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Search results for: “droplet”

  • Droplet Rolling

  • Dancing Droplets

  • Avoiding Droplet Contact

    Avoiding Droplet Contact

    Cold rain splashing on airplane wings can freeze in instants. To prevent that, researchers look for ways to minimize the time and area of contact a drop has. Hydrophobic coatings and textures can do some of the work, but they are easily damaged and don’t always work well when it comes to freezing.

    The new technique shown here uses ring-shaped “waterbowls” to help deflect drops. As the drop impacts and spreads, the walls of the ring texture force the lamella up and off the surface. This reduces both the time and area of contact and, under the right circumstances, cuts the heat transfer between the fluid and surface in half. The technique is useful for more than just preventing freezing, though; it would also be helpful for waterproofing breathable fabrics, where shedding moisture quickly without clogging pores is key to keeping the wearer dry. (Image and research credit: H. Girard et al.; via MIT News and Gizmodo)

  • Transporting Droplets

    Transporting Droplets

    Transporting droplets easily and reliably is important in many microfluidic applications. While this can be done using electric fields, those fields can impact biological characteristics researchers are trying to measure. As an alternative, a group of researchers have developed the concept of “mechanowetting,” a technique that uses surface tension forces to hold droplets on a traveling wave.

    Now visually, it’s a bit tough to see what’s going on here. In the animations, it looks like the droplets are just sticking to a moving surface, but that’s an illusion. The surface the droplet is sitting on is fixed and unmoving. It’s a thin silicone film that covers a ridged conveyor belt. The belt underneath can (and does) move. This creates a traveling wave. Instead of that wave simply passing beneath the droplet, it triggers an internal flow and restoring force that helps the drop follow the wave. The effect is strong enough that small droplets are even able to climb up vertical walls or stick upside-down. (Image, research, and submission credit: E. de Jong et al.)

  • Polygonal Droplets

    Polygonal Droplets

    Spheres are a special shape; they provide the smallest possible surface area necessary to contain a given volume. And since surface tension tries to minimize surface energy by reducing the surface area, drops and soap bubbles are, generally, spherical. There’s subtlety here, though: namely, what if reducing the surface area doesn’t minimize the surface energy?

    That’s the issue at the heart of this study. It looks at microscale oil droplets, like the ones above, that are floating in water and stabilized by surfactants. We’d expect droplets like these to be round, and above a critical temperature, they are. But as the temperature drops, the surfactant molecules along the droplet’s interface crystallize. The drop itself is still liquid, but interface is not.

    This changes the rules of the game. There’s no way for the surfactant molecules to form a sphere when solidified; they simply can’t fit together that way. So instead defects form along the interface and the drop becomes faceted. As the temperature drops further, the energy relationship between the water, oil, and surfactants continues shifting, causing the droplet to change shape – even to increase its surface area – all to minimize the overall energy. The effect is reversible, too. Raise the temperature back up above the critical point, and the interface “thaws” so that the drop becomes round again. (Image and research credit: S. Guttman et al.; via Forbes; submitted by Kam-Yung Soh)

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    Engineering Droplets

    A jet of falling liquid doesn’t remain a uniform cylinder; instead, it breaks into droplets. In this video, Bill Hammack explores why this is and what engineers have learned to do to control the size of the droplets formed.

    The technical name for this phenomenon is the Plateau-Rayleigh instability. It begins (like many instabilities) with a tiny perturbation, a wobble in the falling jet. This begins a game of tug of war. One of the competitors, surface tension, is trying to minimize the surface area of the liquid, which means breaking it into spherical droplets. But doing so requires forcing some of the the liquid to flow upward, against both gravity and the liquid’s inertia. The battle takes some time, but eventually surface tension wins and the jet breaks up.

    That’s not necessary a bad thing. It’s actually key to many engineering processes, like ink-jet printing and rocket combustion, as Bill explains in the full video. (Video and image credit: B. Hammack; submitted by @eclecticca)

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    Fizzy Droplets

    Leidenfrost drops surf on a layer of their own vapor, created by the high temperature of a nearby surface relative to their boiling point. These Leidenfrost drops can self-propel and skitter and skate across a surface, but they’re not the only droplets that do this. In this video, researchers show how a drop of carbonated water on a superhydrophobic (water-repelling) surface also avoids contact. As long as the drop has carbon dioxide to expel, it will maintain a gap relative to the surface and can even surf over a ratcheted surface the way that their Leidenfrost cousins do. (Image and video credit: D. Panchanathan et al., source)

  • Jumping Droplets

    Jumping Droplets

    From butterfly wings to lotus leaves, many surfaces in nature are shaped to repel water. This typically means roughness on the scale of tens of nanometers, which helps trap air between water and the surface. Droplets can still form on these surfaces, but when they merge, the sudden excess of surface energy sends the coalesced droplet flying. With enough height, the tiny droplet can catch the wind and get carried away. It’s like a natural anti-fogging mechanism, and it’s one that engineers are keen to understand and replicate. (Image and research credit: P. Lecointre et al.)

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    The Shaky Life of a Droplet

    An evaporating drop of ouzo goes through several stages due to the interactions of oil, alcohol and water. If you turn the situation around by placing a drop of (blue-dyed) water in a mixture of alcohol and anise oil (top image), you get some similarly odd behavior. The drop of water shimmies and grows as alcohol dissolves into it, carrying the occasional oil droplet with it. Eventually, the droplet grows large enough and buoyant enough that part of it detaches and floats to the surface (middle image). If you increase the alcohol ratio in the surrounding fluid, you speed up this process, causing droplets to stream up to the surface (bottom image). (Image and video credit: O. Enriquez et al., source)

  • The Color of Droplets

    The Color of Droplets

    In nature, color comes from many sources: like the pigmentation of skin and hair, the structural iridescence of a butterfly’s wings, or the refraction of a rainbow from water droplets. Recently, scientists discovered another source of brilliant color in simple, hemispherical water droplets.

    When small droplets form on a transparent surface, they form concave shapes capable of total internal reflection. This means that two light rays entering from the same angle can follow different paths inside the droplet. After reflecting several times, the light rays exit the droplet with a phase difference and how large that phase difference is determines the color. Check out the video below for some brightly colored examples of the effect. The researchers hope the technique will eventually be suitable for creating dye-free, color-changing technologies. (Image credit: F. Frankel; video credit: MIT News; research credit: A. Goodling et al.)

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