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Search results for: “droplet”

  • To Fizz or Not to Fizz

    To Fizz or Not to Fizz

    Place a drop of carbonated water on a superhydrophobic surface and it will slide almost frictionlessly, much the way Leidenfrost drops do. The drop behaves this way thanks to the self-produced layer of carbon dioxide vapor that it levitates on. As the gas escapes, the drop eventually settles back into contact with its surface. But until then, its levitation makes for some fun.

    On the treated half of the glass (left), bubbles form a continuous film against the glass. On the untreated side (right), bubbles nucleate, grow, and rise as expected for a fizzy drink.
    On the treated half of the glass (left), bubbles form a continuous film against the glass. On the untreated side (right), bubbles nucleate, grow, and rise as expected for a fizzy drink.

    Single droplets aren’t the only source of fun, however. In the images above, researchers coated the left half of a wine glass with a superhydrophobic treatment, while leaving the right half of the glass untouched. Once (dyed) carbonated water is poured into the glass, we see a bizarre dichotomy. In the right, untreated half of the glass, carbon dioxide bubbles nucleate, grow, and rise through the glass. But on the left side, the liquid appears still and bubble-less. In fact, the carbon dioxide gas on the left side is forming a continuous bubble film by the surface of the glass! (Image, video, and research credit: P. Bourrianne et al., see also)

  • 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)

  • Beautiful Waves

    Beautiful Waves

    Australian photographer Ray Collins captures some of the most impressively dynamic photographs of ocean waves I’ve ever seen. The textures of the water range from glassy smooth to scaled to violent sprays of droplets. You can easily get lost in every image. For more, check out his website and Instagram. (Image credits: R. Collins; via Colossal)

  • Aerosols and Instruments

    Aerosols and Instruments

    Although COVID has disrupted all of our lives, orchestras saw particular disruption, as little was known about how instruments spread aerosol droplets. In this recent study, a team looked at many wind instruments, as played by professional musicians, for the aerosol load and air flow each instrument creates. They found that, on the whole, wind instruments — like flutes, clarinets, trumpets, and others — create aerosol loads comparable to normal speech. The air flow from each instrument comes primarily from the bell (for brass instruments) or tone holes (for woodwinds) and has a much lower velocity than coughing or sneezing. As a result, the flow decays away to the background air-flow after about 2 meters. (Image credit: trumpet – E. Awuy, trombone – Q. Brosseau et al.; research credit: Q. Brosseau et al.)

    Flow from the bell of a trombone disrupts artificial fog.
    As a musician plays a scale on their trombone, flow from the bell is revealed through artificial fog and laser illumination.
  • 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)

  • Bird Photographer of the Year 2022

    Bird Photographer of the Year 2022

    Try as we might, humans cannot understand fluid dynamics as birds do. Whether they are primarily flyers or swimmers, birds have an innate understanding of lift and other aerodynamic forces that put the best engineers to shame. Shown here are a subset of winners from the 2022 Bird Photographer of the Year competition, each of them showing off fluid dynamics in some fashion. Hummingbirds hover, droplets shine like diamonds, and divers brace for impact. You can peruse more winner at BPOTY’s website. (Image credits: Various; see alt text of individual images)

  • Featured Video Play Icon

    Aerated Faucets

    So much goes on in our daily lives that we never see. But with the power of the smartphones in our pockets, we can catch more than ever before, as illustrated in this video. Here a researcher uses the standard “slo-mo” (240 fps) video mode on a smartphone to look at the flow from a typical kitchen faucet. Household faucets often have an aerator that adds air bubbles to the flow, something that’s particularly visible in slow motion at high flow rates. What you can see depends on more than just the frame rate, though. Without strong illumination — provided in this case by sunlight — you could easily miss the cloud of droplets ejected by the faucet. (Image and video credit: M. Mungal)

  • Inhibiting Marine Lightning

    Inhibiting Marine Lightning

    Thunderstorms over the ocean have substantially less lightning than a similar storm over land. Scientists wondered whether this difference could be due to lower cloud bases over the ocean or differences in the cloud droplets’ nuclei. But a new study instead implicates coarse sea spray as the deciding factor. By tracking the full lifetime of storm systems through remote sensing, the team found that fine aerosols can increase lightning activity over both land and ocean. But adding coarse sea salt from sea spray reduced lightning by 90% regardless of fine aerosols. With sea salt in the mix, clouds seem to develop fewer but larger condensation droplets, providing less opportunity for the electrification necessary to generate lightning. (Image credit: Z. Tasi; research credit: Z. Pan et al.)

  • Encapsulating Drops

    Encapsulating Drops

    Sometimes a droplet needs a little protection while it’s traveling to its destination. When that’s the case, we often try to encapsulate it in a layer of material that won’t be affected by whatever environment the drop is traveling through. In this study, researchers aimed to give their drops not one but two layers of protection — in as simple a way as possible.

    The team began with three layers of liquid. The lowest layer was water, the middle layer was an oil, and the top layer was a mixture of water and isopropyl alcohol. Next, they added glass particles that were denser than the alcohol, but less dense than the oil. This caused the particles to form a clump — a granular raft — along the interface between the alcohol and the oil (not shown). When the layer of particles became heavy enough, it began to sink into the oil, carrying some of the alcohol with them. This conglomeration formed the initial droplet of alcohol mixture encased in an armor of glass beads.

    As this armored droplet sank, it approached the second interface: the oil-water interface. At this juncture, the team observed three different outcomes. When the glass particles were small or light, the armored drop would come to a rest at the oil-water interface. As the drop deformed, water would pierce the armor, causing the whole drop to rupture (Image 1).

    In the second case, heavier particles caused the armored drop to sink through the oil-water interface, but a low oil viscosity meant that the oil film drained from the bottom of the drop before the drop was fully encapsulated. Once again, this let the water through and ruptured the droplet (Image 2).

    In the final case, armored drops with just the right bead density and oil viscosity would sink through the oil-water interface until the oil pinched off behind the drop. This pinch-off allowed the oil to redistribute around the drop, encapsulating it in layers of both oil and particles, thereby protecting it as it continued its journey (Image 3). (Image credits: top – Girl with red hat, experiment – A. Hooshanginejad et al.; research credit: A. Hooshanginejad et al.)

  • Listening to the Sizzle

    Listening to the Sizzle

    The sizzle of frying food is familiar to many a cook, and that sound actually conveys a surprising amount of information. In this study, researchers suspended water droplets in hot oil and observed their behavior, both with high-speed video and with microphones. They found that these vaporizing drops created three types of cavities in the oil: an exploding cavity that breaks the surface, an elongated cavity that remains submerged, and an oscillating cavity that breaks up well below the surface. All three cavities flung oil droplets upward, and all three were acoustically distinct from one another. That means, as the authors suggest, that it might be possible to measure the aerosol droplets generated during frying simply by listening! (Image credit: fries – W. Dharma, others – A. Kiyama et al.; research credit: A. Kiyama et al.; via Cosmos; submitted by Kam-Yung Soh)

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