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Search results for: “water droplet”

  • Imitating a Cough

    Imitating a Cough

    Coughing and sneezing create violent air flows in and around our bodies. As that fast air rushes over mucus layers in our lungs, throat, and sinuses, the resulting flow breaks up the mucus into droplets. To explore the details of that process, researchers built a “cough machine” that sends a rush of air over a thin film of water mixed with glycerol. The setup allows them to observe the physics in a way that’s nearly impossible in a human cough or sneeze.

    Imitating a cough: high-speed video shows how a thin film made of water and glycerol breaks down in a strong airflow. Parts of the film inflate into hollow bags that form thinner weak spots. When the film breaks in those places, it forms rims and ligaments that break up into droplets.
    Imitating a cough: high-speed video shows how a thin film made of water and glycerol breaks down in a strong airflow. Parts of the film inflate into hollow bags that form thinner weak spots. When the film breaks in those places, it forms rims and ligaments that create a spray of droplets.

    As seen above, air flowing past shears the viscous fluid, stretching it out. The leading edge of the film destabilizes and breaks into large drops, but it’s what comes next that really gets things going. Areas of the film inflate to form hollow bags. When sections of the bag thin to about 1 micron, the film ruptures and the bags burst. This triggers a cascade of instabilities in the film’s rim that ultimately rip the film into a spray of tiny aerosol droplets. The researchers found that, despite their tiny size, these droplets collectively carry a large volume of liquid, making them all the more important for understanding transmission of respiratory illnesses. (Image credit: top – A. Piacquadio, experiment – P. Kant et al.; research credit: P. Kant et al.)

  • Viscoelasticity and Bubbles

    Viscoelasticity and Bubbles

    Bursting bubbles enhance our drinks, seed our clouds, and affect our health. Because these bubbles are so small, they’re easily affected by changes at the interface, like surfactants, Marangoni effects, or, as a recent study shows, viscoelasticity.

    A bubble released in pure water pops at the surface, creating a rebounding jet and a daughter droplet.
    A bubble released in pure water pops at the surface, creating a rebounding jet and a daughter droplet.

    In clean water, a bubble’s burst generates a rebounding jet that shoots off one or more daughter droplets, as seen in the animation above. But when researchers added proteins that modify only the water’s surface, they found something very different. As seen below, the bursting bubble no longer generated a jet, and, instead of forming droplets, it made a single, tiny daughter bubble. The difference, they found, comes from the added viscoelasticity of the surface. The long protein molecules resist getting stretched, which damps out the tiny waves that surface tension usually produces on the collapsing bubble cavity. (Image and research credit: B. Ji et al.; submission by Jie F.)

    When the surface of water is viscoelastic, a bursting bubble creates no jet and a daughter bubble instead of a drop.
    When the surface of water is viscoelastic, a bursting bubble creates no jet and a daughter bubble instead of a drop.
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    Aquatic Escape Artists

    Springtails are tiny hexapods found living on the air-water interface. Like other creatures living at the interface, they sometimes need to make a quick escape. For the springtail, that means a high-flying leap, driven by their fork-shaped furcula. The springtail soars into the air, where it contorts its body and uses aerodynamic forces — along with a droplet it carries on its belly — to orient itself. For landing, it uses that droplet as a sticky anchor that helps it adhere to water (or ground) instead of bouncing. Nailing that landing sets it up to make another daring escape as quickly as needed. (Video and image credit: Deep Look; research credit: V. Ortega-Jimenez et al.)

  • Puddle Depth Matters for Stalagmites

    Puddle Depth Matters for Stalagmites

    In a cave, mineral-rich water drips from the ceiling, spreading ions used to build stalagmites. A recent study considers how the depth of a pool affects the droplet’s splash and how material from the droplet spreads. The authors found several scenarios that vary widely depending on pool depth.

    A droplet falling into a shallow pool creates a splash that quickly breaks up into droplets. This flings the red droplet material in many directions.
    A droplet falling into a shallow pool creates a splash that quickly breaks up into droplets. This flings the red droplet material in many directions.

    A drop falling into a shallow pool had a splash that quickly broke up into droplets (above). By dyeing the pool green and the droplet red, they could track where the droplet’s material wound up. The spray of small droplets carried fluid far, but the main point of impact had a strong concentration of the drop’s fluid.

    With a deeper pool, the drop's impact creates a thick crown splash that collapses in on itself. The drop's fluid is quickly mixed into the pool.
    With a deeper pool, the drop’s impact creates a thick crown splash that collapses in on itself. The drop’s fluid is quickly mixed into the pool.

    In contrast, a deeper pool sent up a thick-walled splash crown that collapsed in on itself. This droplet’s material saw lots of mixing with the pool, but only near the point of impact. From their work, the authors concluded that models of stalagmite growth should incorporate pool depth in order to capture how minerals actually concentrate and move. (Image credit: cave – H. Roberson, others – J. Parmentier et al.; research credit: J. Parmentier et al.; via APS Physics; submitted by Kam-Yung Soh)

  • How a Leak Can Stop Itself

    How a Leak Can Stop Itself

    Some leaks can actually stop themselves, and a new analysis shows how. When a vertical pipe has a small hole, water initially spouts out of it, then dribbles, and, finally, drips as the water level in the pipe falls, decreasing the driving pressure of the flow. But the pipe doesn’t have to empty to a level below the hole for the leak to stop. Instead, a final droplet can form a cap over the hole, with its shape providing enough pressure to balance the remaining pressure from fluid in the pipe.

    Water leaking from a vertical pipe transitions from continuous flow to discrete drops (left). Dripping continues until the final droplet forms at t = 0 seconds.
    Water leaking from a vertical pipe transitions from continuous flow to discrete drops (left). Dripping continues until the final droplet forms at t = 0 seconds.

    The researchers found that the final drop’s kinetic energy (as well as its potential energy) was critical to determining which drop would stop the flow. The last drop behaves like a lightly-damped harmonic oscillator; it needs enough potential energy to counter the flow and a small enough inertia that it doesn’t slip away down the pipe. (Image credit: top – G. Crofte, experiment – C. Tally et al.; research credit: C. Tally et al.; via APS Physics)

  • Oil-Covered Bubbles Popping

    Oil-Covered Bubbles Popping

    When bubbles burst, they release smaller droplets from the jet that rebounds upward. Depending on their size, these droplets can fall back down or get lofted upward on air currents that spread them far and wide. Thus, knowing what kind of bubbles produce small, fast droplets is important for understanding air pollution, climate, and even disease transmission.

    The jet from a bubble of clean water.
    The jet from a bubble of clean water is broad and slow, releasing fewer and larger drops.

    In a recent study, researchers compared droplets made by clean, water-only bubbles, and the ones generated from water bubbles with a thin layer of oil coating them. The clean bubbles created jets that were broad and relatively slow moving; this motion produced a few large drops that quickly fell back down.

    The jet from an oil-covered bubble.
    The jet from an oil-covered bubble is skinny and fast-moving. It produces many small droplets.

    In contrast, the oil-slicked bubbles made a narrow, fast-moving jet that broke into many small droplets. These droplets could stay aloft for longer periods, indicating that contaminated water can produce more aerosols than clean. (Image credit: top – J. Graj, bursting – Z. Yang et al.; research credit: Z. Yang et al.; submitted by Jie F.)

  • Ominous Mammatus

    Ominous Mammatus

    Mammatus clouds are fairly unusual and often look quite dramatic. Most clouds have flat bottoms, caused by the specific height and temperature at which their droplets condense. But mammatus clouds have bubble-like bottoms that are thought to form when large droplets of water or ice sink as they evaporate. Although they can occur in the turbulence caused by a thunderstorm, mammatus clouds themselves are not a storm cloud. They appear in non-stormy skies, too. The clouds are particularly striking when they’re lit from the side, as in the image above. (Image credit: J. Olson; via APOD)

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    Pee-Flinging Sharpshooters

    The tiny glassy-winged sharpshooter feeds exclusively on nutrient-poor sap from plant xylem. Since the sap is 95% water, the insects have to consume massive amounts, necessitating lots of urination — up to 300 times their body weight each day! With so much urine to get rid of and so little energy to spare, the sharpshooter has developed an ingenious, low-energy method to expel its waste. The insect forms a droplet on its anal stylus and flings it. A recent study reveals just how clever the insect’s method is.

    Researchers found that sharpshooters fling their droplets 40% faster than their stylus moves. This superpropulsion is only possible because the stylus’s motion is finely tuned to the droplet’s elasticity. Essentially, the insects achieve single-shot resonance with every throw. The energy-savings for the insects is substantial; researchers estimate that making a jet of urine instead would cost four to eight more times energy. (Video credit: Georgia Tech; image and research credit: E. Challita et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • Rocket-Like Supercooled Drops

    Rocket-Like Supercooled Drops

    Many droplets can self-propel, often through the Leidenfrost effect and evaporation. But now researchers have observed freezing droplets that self-propel, too. The discovery came when observing the freezing of supercooled water drops inside a vacuum chamber. The researchers kept losing track of drops that seemingly disappeared. Upon closer inspection, though, they found that the drops weren’t shattering; they were flying away as they froze.

    Inside a drop, freezing starts at a point, the nucleation point, and spreads from there. But the nucleation point isn’t always at the center of the drop. This asymmetry, the researchers found, is at the heart of the drop’s propulsion. When ice nucleates, the phase change releases heat that increases the drop’s evaporation rate, which can impart momentum to the drop. For an off-center nucleation, that momentum is enough to send the drop shooting off at nearly 1 meter per second. (Image credit: SpaceX; research credit: C. Stan et al.; via APS Physics)

  • How Large Particles Get in Sea Spray

    How Large Particles Get in Sea Spray

    When bubbles burst at the ocean’s surface, they eject droplets that can carry high concentrations of contaminants like pollutants, viruses, and microplastics. Previous theories posited that only particles smaller than the microlayer surrounding the bubble could make their way into these drops, but new work shows otherwise.

    As bubbles rise to the surface, they carry particles on their surface, collecting them to a concentration that’s even higher than the surrounding seawater. But which particles make it into the air depend on the details of what happens when the bubble pops. Previously, researchers assumed that the thin microlayer of fluid surrounding the bubble was uniform, but that turns out not to be the case. As the bubble pops, some regions of the microlayer stretch and thin, while others grow thicker. The thicker the microlayer, the larger the particles it can pull along. In their single-bubble experiments, the researchers found that 15- and 30-micrometer plastic beads — representing oceanic microplastics — appeared in high concentrations in ejected droplets.

    This animated simulation shows how fluid along the edge of a bubble makes its way into ejected droplets. Green particles indicate fluid from the left half of the bubble; blue shows fluid from the right side.
    This animated simulation shows how fluid along the edge of a bubble makes its way into ejected droplets. Green particles indicate fluid from the left half of the bubble; blue shows fluid from the right side.

    Environmental scientists are keen to understand these mechanisms because they link our oceans and atmosphere, potentially affecting rainfall, pollution spread, and epidemiology. (Image, video, and research credit: L. Dubitsky et al.; via APS Physics)

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