FYFD

Category: Phenomena

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    Self-Propelled Droplets

    Drops of ethanol on a heated surface contract and self-propel as they evaporate. My first thought upon seeing this was of Leidenfrost drops, but the surface is not nearly hot enough for that effect. Instead, it’s significantly below ethanol’s boiling point. Looking at the drops in infrared reveals beautiful, shifting patterns of convection cells on the drop. The patterns are driven by the temperature difference along the drop; at the bottom, the drop is warmest, and at its apex, it is coldest. Those differences in temperature create differences in surface tension, which drives a surface flow that breaks the drop’s symmetry. The asymmetry, the authors suggest, is responsible for the drop’s propulsion. (Image and video credit: N. Kim et al.)

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    Simulating Schools

    In nature, fish school for many reasons: protection from predators, increased sensing, and hydrodynamic advantages. To capture this complex behavior, researchers are building their own digital fish, governed by known rules. Here, scientists give each fish social rules — based on vision range and preferred distance from a neighbor — and hydrodynamic rules — based on a fish’s wake. With the rules in place, they can then observe the schooling behaviors of their digital fish. Like their real counterparts, these schools show different flocking based on apparent “moods”. (Image and video credit: J. Zhou et al.)

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    Draining a Bottle

    Turn a bottle upside-down to empty it, and you’ll hear a loud glug-glug-glug as the liquid in the bottle empties and air rushes in. In this video, researchers aim a high-speed camera at the very first bubble that forms during the process. Once the bubble reaches the wider area of the bottle, it tends to pinch off in the bottle’s neck. That creates a narrow jet that pierces the bubble and flies all the way to the other side, leaving a column of liquid inside the rising bubble. Increasing the fluid’s viscosity has remarkably little effect, at least until the liquid is extremely viscous. (Image and video credit: H. Mayer et al.)

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    Classifying Waves

    In a lab, researchers create their waves in a long, clear-sided tank, where they can observe how the waves form, travel, and interact. To generate the wave, they use a plate, attached to a piston. Push the water at one end, and a wave forms. The type of wave that forms depends on both the velocity and the stroke length of the piston, as shown in this video. By mapping out these two variables, researchers can observe all different sorts of waves, from peaceful solitary waves to wild, plunging breakers. (Image and video credit: W. Sarlin et al.)

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    In a Box, Shaken

    Tidal areas experience lots of oscillating, back-and-forth flow that builds up patterns in the sand below. In this experiment, researchers investigate a similar situation by filling a box with water and spherical particles, then shaking the box from side-to-side. Inside the box, the particles line up in chains that are perpendicular to the direction of oscillation (think sand ripples parallel to a shoreline). In this simplified system, the team can then look at what forces align the particles, how defects in the pattern shift, and what happens when the oscillation gets bigger. (Image and video credit: T. van Overveld et al.)

  • Vietnam’s Emerald Isles

    Vietnam’s Emerald Isles

    Vietnam’s Hạ Long Bay is home to more than 1,600 islands, many of them made up of mountainous limestone. The area is famous for its karst features, a type of terrain formed from highly porous, water-soluble rock. Over time, water dissolves and fractures the limestone, creating karst landscapes full of caves, springs, sinkholes, and fluted rock outcroppings. The area’s erosion also produces highly fertile soil, leading to a verdant ecosystem with many unique and endemic species. (Image credit: N. Kuring/NASA/USGS; via NASA Earth Observatory)

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    Pumping With Faraday Waves

    Vibrate a liquid pool vertically, and it will form a pattern of standing waves known as Faraday waves. Here, researchers confine those waves to a narrow ring similar in size to the wave. The confinement causes a type of secondary flow — a streaming flow — beneath the water surface. As a result, the wave pattern rotates around the ring. The applications of this rotation are pretty neat. As the team demonstrates, it can drive complex fluid networks and even create a pump! (Image and video credit: J. Guan et al.)

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    A Fractal Raft From a Spinning Top

    File this one under Cool Things I Would Have Never Thought Of. In this video, researchers play around with the flow around a spinning top and end up creating a fractal, granular raft. By immersing a top in dyed fluid, they show the toroidal vortices that form around the spinning toy. Then, instead of dye, they add a stretchy elastomer compound that cures over time. The elastomer stretches into thin ligaments in the swirling flow around the top. Eventually, it breaks apart into spherical drops of all different sizes.

    Once the top is removed, the elastomer drops slowly float to the surface. Surface tension and the Cheerios effect draw the drops together, and because of their many sizes, the rafts that form are fractal. (Image and video credit: B. Keshavarz and M. Geri)

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    DIY Superwalking Droplets

    Over the past few years, we’ve seen lots of research in walking droplets, especially as hydrodynamic quantum analogs. But did you know you can replicate this set-up at home and play with it yourself? This video gives an overview of the equipment you’ll need and a simple procedure to follow to get it up and running. From there, your imagination is the limit! (Image and video credit: R. Valani)

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    “Reconfiguring It Out”

    Leaves flutter and bend in the breeze, changing their shape in response to the flow. Here, researchers investigate this behavior using flexible disks pulled through water. The more flexible the disk and the faster the flow, the more cup-like the disk’s final shape. Adding tracer particles to the water allows them to visualize the flow behind the disk. Every disk leaves a donut-shaped vortex ring spinning in its wake, but the more reconfigured the disk, the narrower the vortex. This, ultimately, reduces drag on the disk. That’s why trees in heavy winds streamline their branches and leaves; that flexibility lowers the drag the tree’s roots have to anchor against. (Image and video credit: M. Baskaran et al.)