FYFD

Tag: droplets

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    “There is a crack in everything…”

    When millimeter-sized drops of water infused with nanoparticles dry, they leave behind complex and beautiful residues. As water continues evaporating, the residues warp, bend, and crack. In this video, researchers set their science to the music of Leonard Cohen. The results resemble blooming flowers and flying water fowl. If you’d like to learn more about the science behind the art, check out the two open-access papers linked below. (Video and image credit: P. Lilin and I. Bischofberger; submitted by Irmgard B.; see also P. Lilin and I. Bischofberger and P. Lilin et al.)

  • Self-Cleaning With Salt Critters

    Self-Cleaning With Salt Critters

    Even freshwater contains trace salts and minerals that cause scaly buildups as they evaporate. Getting rid of the scale usually requires toxic chemicals and/or lots of scrubbing, neither of which are desirable at the industrial level. At the same time, we’re extremely limited in the amount of freshwater that we have available; only about 1% of Earth’s water is liquid and fresh. If we could use salt water in more industrial processes, that would preserve freshwater for drinking and agriculture. But how do we tackle the scaly buildup?

    (A) On microtextured surfaces, salt from evaporating drops can work its way into the gaps, destroying the superhydrophobicity of the surface. (B) In contrast, nanotextured surfaces give the salt nowhere to adhere, resulting in "salt critters" that grow upward and detach.
    (A) On microtextured surfaces, salt from evaporating drops can work its way into the gaps, destroying the superhydrophobicity of the surface. (B) In contrast, nanotextured surfaces give the salt nowhere to adhere, resulting in “salt critters” that grow upward and detach.

    Enter “salt critters.” Researchers found that when salt water evaporated from microtextured surfaces designed to shed water, salt would eventually build up in the gaps, breaking the hydrophobic effect and allowing scale to build up. In contrast, a nanotextured surface left nowhere for the salt to adhere. On these surfaces, evaporating salt water built jellyfish-like salt critters that rose from the surface and, eventually, broke off and rolled away, leaving the surface pristine. (Image credit: S. McBride; research credit: S. McBride et al.; via Physics Today)

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    The Shape of Rain

    In our collective imagination, a raindrop is pendant shaped, wide at the bottom and pointed at the top. But, in fact, a falling raindrop experiences much more complicated shapes. Here, researchers blow a jet of air onto a still droplet, a good facsimile for a raindrop falling through the atmosphere. The jet of air first squishes the drop, then inflates it into a shape known as a bag. The thin sides of the bag stretch and eventually break, spraying tiny droplets. As the disintegration continues, the thick rim of the bag breaks up into big droplets. As the video demonstrates, viscosity and viscoelasticity can affect the break-up, too. (Image and video credit: I. Jackiw and N. Ashgriz)

  • Shaped Splashes

    Shaped Splashes

    When a raindrop hits a leaf, it spreads out into a rimmed sheet that breaks up into droplets. These tiny drops can carry dust, spores, and even pathogens as they fly off. But many leaves aren’t smooth-edged; instead they have serrations or teeth. How does that affect a splash? That’s the question at the heart of today’s study.

    A water drop hits a star-shaped pillar and breaks up.
    A water drop hits a star-shaped pillar and breaks up.

    To simplify from a leaf’s shape, the team studied water dropping onto star-shaped pillars. As seen above and below, the pillar’s edge shaped the splash sheet, with the sheet extending further in the edge’s troughs. This asymmetry extends into the rim also, concentrating the liquid — and the subsequent spray of droplets — along lines that extend from the edge’s troughs and peaks.

    A viscous water-glycerol drop hits a star-shaped pillar, spreads, and breaks into droplets.
    A viscous water-glycerol drop hits a star-shaped pillar, spreads, and breaks into droplets.

    The team found that, in addition to sending drops along a preferred direction, the shaped edge made the droplets larger and faster than a smooth edge did. (Image and research credit: T. Bauer and T. Gilet)

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    “Starlit”

    In “Starlit,” filmmaker Roman de Giuli leverages paint, ink, water, and oil to create astronomical views. Colorful droplets spin past like neon exoplanets. Shards of glitter form comets. Satellite droplets become moons about their larger sibling. (Video and image credit: R. de Giuli)

  • How Water Droplets Charge Up

    How Water Droplets Charge Up

    Rubbing a balloon on your hair can build a significant electrical charge. Water droplets have the same issue when they slide across a hydrophobic, electrically-insulated surface. A new study models why these charges build up and tests the model both experimentally and through simulation. They focused their theory on three effects that determine how much charge builds up. The first is a two-way chemical reaction that continuously creates charge at the interface, with positive charge building in the drop. Secondly, the drop’s contact angle with the surface sets how many protons can build up at the contact line, thereby affecting the electrical field they generate. And, finally, fluid motion at the rear of the drop deflects protons upward, shifting the electrical field. In particular, their model predicts that the higher contact angles of hydrophobic surfaces should increase charge build-up and faster sliding velocities should slow charge build-up, both of which agree with experiments.

    The model should help researchers understand various charging scenarios, like those found on self-cleaning surfaces, in inkjet printing, and in semiconductor manufacturing. In the last scenario, rinsing semiconductor wafers in ultrapure water can build up charges in the kilovolt range, which is enough to damage the product. (Image credit: D. Carlson; research credit: A. Ratschow et al.; via APS Physics)

  • Drops of Fiber Suspensions

    Drops of Fiber Suspensions

    To 3D print with fiber-infused liquids, we need to understand how these drops form, break-up, and splash. That’s the subject of this research poster, which shows drops of a fiber suspension forming and pinching off along the top of the image. In the lower half of the image, drops of the suspension hit a hydrophilic surface and spread. How the drop and its fibers spread will affect the final properties of the printed material. (Image credit: S. Rajesh and A. Sauret; via GoSM)

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    Serpents and Ouroboros

    Beads of condensation on a cooling, oil-slicked surface have a dance all their own in this video. Large droplets gobble up their fellows as they follow serpentine paths; each new droplet donates its interfacial energy to feed the larger drop’s kinetic energy. Eventually, the big drops switch to a circular path, like an ouroboros, the tail-eating serpent of mythology. This transition happens due to the oil shifted by the dancing droplets. You can recreate the effect at home by rubbing a thin layer of oil over glass and setting it atop a hot mug of your favorite beverage. (Video and image credit: M. Lin et al.; research credit: M. Lin et al.)

  • Skittering Drops

    Skittering Drops

    Drip some ethanol on a hot surface, and you’d expect it to spread into a thin layer and evaporate. But that doesn’t always happen, and a recent study looks at why.

    Ethanol is what’s known as a volatile liquid, meaning that it evaporates easily at room temperatures, well below its boiling point. When dropped on a uniformly heated surface above 45 degrees Celsius, the drop contracted into a hemisphere and then began to wander randomly across the surface. Researchers trained an infrared camera on the drop from below (above image), and found an unsteady, roiling motion inside the drop. These asymmetric flows, they concluded, drive the drop’s erratic self-propulsion. They suspect the mechanism may explain why some ink droplets wind up in the wrong place on a page during ink-jet printing. (Image and research credit: P. Kant et al.; via APS Physics)

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    Spreading Frost

    Condensation forms beads of water on a surface. When suddenly cooled, those drops begin to freeze into frost. This video looks at the process in optical and in infrared, revealing the patterns of spreading frost and the tiny ice bridges that link one freezing drop to the next. (Video and image credit: D. Paulovics et al.)