FYFD

Year: 2019

  • Phase-Switching to Avoid Icing

    Phase-Switching to Avoid Icing

    Preventing ice and frost from forming on surfaces – especially airplane wings – is a major engineering concern. The chemical de-icing cocktails currently used in aviation are a short-lived solution, and while superhydrophobic surfaces can be helpful, they tend to be easily damaged and therefore impractical. Another possible solution, shown here, are so-called phase-switching liquids – substances like cyclohexane that have freezing points higher than that of water. This means that they form a solid coating near the freezing temperature of water.

    Water droplets on these coatings move in a random stick-slip walk (above) but they tend not to freeze. This is because freezing requires the droplets to release heat, which melts part of the phase-switching liquid. Now, instead of solidifying to the surface, the droplet moves on a film of the phase-switching liquid. Re-freezing that liquid is tough because it’s thermodynamically unfavorable, and the smoothness of the liquid layer makes it harder for ice to find a nucleation point. In lab tests, the phase-switching liquid surfaces resisted ice and frost more than an order of magnitude longer than conventional materials. (Image and research credit: R. Chatterjee et al.; video credit: Univ. of Illinois at Chicago; submitted by Night King)

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    Agnes Pockels: Surface Science Pioneer

    Today’s FYFD video tells a story I’ve wanted to share for a couple of years now. It’s about the life and work of Agnes Pockels, a woman born in the mid-nineteenth century who, despite a lack of formal scientific training, made major contributions to the understanding of surface tension and to the experimental apparatuses and methodologies used in surface chemistry in general. She accomplished all of this not in a scientific lab, but from her kitchen.

    Pockels’ story is one of curiosity, determination, and meticulous scientific inquiry. Chances are that you’ve never heard of her, but you really should. Check out the full video below to learn more! (Image and video credit: N. Sharp)

  • 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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    Weirs

    Hydraulic engineers use weirs, like the one shown below, to control upstream flow conditions. Weirs can come in many forms, but they essentially look like a small dam with water flowing over the top. They’re used to control both the flow rate and the upstream water level. As Grady from Practical Engineering explains, there are a few characteristics hydraulic engineers can vary to help adapt to changing water conditions. Check out the full video above to learn more about these important engineering features you’ve likely seen but never learned about. (Video and image credit: Practical Engineering)

  • Forming a Waterfall

    Forming a Waterfall

    Many factors can affect a waterfall’s formation – changes in bedrock structure, tectonic shifts, and glacial motion, to name a few. But a new study suggests that some waterfalls may be self-forming. Using a lab-scale experiment, researchers created a homogeneous “bedrock” out of polyurethane foam, which they eroded with a combination of constant water flow and particulates. Even without external perturbations, the flow carved out a series of steps.

    As a pool deepened, particles built up inside, armoring the bed against further erosion. But further downstream, the chute continued to erode, steepening the area between them until a waterfall formed. On the timescale of the experiment, the waterfalls lasted only 20 minutes or so, but that’s equivalent to up to 10,000 years in geological time. (Image credit: M. Huey; research credit: J. Scheingross et al.; via EOS News; submitted by Kam-Yung Soh)

  • Water Impacts

    Water Impacts

    In the clean and simplified world of the laboratory, a droplet’s impact on water is symmetric. From a central point of impact, it sends out a ring of ripples, or even a crown splash, if it has enough momentum. But the real world is rarely so simple.

    Here we see how droplets impact when the wind is blowing against them. The drops fall at an angle, creating an oblique cavity. Rings of ripples spread from the impact, but the ligaments of a splash crown form only on the leeward side. As the wind speed increases, so does the violence of the impact, eventually beginning to trap tiny pockets of air beneath the surface. Those miniature bubbles can spray droplets and aerosols into the air when they finally pop. (Image and video credit: A. Wang et al.)

  • Rogue Waves

    Rogue Waves

    After centuries of tales from sailors, in 1995 the Draupner off-shore platform recorded the first ever evidence of a freak wave – a single, wall-like wave steeper and taller than any other waves around it. Theories have been tossed back and forth for the last quarter century as to how the Draupner wave formed, but now a group of researchers report they have recreated a lab-scale version of this is famous wave. 

    They did so in a wave pool by making two smaller groups of waves cross one another at about 120 degrees (top). The interaction of those wave packets generated a much larger, steeper wave (bottom image sequence) that matched the profile of the Draupner wave. Recreating this past freak wave confirms that wave-crossing can lead to freak waves, which will hopefully help us forecast when conditions may be right for more to occur. (Image credit and research credit: M. McAllister et al., source; via Motherboard; submitted by Kam-Yung Soh)

  • Moving Droplets

    Moving Droplets

    Microfluidic devices – such as those used by individuals with diabetes to monitor their blood glucose levels – are all about transport. Typically, these devices use some kind of externally applied force, like a temperature gradient or electrical field, to force liquids through the device’s narrow channels. But a new study describes a way to move droplets without an external force.

    The researchers built their devices using two slips of glass, coated with an oil-attracting, water-repellent mixture. They attached the glass slips with a narrow spacer at one end, leaving the other end free. This made a narrow, but slightly flexible gap. When the scientists placed an oil drop inside the closed end, it spread on the glass, pulling the two sides closer to one another. Water drops, on the other hand, tried to force the walls apart, in an effort to minimize contact. Both sets of drops, interestingly, moved toward the open end of the device.

    The researchers found that the shapes assumed by the droplets create an internal pressure gradient, which, in both cases, slowly moves the drops. They call this method bendotaxis, a type of self-propulsion driven by the drops’ ability to bend the material they’re touching. It’s not a fast way to transport fluids – the drops moved only a few micrometers per second – but it may be useful for applications like drug deliveries where the liquid needs to be administered slowly over a longer period. (Image credit: TesaPhotography; research credit: A. Bradley et al.; via APS Physics; submitted by Kam-Yung Soh)

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    Freezing Drop Impact

    At the altitudes where aircraft fly, it’s often cold enough for water drops to freeze in seconds or less. Once attached to a wing, such frozen drops disrupt the flow, reducing lift and increasing drag. To help understand how such droplets freeze, scientists study droplet impact on cold surfaces. Starting at room temperature (counter-clockwise from upper left), a drop will spread on the surface, then retract. When the temperature is colder, parts of the droplet freeze before retraction completes, leaving a thin sheet with a thicker center. At even colder temperatures, the droplet’s rim destabilizes and freezing occurs before the droplet has time to retract fully. And at the coldest temperatures, the droplet breaks apart into a frozen splash. (Image and video credits: V. Thievenaz et al.)

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    Sniffing

    In many ways, smell is a strange sense. The very act of sniffing – pulling air and odor molecules into our noses – changes what remains behind in a way that sight and sound do not. Humans aren’t great sniffers, but dogs have an exquisite sense of smell, and in this video, Deep Look describes how and why that is. From special scent organs to their experimental protocols, dogs are well-adapted to reading the world by smell. (Image and video credit: Deep Look)