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  • The Japanese Pufferfish

    The Japanese Pufferfish

    [original media no longer available]

    If you’ve ever dived or snorkeled over a sandy lake or ocean bottom, you’ve probably seen some neat patterns there. But it’s hard to compete with the Japanese pufferfish for pure artistry. This small fish creates enormous and elaborate designs in the sand in order to attract a mate. The male fish moves the sand into place by flapping his fins very close to the surface. Above a critical flapping velocity, his fins generate vortices capable of picking up sand, as seen below. With repeated passes, the fish is able to excavate the trough that is key to his creation. It’s a constant fight against the current, though. 

    Puffers aren’t the only ones who flap their fins to move the sands. Rays and flounders use this technique to bury themselves and hide (Video credit: BBC Earth; image credit: A. Sauret, source; research credit: A. Sauret et al.)

  • A Real Tatooine

    A Real Tatooine

    Since at least the release of “Star Wars”, we have wondered what life would be like on a circumbinary planet – a planet orbiting two stars. In the past few decades, we have discovered several such planets, but we are still in the early days of modeling the climate of these worlds. One recent study uses the stars of the Kepler 35 system, which are only slightly less luminous than our sun, to explore the climate of an Earth-like water planet.

    According to the study, this fictional planet would maintain Earth-like habitability at a distance of 1.165–1.195 astronomical units from its suns’ center of gravity – just a little further out than our own orbital distance. Variables like the planet’s mean global surface temperature and precipitation vary with two distinct periods – the time required for the stars to orbit one another and the time it takes for the planet to orbit its stars. Both factors affect how much sunlight the planet receives. The planet’s climate response to these changes is complex and varies depending on location, but the overall variations observed in the climate are small. It does show, however, that places like Tatooine don’t have to be desert planets! (Image credit: Tatooine – Star Wars; Kepler 35 system – L. Cook; research credit: M. Popp and S. Eggl)

  • Sorting by Bubble

    Sorting by Bubble

    Microfluidic devices, also known as labs-on-a-chip, require clever techniques for processes like sorting particles by size. One such technique uses an oscillating bubble to sort particles. When the bubble vibrates back and forth (left) it creates what’s known as a streaming flow – large regions of recirculation (shown as gray ellipses in the right image). If the bubble is placed inside a channel, we say that two flows have been superposed; the device combines both the left-to-right flow of the channel and the recirculating streaming flow.

    Introduce a micron-sized particle into this combined flow, and it will get carried to the bubble and then bounced around by its effects (left). In fact, the larger the particle is, the more the bubble deflects it relative to the flow. You can see this in the image on the right as well. Here the frame rate has been matched to the bubble’s vibration, so the bubble appears stationary, and the particle paths look smooth. The gray lines show the fluid’s path, and individual solid particles are introduced at the left. The largest particle gets strongly deflected as it passes the bubble and exits at the top-right. A fainter, smaller particle follows after it. Being smaller, the bubble’s deflection on it is weaker, and this second particle exits along a path to the center-right. The result is a fast and simple method for particle sorting. (Image and research credit: R. Thameem et al., source)

  • Capillary Action in Microgravity

    Capillary Action in Microgravity

    On Earth, gravity dominates over many fluid effects, but in microgravity a different picture emerges. This animation shows a two-channel apparatus partially filled with silicone oil being dropped. While in free-fall, the liquid experiences microgravity conditions and the height of the fluid in the two connected channels changes. The oil meniscus climbs up the walls of the tubes thanks to capillary action. This is the result of intermolecular forces between the liquid and solid walls. Capillary action is most effective in narrow tubes where surface tension and the adhesion between the liquid and solid can actually propel liquid up the walls, as seen here. On Earth we mostly ignore capillary action, except in very small spaces, but for space systems, it is a major force to reckon with in designing flows. (Image credit: NASA Glenn Research Center, source)

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    Building Labs on a Chip

    In their second video on microfluidics, the Lutetium Project takes viewers inside the process of creating microfluidic circuits, also known as labs-on-a-chip. When you want to build pipes only a few microns across, you need to use special techniques. As the video shows, manufacturing starts with photolithography, a process used to selectively mask parts of the substrate which are then etched away chemically. This creates a mold that’s later covered in a polymer solution. Once hardened, the polymer is removed from the mold, treated and attached to a glass slide. The result is a tiny fluid circuit that’s only a few square centimeters in total size. To really appreciate the process, check out the video, which helpfully takes you inside the clean room to see the chip manufacturing process firsthand. (Video and image credit: The Lutetium Project)

  • Stabilizing Films

    Stabilizing Films

    Liquids don’t typically survive very long as thin films. If you try to make one from water, gravity drains it away immediately. (Not so in space.) To make a liquid film stick around, we add surfactants like soap. These extra molecules congregate at the surface of the film and provide a stabilizing force to oppose gravity’s drainage. Exactly what that stabilizing force is depends on the surfactant.

    Surfactants that are insoluble are often quite viscous. These molecules distribute themselves across the interface and then they stay. They resist both gravity or even just moving thanks to their high viscosity. That produces a soap film pattern like the one on the right – symmetric and slow to change.

    Other surfactants may be soluble in the film and have no appreciable viscosity themselves. These surfactants constantly move and shift on the interface as surface tension variations occur. When weak spots form, the surfactant molecules shift, via the Marangoni effect, to stabilize the film. This creates a film pattern like the familiar one on the right, with an ever-shifting palette of colors. (Image and research credit: S. Bhamla et al., source; submission by S. Bhamla)

  • A Drip’s Vortex

    A Drip’s Vortex

    Drip food coloring into water and you can often see a torus-shaped vortex ring after the drop’s impact. That vortex rings form during droplet impact has been well known for over a century, but only recently have we begun to understand the process that leads to that vortex ring. Part of the challenge is that the vortex formation is very small and very fast, but recent work with x-ray imaging has allowed experimentalists to finally capture this event.

    When a drop impacts a pool, surface tension draws some of the pool liquid up the sides of the drop. At the same time, the impact causes ripple-like capillary waves down the sides of the drop. This causes pool liquid to penetrate sharply into the drop, triggering the spirals that mark the forming vortex ring. When drops impact with even higher momentum, multiple vortex spirals can form, as seen on the lower right image. The authors observed as many as four rings during an impact. For more, check out the (open access) article.  (Image and research credit: J. Lee et al., source)

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    Ferrofluid Microlandscapes

    Ferrofluids are an ever-fascinating topic. Consisting of ferromagnetic nanoparticles suspended in a carrier fluid, ferrofluids are known for their bizarre behaviors in the presence of a magnetic field, like their tendency to form pointed peaks reminiscent of Bart Simpson’s hair. In a new Concept Zero video, photographer Linden Gledhill creates fascinating micro-landscapes using ferrofluids suspended in solvents. Driven by magnetic fields, the ferrofluids take on many shapes that change as the solvent and eventually the ferrofluid’s carrier fluid evaporate. Check out the full video above and, if you’re looking for some new decorations for your walls, you can check out the project’s fine art gallery.   (Video and image credit: L. Gledhill and Concept Zero; submitted by L. Gledhill)

  • Self-Propelled Hovercraft

    Self-Propelled Hovercraft

    When placed on an extremely hot substrate, some drops levitate and can be propelled over specially textured surfaces. Inspired by this work, researchers are using similar principles to explore manipulation of levitating plates using surface texture. Their apparatus consists of a semi-porous, grooved surface that ejects air upward to levitate Plexiglas objects – think air hockey table with grooves. With enough airflow, the Plexiglas levitates. The grooves force air in a particular direction – in the case of the herringbone pattern, this is in the direction of opening – and, as the air moves, it drags its Plexiglas hovercraft along. As shown in the second animation, grooves can do more than move the glass linearly; with patterns offset by 90-degrees, they can make the hovercraft rotate.

    Here’s an interesting next step for anyone out there with an air hockey table and a 3D printer: does the directional manipulation work if the grooves are on the object and not the table? In other words, can you create an air hockey puck that preferentially goes to your opponent’s goal? (Image and resource credit: D. Soto et al., source)

  • 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.)

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