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Tag: superhydrophobic

  • Staying Dry Underwater

    Staying Dry Underwater

    Many insects are known to quest underwater, but few are as adept at it as the alkali fly. This species has taken common attributes among flies – being covered in tiny hairs and a waxy layer – and really upped the ante. Their extra hairiness and extra waxiness make them extremely difficult to get wet, even in the excessively salty and alkaline waters of California’s Mono Lake, which are enough to defeat all but algae, brine shrimp, bacteria, and alkali flies.

    Staying dry is a challenge, but only one of many this insect tackles. The combination of hair and wax over the insect makes it superhydrophobic, coating it in a silvery layer of air as it crawls below the surface. All that air is buoyant, so to walk underwater, the fly has to exert forces up to 18 times its body weight just to keep from popping back up to the surface.

    The shimmering bubble also helps the fly breathe. Insect respiratory systems use openings all over the exoskeleton to exchange oxygen with the ambient atmosphere via diffusion. While diffusion of oxygen does still happen underwater, it’s a much slower process there. The air sheath around the fly creates a large surface area for oxygen to diffuse, which helps counter the lower diffusion rate. Inside the sheath, the fly breathes as it normally does. (Image and research credit: F. van Breugel and M. Dickinson; via Gizmodo; submitted by @1307phaezr)

  • Moving Fluids in the Right Direction

    Moving Fluids in the Right Direction

    One challenge in creating miniature labs-on-a-chip is keeping fluids moving in the desired direction. The top image above shows red and blue droplets being moved toward one another on the top and bottom of a vibrating surface. Eventually, they meet and mix in the middle. To force the fluids in the right direction, the surface is highly textured, as seen in the lower image. These tiny posts and arcs help trap air between the surface and the drop. This makes the drop’s contact area with the superhydrophobic substrate quite small. The arcs provide directionality, and, as the surface shakes, the drops inch along, releasing the arc on the trailing edge as they make contact with a new one. In effect, the droplets walk themselves just where their designers want them to go. (Image and research credit: T. Duncombe et al.; via SciTechDaily)

  • Elastic Bounces

    Elastic Bounces

    A rigid ball accelerated by a moving surface can only ever move as fast as the surface propelling it. But that’s not true for squishy objects like a water droplet. The composite image above shows the trajectory of a water droplet launched from a moving superhydrophobic surface. As the surface starts rising, it squishes the droplet like a pancake, triggering a deformation cycle where the droplet will squish and extend repeatedly. How quickly the drop changes shape depends on factors like its size and surface tension. The researchers found that a droplet’s launch was strongly affected by the ratio of the droplet’s shape-changing frequency and the frequency of the plate’s motion. When the drop’s shape changed three times faster than the surface’s motion, it would catapult off the surface with 250% of the kinetic energy of a rigid ball!

    Launching elastic balls works the exact same way as droplets, indicating that the phenomenon depends on the way the projectiles deform. The process is similar to jumping on a trampoline. If a trampolinist times her jump just right, she’ll get more energy from the trampoline and fly higher. The droplet does the same when its deformation is properly tuned to its catapult. (Image credit: C. Raufaste et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Superhydrophobic Splashes

    Superhydrophobic Splashes

    Superhydrophobic surfaces have a complicated microscale structure that changes how water interacts with them, like the hairs on a lotus leaf or the scales of a butterfly’s wing. The photo above shows snapshots at each millisecond as a water drop hits a superhydrophobic surface covered in rows of 18 micron-tall posts. The drop hits with enough speed to drive some water into the space between posts, as shown by the dark area near the center of the splash. As the rest of the droplet spreads, four microjets form along the directions of the micropost array. Those jets remain apparent until the drop reaches its maximum radius and starts to recoil. The rectangular shape of the post array affects how the water pulls away from the surface, or depins, causing the round droplet to instead take on a square-like shape as it pulls back. (Image credit: M. Reyssat et al.)

  • A Water Balloon on a Bed of Nails

    A Water Balloon on a Bed of Nails

    If you dropped a water balloon on a bed of nails, you’d expect it to burst spectacularly. And you’d be right – some of the time. Under the right conditions, though, you’d see what a high-speed camera caught in the animation above: a pancake-shaped bounce with nary a leak. Physically, this is a scaled-up version of what happens to a water droplet when it hits a superhydrophobic surface.

    Water repellent superhydrophobic surfaces are covered in microscale roughness, much like a bed of tiny nails. When the balloon (or droplet) hits, it deforms into the gaps between posts. In the case of the water balloon, its rubbery exterior pulls back against that deformation. (For the droplet, the same effect is provided by surface tension.) That tension pulls the deformed parts of the balloon back up, causing the whole balloon to rebound off the nails in a pancake-like shape. For more, check out this video on the student balloon project or the original water droplet research. (Image credits: T. Hecksher et al., Y. Liu et al.; via The New York Times; submitted by Justin B.)

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  • Superhydrophobic Coatings

    Superhydrophobic Coatings

    Superhydrophobic–or water repellent–materials are much sought after. Their remarkable ability to shed water is actually mechanical in nature–not chemical. Surfaces with a highly textured microstructure, like a lotus leaf or a butterfly wing, shed water naturally because air trapped between the high points prevents the water from contacting most of the solid surface. The result is that a drop sitting on the surface will have a very high contact angle and be nearly spherical. Instead of wetting the surface and spreading out, it can slide right off, as seen in the animations above. Here researchers have treated the coins and the right half of the cardboard with a spray-on coating that creates superhydrophobic microscale roughness. Similar coatings are commercially available, but such coatings are delicate and lose their hydrophobicity over time as the microstructure breaks down. (Image credits: Australian National University, source)

  • Floating on a Granular Raft

    Floating on a Granular Raft

    A thin layer of hydrophobic particles dispersed at an oil-water interface is strong enough to prevent a water droplet from coalescing. The researchers refer to this set-up as their granular raft. As the red-dyed water droplet gets larger (top row), it deforms the raft more and more, but the grains continue to keep the drop separate from the fluid beneath (middle row). When water is removed from the droplet, wrinkles form on the raft as the drop’s volume shrinks. This is because the contact line – where the droplet, grains, and air meet – is pinned. The grains already touching the drop are held there by adhesion. But since the drop is shrinking, the area on the raft has to shrink, too – thus wrinkles! (Photo credits: E. Jambon-Puillet and S. Protiere, original)

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    Fish, Feathers, and Phlegm

    Inside Science has a new documentary all about fluid dynamics! It features interviews with five researchers about current work ranging from the physics of surfing to the spreading of diseases. Penguins, sharks, archer fish, 3D printing, and influenza all make an appearance (seriously, fluid dynamics has everything, guys). If you’d like to learn more about some of these topics, I’ve touched on several of them before, including icing, penguin physics, shark skin, archer fish, and disease transmission via droplets.  (Video credit: Inside Science/AIP)

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    Making Droplets Stick

    Lots of plants have evolved leaves that are superhydrophobic – that is, water repellent. For a plant, this makes a lot of sense. A superhydrophobic leaf will make water bounce and run off, draining down to where the plants roots can drink it up. But this same feature can be a frustration to farmers who spread pesticides by spraying plants. They need the pesticide to stick to the leaves if it’s to deter insects, and the superhydrophobicity of the leaves forces them to spray more pesticides in the hopes of getting some to stick. Researchers at MIT are looking to change this status quo with a few biodegradable polymer additives that can counter the leaves’ superhydrophobic tendencies and help droplets stick to the surface. This could reduce the amount of pesticides needed to protect crops. (Video credit: MIT)

  • Easy Squeezing

    Easy Squeezing

    Nearly everyone has struggled with the frustration of trying to get ketchup, toothpaste, or peanut butter out of a container. These fluids and fluid-like substances are notoriously difficult to budge because they prefer to wet and adhere to solid surfaces. One way to limit this adhesion is to use a superhydrophobic surface, like the one shown in the middle image. These surfaces use micro- and nanoscale roughness to trap air pockets underneath a liquid and reduce the amount of contact between the liquid and solid. But such surfaces are delicate and prone to failure. The slippery alternative offered by LiquiGlide is a liquid-impregnated surface, shown in the bottom image. Like a superhydrophobic surface, it consists of a textured solid but one that’s filled with a liquid lubricant that preferentially wets the solid. As a result, the liquid to be shed has little to no contact with the actual solid surface and therefore slides easily off! (Image credit: LiquiGlide, source; research credit: K. Varanasi et al.; suggested by cnsidero)