retraction gives the drop angular momentum. As it leaves the surface, the spin continues. In some configurations, the researchers could make the drop spin at more than 7300 rpm. (Image and research credit: H. Li et al; via Science; submitted by Kam-Yung Soh)

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    The Sharpshooter Insect

    The sharpshooter is a small, sap-sucking insect capable of consuming more than 300 times its body weight in fluid each day. To sustain that level of intake, the insect also has to have a robust mechanism for expelling excess fluid, and that particular talent has earned the insect the nickname of the “pissing fly”. Together a group of sharpshooters can expel enough fluid to imitate rain (top).

    Individually, the insects form a droplet on hydrophobic hairs near their anus. Once the droplet is large enough, those hairs bend like a spring, and the droplet gets catapulted off the insect with an acceleration greater than 20g. That makes it among the fastest reactions in the natural world – more than twenty times the acceleration of a cheetah. Understanding this mechanism is valuable for engineers building robotics as well as for finding ways to counter the agricultural menace the sharpshooters present when it comes to spreading diseases among infected crops. (Image and video credit: E. Challita et al.; via WashPo; submitted by Marc A.)

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    Massive Worthington Jet

    The FloWave facility in Scotland is one of the coolest ocean simulators out there. Equipped with 168 individual wave makers and 28 submerged flow-drive units, it’s capable of recreating almost any ocean conditions imaginable. So naturally the Slow Mo Guys used it to create a giant spike wave.

    Essentially, this is an oversized Worthington jet, the same as the ones you see after a droplet hits the surface. But with several thousand tonnes of crystalline clear water, the effect of that wave focusing is pretty spectacular. When you’re watching the high-speed footage, be sure to pay attention to the details, like the glassy surface of the collapsing jet, or the way holes open and expand as the splash curtain comes down around Dan’s head (above). Longtime readers will recognize many familiar features. (Image and video credit: The Slow Mo Guys)

  • Exploding a Drop

    Exploding a Drop

    Leidenfrost drops levitate over a hot substrate on a thin layer of their own vapor, constantly replenished as the drop evaporates. For the most part, previous studies have focused on pure droplets, but a new one looks at what happens when you add surfactants – and the results are, well, explosive.

    Surfactants are a type of chemical that like to gather at the surface of a drop, and, unlike water, they’re nonvolatile – they don’t evaporate easily. So as the Leidenfrost drop evaporates and shrinks, the surface of the drop becomes more and more crowded with surfactant molecules. Eventually, they form an elastic shell around the remaining water, making evaporation more difficult.

    Inside the droplet, the temperature continues to rise, eventually reaching a point where bubbles of vapor can nucleate inside. When that happens, the bubbles expand almost instantaneously and the internal pressure spike bursts the shell, causing the entire droplet to explode. (Image and research credit: F. Moreau et al.)

  • Avoiding Ice

    Avoiding Ice

    Keeping ice from forming on a surface is a major engineering challenge. Typically, there’s no controlling certain factors – like the size and impact speed of droplets – so engineers try to tame ice by changing the surface. This can be through chemicals – as with deicing fluids used on aircraft – or by tuning the surface itself.

    One way to do this is by making the surface superhydrophobic – or extremely water repellent. These surfaces are rough on a nanoscale level, but they’re delicate, and once ice gets a grip on them, it’s even harder to remove. In a recent study, however, researchers used particles with both hydrophobic and hydrophilic – water-attracting – properties to create a superior ice-resistant surface. The combination of hydrophobic and hydrophilic aspects to the particles made supercooled droplets break up on contact with the surface. This made the drops smaller and decreased their contact time, making it harder for them to stick and freeze. (Image credit: Pixabay; research credit: M. Schwarzer et al.; via Chembites; submitted by Kam-Yung Soh)

  • An Inverted Leidenfrost Drop

    An Inverted Leidenfrost Drop

    Leidenfrost drops – liquid drops that levitate on a layer of their own vapor over a hot surface – have been all the rage in recent years. We’ve seen how they can be guided, trapped, and self-propelled. What you see here is a bit different. This is a droplet of room-temperature ethanol deposited on a bath of liquid nitrogen. What levitates the droplet in this case is vaporous nitrogen evaporating from the bath.

    The droplet is quickly cooling down; it freezes after its second or third bounce off the side walls of the beaker. What causes the droplet to self-propel is an asymmetry of the thin vapor layer beneath the droplet. As soon as some instability causes a slight difference in the thickness of the vapor layer, that triggers the propulsion, which the drop maintains even after freezing. (Image and research credit: A. Gauthier 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)

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    Sonic Tractor Beam

    Acoustic levitation uses the radiation forces generated by sound waves to trap small, lightweight particles at the nodes of standing waves. We’ve seen this a number of times previously, both with solid objects and liquid droplets. What makes this example particularly impressive, though, is that these researchers use an array of speakers to manipulate multiple objects at once. Check out the video above for a whole series of clips from the research. (Video credit: Science; research credit: A. Marzo and B. Drinkwater)

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