FYFD

Category: Research

  • Superhydrophobic Drag

    Superhydrophobic Drag

    Using air or bubbles to reduce drag on boats is a popular idea, whether using supercavitation, the Leidenfrost effect, or superhydrophobic coatings. But most of the experiments done thus far use spheres rather than realisitic boat shapes. In this study, the researchers used two model boats — one with a hydrofoil and the other in a conventional motorboat shape — and applied superhydrophobic coatings to different parts of the model to see how superhydrophobicity affected the overall drag.

    Perhaps surprisingly, they found that superhydrophobic coatings can actually increase the drag! The effect was particularly stark for the hydrofoil boat (Image 2), where the surface jets (lower half) caused by the superhydrophobic coating slowed the boat by 30% compared to its unmodified speed (upper half).

    For the speedboat, a superhydrophobic hull made no overall difference in its drag, though it changed how water splashed in its wake. And coating the boat’s propeller was particularly detrimental, resulting in a speed up to three times slower. Overall, the study suggests that superhydrophobic coatings may be useful in some circumstances, but they have to be applied carefully, as they can have negative impacts, too. (Image credits: top – S. Anghan, others and research credit: I. Vakarelski et al.)

  • Bendable Ice

    Bendable Ice

    Ice — as we typically encounter it — is extremely brittle and easily broken. That’s due to defects in the ice, places where atoms have settled into a spot that does not match the perfect crystalline alignment. Because tiny defect-free threads of ice made by researchers turn out to be wildly flexible!

    To make these perfect ice strands, each of which is a tiny fraction of the thickness of a human hair, researchers applied an electric voltage to a needle in a water-vapor-filled chamber. The technique condensed ice microfibers with perfect crystal structures in a matter of seconds. When bent, the microfibers actually shift from one crystalline arrangement to another in order to carry stress, and once the force is removed, the thread reverts back to its initial straight form. (Image and research credit: P. Xu et al.; via Science News; submitted by Kam-Yung Soh)

  • Dissolving Pinnacles

    Dissolving Pinnacles

    Limestone and other water-soluble rocks sometimes form sharp stone pinnacles like the ones seen here in Borneo. Scientists have recreated these structures in the laboratory simply by immersing water-soluble substances (essentially blocks of candy) into water. Without any background flow, the blocks will slowly form these pinnacle forests as material dissolves into the nearby water, creating a heavy solute-rich fluid that sinks down the exterior of the block. The convection generated by this dissolution drives the material into these sharp shapes, as shown mathematically in this recent study. (Image credit: N. Naim; research credit: J. Huang and N. Moore; via APS Physics)

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    Burning Virtual Forests

    Wildfires are growing ever more frequent and more destructive as the climate crisis worsens. Unfortunately, simulating and predicting the course of these fires is incredibly difficult, requiring a combination of ecology, meteorology, combustion science, and more. To handle so many variables, model builders often turn to statistics that allow them to simulate an entire forest but at the cost of representing individual trees as a few pixels or a cone.

    In this video, researchers show a new wildfire simulation based on a computationally efficient but more realistic depiction of trees. With individual, three-dimensional trees, the simulation can capture effects that are otherwise hard to examine – like the difference in burn rate for coniferous and deciduous forests and the likelihood that a fire can jump a firebreak of a given size. Their weather, fire, and atmospheric models are even able to simulate the birth of fire-generated clouds! Check out the full video to see more and then head over to their site if you’d like to dig into the methodology. (Video and research credit: T. Hädrich et al.; see also)

  • Liquid Bridges

    Liquid Bridges

    In 1893, Baron Armstrong demonstrated a peculiar phenomenon — a liquid bridge of water suspended between two beakers with a strong electric charge between them (Image 1). More than a century later, the details of the mechanism remain challenging to pin down thanks to the setup’s combination of electohydrodynamics, heat transfer (Image 2), evaporation, and chemistry (the electrodes can split water).

    Researchers have pinned down a few details, though, like that the break-up of the liquid bridge (Image 3) depends on its effective length and that the effective length grows as applied voltage increases. Researchers also found that inducing an external flow can extend the bridge’s lifetime, though it does not affect the length at which it breaks up. Interestingly, the phenomenon is not limited to water (and its odd chemistry); ethanol and glycerol have been used for liquid bridges, too! (Image and research credit: X. Pan et al.)

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    Schooling Relies on Vision

    For fish, collective motions like schooling rely on a few mechanisms, including flow sensing and — as beautifully demonstrated in this experiment — vision. Researchers used an infrared camera to track fish motions both in light and dark conditions and compared how orderly the school of fish was in each. As expected, the school’s motion was much more orderly when the fish could see one another clearly. Interestingly, the researchers then ran an experiment in which the illumination rose continuously from dark to fully bright. The fish school’s organization grew continuously with the light! The better they could see one another, the more organized their schooling. (Video and research credit: L. Baptiste et al.)

  • Turbulence in Flight

    Turbulence in Flight

    Eagles and other birds spend much of their lives in the turbulence of our atmospheric boundary layer. Some of their interactions with turbulence — like using topographical effects to aid their flight — are well-known, but much remains uncertain. One team of researchers looked at a tagged golden eagle’s flight data, compared with known wind conditions, and looked for evidence of turbulence’s influence. To do this, they drew on years of research into how turbulence interacts with inertial particles — particles that are heavier than the surrounding fluid and thus unable to follow the flow exactly.

    What they found is that turbulence seems to be baked into many aspects of the eagle’s flight. Even the basic accelerations of the eagle’s body during flight showed characteristics that match those of turbulent flows. The findings suggest that turbulence — rather than something to be avoided — is an integral part of flight for birds, an energy source they’ve learned to exploit. (Image credit: J. Wang; research credit: K. Laurent et al.; submission by G. Bewley)

  • Splashing on Spheres

    Splashing on Spheres

    The splash of a droplet is a surprisingly complex phenomenon, depending not only on the droplet’s characteristics but also the surrounding air pressure, the roughness and temperature of the impact surface, and the surface’s curvature. In this study, researchers investigated the effects of surface curvature on splashing, finding that it’s harder for a drop to splash on spheres of smaller radius than ones with a larger radius of curvature.

    In Image 1, the falling droplet coats the 2-mm sphere with no sign of splashing. But as the radius gets larger (Images 2 and 3), splashing becomes more and more pronounced. They found that the splash suppression is due to a modification of the lift force on the leading edge of the lamella, the thin liquid layer created as the drop impacts and spread. (Image, research, and submission credit: T. Sykes et al.; also available here)

  • Wild Patterns in Ionic Liquids

    Wild Patterns in Ionic Liquids

    Ionic liquids are essentially salts in a liquid form. In these images, a mixture of water and ionic liquid separates when heated. This phase separation causes the initial mixture to break into two regions: one low in ionic liquid and one rich in ionic liquid. Because the surface tensions of these two phases are different from one another, complex flow patterns form. (Image and research credit: M. Pascual et al.)

  • The Shapes of Melting Ice

    The Shapes of Melting Ice

    Water is an odd substance because it is densest at 4 degrees Celsius, well above its melting point at 0 degrees Celsius. This density anomaly means that melting ice takes on very different shapes, depending on the temperature of the water surrounding it. At low temperatures (under 4 degrees Celsius), the cold water melting off the ice is denser than the surroundings, so it sinks. The sinking fluid melts lower portions of the ice faster, leading to an inverted pinnacle (Image 1).

    In contrast, at higher temperatures (above 7 degrees Celsius), the meltwater is lighter than the surroundings and therefore rises, creating an upward-pointing pinnacle (Image 3). At intermediate temperatures, some areas of the ice see rising meltwater and some see sinking. This complicated flow pattern sets up vortices that result in a scalloped edge along the ice (Image 2). (Image and research credit: S. Weady et al.; via APS Physics)