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  • Holding Fast in the Flow

    Holding Fast in the Flow

    Many tiny creatures in the natural world face living in fast flows. The larvae of the net-winged midge, for example, forage their way through fast-flowing Alpine springs with speeds of 3 m/s or more. You or I would find standing in such water a challenge, but these larvae are unbothered, thanks to the clever suction-cup-like appendages that help anchor them to rough rocks.

    The larvae generate their strong attachment with an outer rim flexible enough to conform to uneven surfaces. When they activate the central piston of the suction cup, this creates a seal strong enough to withstand forces up to 600 times the larvae’s body weight. But holding on to one spot forever is hardly useful, so the larvae also have a V-shaped notch in the cup controlled by dedicated muscles. When activated, this quickly breaks the seal, allowing the larvae to relocate. (Image and research credit: V. Kang et al.; via The Engineer; submitted by Marc A.)

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    Superwalking Droplets

    Vibrate a pool of silicone oil and you can generate walking droplets. Drive the vibration at two simultaneous frequencies and you can support much larger droplets, known as superwalkers. These superwalkers have their own intriguing dynamics, a few of which are featured in this video.

    Superwalkers can create promenading pairs, chase one another, orbit, and even form ordered and disordered crystals. They can even generate stop-and-go traffic patterns. As with regular walkers, these complex behaviors come from the interaction of bouncing droplets with their ripples and those of their neighbors. (Image, video, and research credit: R. Valani et al.)

  • Morphing Wings Using Real Feathers

    Morphing Wings Using Real Feathers

    Although humanity has long been inspired by bird flight, most of our flying machines are nothing like birds. Engineers have struggled to recreate the ease with which birds are able to morph their wings’ characteristics as they change from one shape to another. Now researchers have built a biohybrid robot, PigeonBot, that uses actual pigeon feathers as part of its morphing design.

    Many species of birds, including pigeons, have Velcro-like hooks in the microstructure of their feathers. These hooks help the flight feathers stick to one another and create a continuous wing surface that air cannot easily slip through, even as the wing drastically changes shape. By using actual feathers, PigeonBot shares this advantage.

    PigeonBot also has a somewhat minimalist design in its articulation, using only a wrist and finger joint in each wing to control shape. The feathers are connected through an elastic ligament, which — along with their microstructure — allows them to smoothly change shape under aerodynamic loads. The end result is a remarkably capable and agile biorobot researchers can use to better understand how birds control their flight. (Image and research credit: L. Matloff et al. and E. Chang et al.; via NPR and Gizmodo)

  • Breaking the Euler Equations

    Breaking the Euler Equations

    Mathematicians like to break things. Or, more exactly, they like to know when the equations we use to describe physics break down. One popular target in fluid mechanics are the Euler equations, which describe the motion of frictionless, incompressible flows. Mathematicians have been on the hunt for centuries for situations where these equations predict singularities, points where the velocity or vorticity of a fluid change infinitely quickly. Since that can’t happen in reality (at least as far as we understand it), these singularities indicate weaknesses in our mathematical description and may help uncover fundamental flaws in our understanding.

    Despite centuries of effort, the Euler equations withstood mathematical assault… until recently. Since 2013, a series of mathematicians have been successfully chipping away at the Euler equations’ seeming perfection with a series of scenarios that seem to lead to singularities. One is similar to stirring a cup of tea, except that you stir the upper part of the cup in one direction and the bottom half in the opposite. As the flow develops, a singularity occurs where the secondary flows of these two stirring motions collide. For more, check out these two articles over at Quanta. (Image credit: L. Fotios; see also Quanta Magazine 1, 2)

  • Captured by Waves

    Captured by Waves

    Acoustic levitation and optical tweezers both use waves — of sound and light, respectively — to trap and control particles. Water waves also have the power to move and capture objects, as shown in this award-winning poster from the 2019 Gallery of Fluid Motion. The central image shows a submerged disk, its position controlled by the arc-shaped wavemaker at work on the water’s surface. The complicated pattern of reflection and refraction of the waves we see on the surface draws the disk to a focal point and holds it there.

    On the bottom right, a composite image shows the same effect in action on a submerged triangular disk driven by a straight wavemaker. As the waves pass over the object, they’re refracted, and that change in wave motion creates a flow that pulls the object along until it settles at the wave’s focus. (Image and research credit: A. Sherif and L. Ristroph)

  • Testing Waves in High Gravity

    Testing Waves in High Gravity

    Where waves crash and meet, turbulence is inevitable. But exactly how large waves interact — whether in the ocean, in plasma, or the atmosphere — is far from understood. A new experiment is teasing out a better physical understanding by tweaking a variable that’s been hard to change: gravity.

    To do so, the researchers conduct their experiments in a large-diameter centrifuge (shown above) where they can create effective gravitational forces as high as 20 times Earth’s gravity. This increases the range of frequencies where gravity-dominated waves occur by an order of magnitude.

    By studying this extended frequency range, the authors found something unexpected: the timescales of wave interactions did not depend on wave frequency, as predicted by theory. Instead, those interactions were dictated by the longest available wavelength in the system, a parameter set by the size of the container. It will be interesting to see if future work can confirm that result with even larger containers. (Image credit: ocean waves – M. Power, others – A. Cazaubiel et al.; research credit: A. Cazaubiel et al.; via APS Physics; submitted by Kam-Yung Soh)

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    Spinning Ink Out of Markers

    I have to say I’m grateful that my classmates in school never discovered the mess-generating superpower of felt-tipped markers. As the Slow Mo Guys demonstrate here, when you spin or fling these markers, ink will stream out of them. That’s due, in part, to the air vents present near the tips. Markers (and other pens) have those to equalize the pressure between the outside and the ink reservoir; otherwise, the ink won’t flow to the felt tip as it should. Is anyone else surprised by the sheer volume of liquid ink apparently contained in these pens? (Image and video credit: The Slow Mo Guys)

  • Self-Assembly Under Stratification

    Self-Assembly Under Stratification

    Sometimes mistakes lead to great discoveries. After leaving a failed outreach demo overnight, researchers discovered a new mechanism for self-assembling particles. In the initial set-up, a layer of fresh water is poured atop a layer of denser, saltier water. This creates what’s known as a stably stratified fluid, with progressively denser mixtures of salt water as one moves downward. If you pour in particles of an intermediate density (heavier than fresh water and lighter than salt water), they’ll form a layer at one height, and, if you wait overnight, those particles will slowly form a disk-like raft.

    A spheroidal particle causes attractive flow at its equator and repulsive flow at its poles.

    This self-assembly is driven by fluid dynamics — not by any attraction between the particles. Because the particles are unable to absorb salt, their boundaries distort the concentration gradients in the surrounding fluid. This generates subtle currents at the particle boundaries, like in the picture above, where flow moves toward the particle at the equator and away at the poles. Larger particle clusters generate stronger flows, allowing them to attract even more particles.

    Although the speeds involved are quite slow, this mechanism may play an important role in nature, where stratified flows are common. The researchers speculate, for example, that the effect could be important in the clustering of microplastics in the ocean. (Image and research credit: R. Camassa et al.; see also R. McLaughlin; submitted by Kam-Yung Soh)

  • Siberia’s Rivers

    Siberia’s Rivers

    Each winter the Kolyma River in Siberia freezes to a depth of several meters. But by June the river thaws and discharges its annual 136 cubic kilometers of  water into the Arctic. The dark color of the river comes from the sediment and organic material it carries. The Kolyma is the world’s largest river underlain with continuous permafrost. Parts of the river system’s permafrost date back to the Pleistocene more than 12,000 years ago. Since much of its organic matter comes from its permafrost, researchers expect the amount of organic material in the Kolyma’s discharge to increase as the permafrost degrades in our warming climate. (Image credit: NASA Earth Observatory)

  • Recreating Volcanic Lightning

    Recreating Volcanic Lightning

    Some natural phenomena, like volcanic eruptions or tornado formation, don’t lend themselves to fieldwork — at least not at the height of the action. The danger, unpredictability, and destructiveness of these environments is more than our equipment can survive. And so researchers find clever ways to recreate these phenomena in controllable ways. The latest example comes from a lab in Germany, where researchers are recreating volcanic lightning.

    To do so, they heat and pressurize actual volcanic ash in an argon environment and let the mixture decompress into a jet, like a miniature eruption. The lightning that accompanies the jet is thought to depend on friction between ash particles, which build up electric charges when rubbed, just like a balloon rubbed against one’s hair. When the charges get large enough, lightning discharges the build-up.

    Small-scale experiments like this one allow researchers to vary the temperature and water content of the ash and observe how this changes the lightning. Drier ash generates more lightning, but it’s hard to distinguish whether this is inherent to the ash or the result of the denser jets that form without the added eruptive force of steam. (Image credit: eruption – M. Szeglat, lab lightning – Sönke Stern/Ludwig-Maximilians-Universität München/Gizmodo; research credit: S. Stern et al.; via Gizmodo)

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