FYFD

Tag: droplets

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    How Insects Fly in the Rain

    Getting caught in the rain is annoying for us but has the potential to be deadly for smaller creatures like insects. So how do they survive a deluge? First, they don’t resist a raindrop, and second, they have the kinds of surfaces water likes to roll or bounce off. The key to this second ability is micro- and nanoscale roughness. Surfaces like butterfly wings, water strider feet, and leaf surfaces contain lots of tiny gaps where air gets caught. Water’s cohesion — its attraction to itself — is large enough that water drops won’t squeeze into these tiny spaces. Instead, like the ball it resembles, a water drop slides or bounces away. (Video and image credit: Be Smart)

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  • Charged Drops Don’t Splash

    Charged Drops Don’t Splash

    When a droplet falls on a surface, it spreads itself horizontally into a thin lamella. Sometimes — depending on factors like viscosity, impact speed, and air pressure — that drop splashes, breaking up along its edge into myriad smaller droplets. But a new study finds that a small electrical charge is enough to suppress a drop’s splash, as seen below.

    Video showing three different droplets, each with a different electrical charge, impacting an insulated surface. From left to right, the charges are: 0.0 nC, 0.08 nC, and 0.1 nC. The uncharged drop splashes, the low charge drop splashes less, and the final charged droplet spreads without splashing.

    The drop’s electrical charge builds up along the drop’s surface, providing an attraction that acts somewhat like surface tension. As a result, charged drops don’t lift off the surface as much and they spread less overall; both factors inhibit splashing.* The effect could increase our control of droplets in ink jet printing, allowing for higher resolution printing. (Image and research credit: F. Yu et al.; via APS News)

    *Note that this only works for non-conductive surfaces. If the surface is electrically conductive, the charge simply dissipates, allowing the splash to occur as normal.

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    Drops on the Edge

    Drops impacting a dry hydrophilic surface flatten into a film. Drops that impact a wet film throw up a crown-shaped splash. But what happens when a drop hits the edge of a wet surface? That’s the situation explored in this video, where blue-dyed drops interact with a red-dyed film. From every angle, the impact is complex — sending up partial crown splashes, generating capillary waves that shift the contact line, and chaotically mixing the drop and film’s liquids. (Video and image credit: A. Sauret et al.)

  • Simulating a Sneeze

    Simulating a Sneeze

    Sneezing and coughing can spread pathogens both through large droplets and through tiny, airborne aerosols. Understanding how the nasal cavity shapes the aerosol cloud a sneeze produces is critical to understanding and predicting how viruses could spread. Toward that end, researchers built a “sneeze simulator” based on the upper respiratory system’s geometry. With their simulator, the team mimicked violent exhalations both with the nostrils open and closed — to see how that changed the shape of the aerosol cloud produced.

    The researchers found that closed nostrils produced a cloud that moved away along a 18 degree downward tilt, whereas an open-nostril cloud followed a 30-degree downward slope. That means having the nostrils open reduces the horizontal spread of a cloud while increasing its vertical spread. Depending on the background flow that will affect which parts of a cloud get spread to people nearby. (Image and research credit: N. Catalán et al.; via Physics World)

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    “The Ballet of Colors”

    Thomas Blanchard’s short film “The Ballet of Colors” plunges viewers into a warm spectrum of roiling oil and paint. Fluid dynamically speaking, it could be subtitled “the Plateau-Rayleigh instability” thanks to its focus on retracting paint ruptures and ligaments breaking into droplets. Unlike some other videos of this genre, Blanchard uses a high-speed camera here, filming the action at 1,000 frames per second, and the result is smooth, crisply focused, and absolutely delectable. (Video and image credit: T. Blanchard et al.)

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  • A Drop’s Shape Effects

    A Drop’s Shape Effects

    Falling raindrops get distorted by the air rushing past them, ultimately breaking large droplets into many smaller ones. This research poster shows how variable this process is by showing two different raindrops, both of the same 8-mm initial diameter. On the left, the drop is prolate — longer than it is wide — and on the right, the drop is oblate — wider than it is long. Moving from bottom to top, we see a series of snapshots of each drop’s shape as it deforms and, eventually, breaks into smaller drops. The overall process is similar for each: the drop flattens, dimples, and then inflates like a sail, with part of the drop thinning into a sheet and ultimately breaking into smaller droplets. Yet, each drop’s specific details are entirely different. (Image credit: S. Dighe et al.)

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    “Trinity”

    Inspired by the film Oppenheimer, artist Thomas Blanchard created “Trinity,” a short film imagining a nuclear explosion with macro-scale fluid motion. There’s clever video editing and compositing in this video, but no CGI. Instead, Blanchard filmed fire, sparklers, alcohol inks, pigments and more up close and in stunning detail. As always, his work is a reminder of the amazing possibilities of analog-based art. (Video and image credit: T. Blanchard)

  • Quick-Drying, Fast-Cracking

    Quick-Drying, Fast-Cracking

    Water droplets filled with nanoparticles leave behind deposits as they evaporate. Like a coffee ring, particles in the evaporating droplet tend to gather at the drop’s edge (left). As the water evaporates, the deposit grows inward (center) and cracks start to form radially. After just a couple minutes, the solid deposit covers the entire area of the original droplet and is shot through with cracks (right).

    Researchers found that the cracks’ patterns and propagation are predictable through a model that balances the local elastic energy and and the energy cost of fracture. They also found that the spacing between radial cracks depends on the deposit’s local thickness. Besides explaining the patterns seen here, these cracking models could help analyze old paintings, where cracks could hide information about the artist’s methods and the artwork’s condition. (Image and research credit: P. Lilit et al.; via Physics Today)

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    Explosively Jetting

    Dropping water from a plastic pipette onto a pool of oil electrically charges the drop. Then, as it evaporates, it shrinks and concentrates the charges closer and closer. Eventually, the strength of the electrical charge overcomes surface tension, making the drop form a cone-shaped edge that jets out tiny, highly-charged microdrops. Afterward, the drop returns to its spherical shape… until shrinkage builds up the charge density again. This microjetting behavior can carry on for hours! (Video and image credit: M. Lin et al.; research preprint: M. Lin et al.)

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    The Mystery of the Binary Droplet

    What goes on inside an evaporating droplet made up of more than one fluid? This is a perennially fascinating question with lots of permutations. In this one, researchers observed water-poor spots forming around the edges of an evaporating drop, almost as if the two chemicals within the drop are physically separating from one another (scientifically speaking, “undergoing phase separation“). To find out if this was really the case, they put particles into the drop and observed their behavior as the drop evaporated. What they found is that this is a flow behavior, not a phase one. The high concentration of hexanediol near the edge of the drop changes the value of surface tension between the center and edge of the drop. And that change is non-monotonic, meaning that there’s a minimum in the surface tension partway along the drop’s radius. That surface tension minimum is what creates the separated regions of flow. (Video and image credit: P. Dekker et al.; research pre-print: C. Diddens et al.)