FYFD

Category: Research

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    Blowing Vortex Rings from Bubbles

    When bubbles burst, we often pay attention to the retracting film and forming droplets, but what happens to the air that was inside? By placing a little smoke inside them, we can see. The air inside these bubbles is slightly pressurized compared to the ambient, and as such a bubble ruptures, its air gets pushed out the expanding hole. That momentum makes the air curl as it forces its way into the surrounding air, creating a stack of vortex rings. The researchers observed as many as six stacked vortices from bubbles just under 4 cm in diameter. (Image and research credit: A. Dasouqi and D. Murphy; video credit: Science; see also A. Dasouqi and D. Murphy)

  • Kneading Dough

    Kneading Dough

    Kneading bread dough is something of an art. The process binds flour, water, salt, and yeast into a network that is both elastic and viscous. It also traps pockets of air that will determine the texture of the final loaf. Underknead and the bubbles won’t form; overknead and the result will be a dense loaf that doesn’t rise in the oven.

    Capturing all of that physics in a realistic model is tough, but researchers have done so and validated their digital dough against experiments. The group focused on simulating industrial mixers, which knead dough with a moving, spiral-shaped rod rotating around a stationary vertical one. They found the industrial set-up did not mix as well as kneading by hand, but that could be improved by swapping the stationary rod for a second spiral one. (Image credit: G. Perricone; research credit: L. Abu-Farah et al.; via Physics World; submitted by Kam-Yung Soh)

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    Blooming Deposits

    Evaporate a droplet full of silica nanoparticles, and you’ll get beautiful, flower-like films. As the water evaporates, dry nanoparticles build up in a solid deposit. The evaporation creates a pressure gradient that pulls toward the center of the drop, forcing the deposit to bend. As stress builds in the deposit, cracks form petal-like segments. The number of cracks is indicative of how much of the drop was solid material; the higher the volume fraction of particles is, the fewer cracks form and the less the deposit bends. (Image, video, and research credit: P. Lilin et al.)

  • Creating Star Wars-Like Volumetric Displays

    Creating Star Wars-Like Volumetric Displays

    Despite their ubiquity in science fiction, volumetric displays — three-dimensional displays visible from any angle — have been tough to create in real life. But a team from the University of Sussex has made impressive strides using a system based on acoustic levitation.

    Here’s how it works: an array of ultrasonic speakers levitates and moves small plastic beads at up to 9 m/s. Simultaneously, LED lights project colors onto the sphere. Thanks to the human brain’s ability to create persistent images from the motion, we’re able to see simple displays like the figure-8 and smiley face above with the naked eye. To form something more complicated, like the spinning globe seen in the final image, the bead must be filmed using a camera with a slow shutter speed. But with that, the display looks incredible.

    There’s obviously a ways to go before your R2 unit can project holographic messages for you, but all the basic ingredients for that technology are here. Check out the coverage on Scientific American and the original research paper for more. (Image credit: Star Wars – Lucasfilm; others – E. Jankauskis; research credit: R. Hirayama et al.; via SciAm) 

  • Martian Landslides

    Martian Landslides

    Sometimes there are advantages to studying planetary physics beyond Earth. Mars does not have plate tectonics, vegetation, or the level of erosion we do, allowing geological features like those left behind by landslides to persist undisturbed for millions of years. And, thanks to a suite of orbiters, we’ve mapped most of Mars at a resolution better than many parts of our own planet. All together, this gives researchers a treasure trove of geological data from our nearest neighbor.

    One peculiar feature of many landslides is their long runout. Even over relatively flat ground, some landslides cover extreme distances from their point of origin. On Earth, we often see this behavior near glaciers, so scientists theorized that the presence of ice was somehow necessary for the landslide to cover such a long distance. But previous laboratory experiments with dry, ice-free grains showed the same behavior: long runouts marked with ridges running parallel to the flow. The mechanism behind the ridges is still somewhat unclear, but it seems to be connected to fluid dynamical instabilities that form between fast-flowing particles of differing density. But such results have been confined to lab-scale experiments and numerical simulations.

    A new report, however, shows that landslides on Mars share the same characteristic spacing and thickness between their ridges. This evidence suggests that the same ice-free mechanism could account for the long run-out of landslides on Mars and other planets. (Image credit: NASA/JPL-Caltech/University of Arizona; research credit: G. Magnarini et al.; via The Conversation; submitted by Kam-Yung Soh)

  • Drying Out

    Drying Out

    Look closely at old paintings, and you’ll notice arrays of tiny, straight cracks that form as the paint dried. This sort of pattern formation during drying is not unusual. Here we see the patterns formed when a thin layer of hydrogel sandwiched between two glass plates dries. As the water evaporates, stress builds at the interface between the air and gel, causing bubbles to form. The bubble size and shape depend on the size on the gap between the plates and the characteristics of the gel. The resulting patterns can be entirely disordered, or they can form worm-like designs that curl throughout the domain. (Image and research credit: R. Pic et al.)

  • Robotic Research Facilities

    Robotic Research Facilities

    One of the major challenges in fluid dynamics is the size of the parameter spaces we have to explore. Because many problems in fluid dynamics are non-linear, making small changes in the initial set-up can result in large differences in the results. Consider, for example, a simple cylinder towed through a water tank. As the cylinder moves, vortices will form around it and shed off the back, causing the cylinder to vibrate. The details of what will happen will depend on variables like the cylinder’s size and flexibility, the speed it’s being towed at, and which directions it’s allowed to vibrate in. Mapping out the parameter space, even sparsely, could take a graduate student hundreds of experiments.

    To speed up this process, engineers are now building robotic facilities like the Intelligent Towing Tank (ITT) shown above. Like graduate students, the ITT can work into the wee hours of the night, but, unlike graduate students, it never needs to eat, sleep, or stop experimenting. Now, one could use a facility like this to brute-force the answers by testing every possible combination of parameters, but even working 24 hours a day, that would take a long time. Instead, researchers use machine learning to guide the robotic facility into choosing test parameters in a way that optimizes the factors the researchers define as important.

    Essentially, the system starts with experiments chosen at random within the parameter space, and then uses those results to select areas of interest until it’s gathered enough data to satisfy the limits specified by human researchers. In theory, a well-designed algorithm can dramatically reduce the number of experiments needed to explore a parameter space. (Image and research credit: D. Fan et al.; submitted by Kam-Yung Soh)

  • Falling Drops and Forming Stalagmites

    Falling Drops and Forming Stalagmites

    The vast stalactites and stalagmites found in caves take millennia to form. Mineral-rich water seeps down the icicle-like stalactites and then drips onto stalagmites below, each drop depositing a little more calcite onto the growing rock. By observing this dripping action first-hand, researchers found that most falling drops create a splash that’s much smaller than the width of the stalagmite they fall onto. So how do stalagmites end up so wide?

    It turns out that there’s a large variance in where drops hit the stalagmite. There’s no wind in these caves to push the droplets, so researchers concluded the drop’s trajectory depends on the vortices it sheds as it falls. A drop that falls from a short height will have a vertical trajectory. But once the drop is falling tens of meters, it can end up as many as several centimeters to the side of where it would fall in a vacuum. This scatter-shot variation in drop impacts is what enables stalagmites to grow so wide. (Image and research credit: J. Parmentier et al., source; via NYTimes; submitted by Kam-Yung Soh)

  • Understanding Wildfire

    Understanding Wildfire

    Wildfires are an ongoing challenge in the western United States, where droughts and warmer conditions have combined with a century of fire suppression to form perfect conditions for monstrous fires. It’s long been understood that ambient winds can drive spreading fire, but the connection between wildfire and wind is more complicated than this.

    The heat of a fire drives buoyant air to rise, creating tower-like updrafts in a flame front. We see this both in the shape of the grass fire above, and in the wind vectors of a simulated grass fire in the lower image. Between those towers are troughs where cooler ambient wind is drawn in to replace the rising air. How a fire spreads will depend on the speed, direction, and temperature of these winds. A hot wind fed by the fire’s heat will raise the temperature of fuel in unburned areas, bringing it closer to ignition. In contrast, cooler ambient winds can hinder a fire by keeping nearby grass and twigs too cool to ignite. (Image credit: fire – M. Finney/US Forest Service; simulation – R. Linn; research credit: R. Linn et al.; for more, see Physics Today)

  • Surfing Honeybees

    Surfing Honeybees

    Honeybees have superpowers when it comes to their aerodynamics and impressive pollen-carrying, but their talents don’t end in the air. A new study confirms that honeybees can surf. Wet bees cannot fly–their wings are too heavy for them to get aloft when wet–but falling into a pond isn’t the end for a foraging honeybee.

    Instead, the bee flaps its wings, using them like hydrofoils to lift and push the water. This action generates enough thrust to propel the bee three body lengths per second. It’s a workout the bee can only maintain for a few minutes at a time, but researchers estimate honeybees could cover 5-10 meters in that time. Once ashore, the bee spends a few minutes drying itself, and then flies away no worse for the wear. (Image and research credit: C. Roh and M. Gharib; via NYTimes; submitted by Kam-Yung Soh)