FYFD

Category: Research

  • 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)

  • Swimming Like a Balloon

    Swimming Like a Balloon

    For humans, swimming is relatively easy. Kick your legs, wheel your arms, and you’ll move forward. But for microswimmers, swimming can be more complicated. For them, the world is a viscous place, and the rules that we swim by can’t help them get around. In a highly viscous world, flows are reversible. Kick one limb down and you might move forward, but when you pull the limb up, you’ll be sucked right back to where you started. So microswimmers must use asymmetry in their swimming. In other words, their recovery stroke cannot be the mirror-image of their power stroke.

    A new study suggests that simple elastic spheres could make good microswimmers through cyclic inflation and deflation. When the sphere deflates, it buckles, making a shape unlike its inflating one. This difference in shape change is enough to propel the sphere a little with each cycle. Right now the test system is a macroscale one, but the researchers hope to continue miniaturizing. (Image and research credit: A. Djellouli et al.; via APS Physics; submitted by Kam-Yung Soh)

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    Solving Mazes

    Earlier this fall, I attempted my first corn maze. It didn’t work out very well. Early on I unknowingly cut through an area meant to be impassable and thus ended up missing the majority of the maze. Soap, as it turns out, is a much better maze-solver, taking nary a false turn as it heads inexorably to the exit. The secret to soap’s maze-solving prowess is the Marangoni effect.

    Soap has a lower surface tension than the milk that makes up the maze, which causes an imbalance in the forces at the surface of the liquid. That imbalance causes a flow in the direction of higher surface tension; in other words, it tends to pull the soap molecules in the direction of the highest milk concentration. But that explains why the soap moves, not how it knows the right path to take. It turns out that there’s another factor at work. Balancing gravitational forces and surface tension forces shows that the soap tends to spread toward the path with the largest surface area ahead. That’s the maze exit, so Marangoni forces pull the soap right to the way out! (Video credit: F. Temprano-Coleto et al.; research credit: F. Peaudecerf et al.)

    ETA: Based on the latest research results, gravity may play less of a role than originally thought. Instead, it seems as though the soap chooses its path in part through pre-existing background levels of surfactant. As the dye advances, it compresses the background surfactant, decreasing the local surface tension until it is in equilibrium with dyed area. Because longer paths take longer to reach that equilibrium, the dye spreads preferentially toward the largest surface area.

  • Resisting Coalescence

    Resisting Coalescence

    When a droplet falls on a pool, we expect it to coalesce. There are exceptions, like bouncing droplets, but in general a droplet only sticks around for a split second before being engulfed. And yet, from morning coffee (top image) to walks in the woods, we frequently see millimeter-sized droplets sticking around for far longer than it seems like they should. New research offers a clue as to why: it’s thanks to a temperature difference. 

    When there’s an appreciable temperature difference between the drop and the pool, it causes rotating convective vortices (bottom image) in both the drop and the pool. When the temperature difference is large, the vortices are strong enough that their motion recirculates air inside the tiny gap between the drop and the pool. This supports the weight of the drop and keeps the two liquids separate. But the convection also redistributes heat, and eventually the drop and pool become similar enough in temperature that the circulation dies out, the air gap drains, and the two coalesce. (Image and research credit: M. Geri et al.; via MIT News; submitted by Antony B.)

  • Stopping a Bounce

    Stopping a Bounce

    One way to damp a bouncing ball is to partially fill it with a fluid (a) or granular material (b). For the fluid, the initial impact sloshes the liquid. That doesn’t change the trajectory of the initial bounce noticeably, but it interferes with the second impact, drastically damping the rest of the ball’s bounces until it comes to a stop. A grain-filled ball is similar, at least to begin with. The initial bounce sends the grains flying, forming a granular gas inside the ball. This doesn’t affect the trajectory of the first bounce, but the second impact collapses the granular gas. All the impacts of the grains with one another dissipate the energy of the bounce, and the ball comes to a complete stop. This suggests that a partially-grain-filled container can make a good damper in sport or industrial applications. It also suggests that it might be even better for water-bottle flipping than water is. (Image and research credit: F. Pacheco-Vázquez & S. Dorbolo)

  • Revealing Stress

    Revealing Stress

    What goes on inside of a granular material like sand when an object moves through it? Individual grains will shift and may impact one another or simply slide past. Researchers use special photoelastic materials to see these forces in action. A photoelastic material responds to changes in stress by polarizing light, revealing areas of stress concentration. For an entire network of photoelastic beads, forces between the grains appear like a web of lightning. Individual strands are known as force chains. Bright lines indicate areas where grains are jammed against one another in opposition to the object’s movement. As the intruder is pulled against the force chain network, grains shift and new force chains form. (Image credit: Y. Zhang and R. Behringer, source)

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    Pigeon Flutter

    Birds are well-known for their vocalizations, but this isn’t their only way to produce noise. A new study on crested pigeons finds that the birds’ wings produce distinctive high and low notes during take-off. A low note takes place during each upstroke, and a high note is heard during the downstroke. A major source of the noise is the highly modified P8 feather. When airflow over the feather is fast enough, it sets off twisting and torsion in the feather through aeroelastic flutter. It’s this vibration that causes the noise. By playing back the notes at different speeds, researchers found that the crested pigeons use the notes’ timing as an alarm. When the cycle of high and low repeats in quick succession, they respond by taking off to escape the perceived danger.

    Other bird species are also known to use aeroelastic flutter to make noise. Check out these hummingbirds, which use flutter in their mating displays.   (Video credit: Science; research credit: T. Murray et al.)

  • Emulsions By Condensation

    Emulsions By Condensation

    Oil and water are hard to mix, as any salad dressing aficionado will attest. Technically, the two fluids are immiscible – they won’t mix with one another – but one way around this is to emulsify them by distributing droplets of one in the other. This is usually accomplished by shaking or using sound waves to vibrate the mixture, but the results are typically short-lived. The larger a droplet is, the more gravity affects it, causing the buoyant oil to rise and separate from the water.

    The key to making an emulsion last is creating tiny droplets, which a new study accomplishes energy efficiently through condensation. Instead of mixing the oil and water immediately, the researchers used a surface covered in a mixture of oil and surfactant and cooled it in a humid chamber. As the temperature dropped, water condensed onto the oil and became encapsulated, creating nanoscale emulsion droplets. At such a tiny scale, buoyant forces are unable to overcome surface tension, so the emulsion remains stable for months. (Image credit: MIT, source; research credit: I. Guha et al.; via MIT News)

  • Symmetric Wakes

    Symmetric Wakes

    Nature is full of remarkable patterns and moments of symmetry. This image shows the wake behind two rotating cylinders. Half of the cylinders are visible at the far left. The flow moves left to right. The cylinders are rotating at the same rate but in opposite directions, clockwise for the cylinder on top and counter-clockwise for the bottom one. At this speed relative to the freestream, there is a beautiful symmetry to the vortices in the wake, but the researchers found that even a slight deviation from this condition quickly destroyed the pattern. The flow is visualized here by introducing tiny hydrogen bubbles via electrolysis. The bubbles are small enough that their buoyancy has no appreciable effect. (Image credit: S. Kumar and B. Gonzalez)

  • Schooling Together

    Schooling Together

    Since the 1970s, fluid dynamicists have chased the idea that fish swim in schools for hydrodynamic advantage. The original 2D conception of the idea placed fish in a diamond pattern so that their wakes would constructively interfere and improve swimming efficiency. In nature, that exact pattern is rarely seen, possibly due to 3D effects or the difficulty of maintaining the exact orientation. Fish do, however, show signs of grouping themselves for efficiency – especially when they’re forced to swim quickly. 

    A recent study found that tetras, a type of small fish often used as pets, prefer a staggered diamond configuration (left) when free-swimming at low speeds around one body length per second. At higher speeds, around four body lengths per second, groups of tetras preferred a side-by-side or “phalanx” configuration (right). Here the fish tended to synchronize their tail-beat frequency with their neighbors, essentially working together for a mutually beneficial wake structure. The researchers found that this configuration was much more efficient than a lone swimmer or uncoordinated group, implying that fish do school for energy-savings when they’re swimming fast. (Image and research credit: I. Ashraf et al., source; via Hakai; submitted by Kam-Yung Soh)