FYFD

Category: Research

  • The Challenges of Being Small

    The Challenges of Being Small

    For juvenile fish, feeding is a challenge. Their small size — often less than 5 mm in length — makes hydrodynamically capturing prey much harder because of viscosity’s relatively larger effect on them. But size may not be the only factor in determining their success, as a new study shows.

    Researchers studied feeding behaviors of two, equally-sized species’ larvae: zebrafish and guppies. The biggest difference between these two species is their developmental time prior to beginning to hunt on their own. Guppies develop five times longer than zebrafish larvae before they start feeding.

    Both fish have the same hydrodynamic limitations to overcome. If you look closely at the first image, you’ll see fluid being pushed ahead of the fish as it swims. The researchers refer to this as a bow wave, and it effectively announces to any prey that the fish is approaching. To sneak up on prey, the fish has to be able to generate enough suction force to pull its food in from beyond the bow wave’s reach. The experiments showed that guppies were able to do this reliably, while zebrafish could not. The subsequent difference in their feeding success was stark: the guppies’ success rate was almost five times that of the zebrafish! (Image and research credit: T. Dial and G. Lauder, source; via G. Lauder)

  • When Shear Meets Slip

    When Shear Meets Slip

    One of the classic concepts students learn early in their fluids education is the no-slip condition. In essence, this idea says that friction between a solid object — say, a wall — and the fluid immediately next to it is such that no movement is possible where they meet. The fluid cannot “slip” along the surface, hence “no-slip”. It’s a simple concept, but one that can create a lot of complexity in practice.

    Imagine, for example, a fluid sandwiched between two surfaces: one stationary and one moving at a constant speed. This movement creates a shear flow, in which the velocity of the fluid varies from the speed of the moving plate all the way down to zero, the speed of the stationary plate. If we placed a little platelet in the middle of this flow, we’d expect it to rotate because of the faster flow on one side.

    But a new paper finds something rather different, at least when considering an extremely small nanoplatelet. With a tiny enough plate, individual molecules can slip along the surface, and when that happens, instead of rotating, the nanoplatelet aligns itself with the flow. That alignment means the added particle would disturb the flow less, creating a lower viscosity and better flowability. (Image and research credit: C. Kamal et al.; submitted by Simon G.)

  • Shake It!

    Shake It!

    Vibrate a pool of water, and you’ll get Faraday waves, ripple-like excitations that form their own distinctive pattern compared to the driving vibration. But you don’t have to vibrate a pure liquid to see Faraday waves. A recent study observed them in vibrated earthworms!

    Odd as this may sound, the results make sense. When anesthetized (as they were in the experiments), earthworms are essentially a liquid wrapped in an elastic membrane, which is not so different from a droplet held together by surface tension.

    But why vibrate earthworms in the first place? It turns out earthworms are a good model organism for studies of vertebrate neural systems, so observing how vibrations propagate through them can provide insight into how our own nervous systems transmit information. (Image, research, and submission credit: I. Maksymov and A. Pototsky)

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    Leaping Hoops

    Some water-walking insects are able to leap off a watery interface. One way to model these creatures is with elastic hoops, which can also propel themselves off the water’s surface. In this video, researchers explore some of the factors that affect the jump, like hoop geometry, material, and hydrophobic coatings. Wider hoops jump better than thinner ones because they can store more elastic energy. Hydrophobic hoops also leap higher, because less energy gets wasted in splash creation. Since most water-walking insects have hydrophobic legs already, that’s a bonus for jumping off the surface! (Image, video, and research credit: H. Jeong et al.)

  • Branching Sparks from Senko-hanabi

    Branching Sparks from Senko-hanabi

    Senko-hanabi are a Japanese firework, somewhat similar to a sparkler. But instead of being driven by burning powder, the senko-hanabi’s sparks come from bursting liquid droplets undergoing an exothermic reaction with air.

    Chemistry aside, the effect is similar to what goes on in soda water. As bubbles within the liquid nucleate and move to the surface, they burst, generating smaller droplets. As the researchers explain, the same cascade carries on in the smaller drops, creating the branching sparks the firework is known for.

    For more slow motion views of the fireworks and sparks, check out the video below or those produced by the researchers. (Image and research credit: C. Inoue et al. and C. Inoue et al.; video credit: NightHawkInLight; submitted by Jason C.)

  • The Vortex Beneath a Drop

    The Vortex Beneath a Drop

    While we’re most used to seeing levitating Leidenfrost droplets on a solid surface, such drops can also form above a liquid bath. In fact, the smoothness of the bath’s surface, combined with mechanisms discussed in a new study, means that drops will levitate at a cooler temperature over a liquid than they will over a solid surface.

    Researchers found that a donut-shaped vortex forms in the bath beneath a levitating droplet, but the direction of the vortex’s circulation is not always the same. For some liquids, the flow moves radially outward from beneath the drop. In this case, researchers found that the dominant force was shear stress caused by the vapor escaping from under the droplet.

    With other droplet liquids, the flow direction instead moved inward, forming a sinking plume beneath the center of the drop. In this situation, researchers found that evaporative cooling dominated. As the liquid beneath the droplet cooled, it became denser and sank. At the same time, the lower temperature changed the bath’s local surface tension, creating the inward surface flow through the Marangoni effect. (Image credit: F. Cavagnon; research credit: B. Sobac et al.)

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    Mimicking Supernovas

    The Hubble archives are full of incredible swirls of cosmic gas and dust, many of which were born in supernovas. Predicting the forms these massive explosions will generate is extremely difficult, thanks in large part to the complicated fluid dynamics generated by their blast waves. But new lab-scale experiments may help shed light on those underlying processes.

    Researchers mimic supernovas in the lab by launching blast waves through an interface between a dense gas (shown in white) and a lighter one (which appears black). As the blast wave passes, it drives the dense fluid into the lighter one, triggering a series of instabilities. Notice how any initial perturbations in the interface quickly grow into mushroom-like spikes that rapidly become turbulent. This behavior is exactly what’s seen in supernovas (and in inertial confinement fusion)! (Video credit: Georgia Tech; research credit: B. Musci et al.; submitted by D. Ranjan)

  • Mossy Vortex Rings

    Mossy Vortex Rings

    Many plants have evolved an ability to move remarkably quickly. Often, this capability is driven by water. Here we see the moss Sphagnum affine, which disperses its spores explosively. The process is triggered by the spore capsule gradually drying out; its shape changes from round to cylindrical, pressurizing the capsule. Once the internal pressure is high enough to overcome the strength of the capsule’s upper membrane, the capsule bursts, sending a plume of spores aloft. The sudden release of spore-laden air forms a vortex ring, which lifts the spores higher far more efficiently than they would be otherwise. (Image credit: capsule dry-out – J. Edwards et al., spore dispersal – J. Edwards et al. 2010; research credit: J. Edwards et al.)

  • Scaling High-Speed Impacts

    Scaling High-Speed Impacts

    The impact of a solid object into a bed of grains is a major topic in many fields from ballistics to astronomy. Researchers study these impacts experimentally using photoelastic disks, which display visible stress patterns when placed between polarizers. The lightning-like patterns you see above reveal how forces propagate inside the grains as the object hits.

    Researchers focused on the peak forces generated during high-speed impacts, an area that hasn’t been well-captured by existing impact models. They found that this peak force obeys its own scaling laws that depend on factors like impact speed, impacter size, grain stiffness, and grain density. (Image and research credit: N. Krizou and A. Clark)

  • Dissolving Caramel

    Dissolving Caramel

    In nature, erosion patterns are driven by the interactions of flow and topography. Here, researchers study that process in the lab by placing an inclined block of caramel in quiescent syrup and watching as it dissolves. Initially, the bottom surface of the block develops regularly-spaced plumes — the dark lines seen in the first image. But because the caramel-laden plumes are heavier than the surrounding fluid, the flow quickly becomes unstable. The plumes cross one another and begin to carve chevrons into the caramel.

    The chevrons appear to march their way upward in the video. They slowly grow and change into a distinctly scalloped pattern. Scallops like these are often seen by geologists in caves and icebergs, and the authors argue that their results and modeling indicate the importance of buoyant flow effects on such natural formations. (Image and research credit: C. Cohen et al.)