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)
Photographer Lucie Pollet caught this image of her freediving friend ascending through a plume of bubbles and sunlight. I love the otherworldliness of the image, like the diver is an astronaut in the dark of space. The illumination of the bubbles is spectacular, too, and reminds me of the way penguins use supercavitation to help escape predators. (Image credit: L. Pollet; via Oceanographic Magazine)
During digestion, our intestines use two different patterns of muscle contraction to move food through our bodies. Scientists have long wondered why we have this added complexity. Using numerical simulations of the fluid flow created by these contractions, researchers have uncovered the answer.
Our intestines use peristalsis, a forward-with-occasional-backward flow pattern, as the main driver. The strength of the muscle contractions determines how fast the average flow speed is. When the speed is slow, our bodies have more time to absorb nutrients, but that also allows more time for bacteria to flourish on those same nutrients. The other flow pattern, segmentation, creates a weaker flow overall but with much more mixing, which again enhances nutrient uptake.
Switching between the two patterns, the researchers found, gives the body the best of both. Segmentation can enhance mixing and provide good nutrient uptake, then peristalsis can move the contents along quickly enough that bacteria don’t have time to grow before getting flushed out. (Image credit: Kindel Media; research credit: A. Codutti et al.; via APS Physics)
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.)
Mixing two fluids is a tougher task than you might think. One of my favorite asides from a fluids lecture concerned how to mix fruit into yogurt in an industrial setting. Mix too quickly, and you’ll obliterate the yogurt’s consistency, but mix too little and you may as well sell it as fruit-on-the-bottom. Apparently that particular problem got solved by sending the fruit and yogurt flowing through a series of specially-shaped ducts to slowly and carefully mix them together.
In this study, researchers tackle a similar problem — mixing two fluids in a circular cross-section — through optimization. As you can see above, circular stirrers on their own don’t do a great job of mixing. So the researchers searched for the right combination of stirrer shape, mixing speed, and mixing trajectory to give the best mixing for a set mixing time and energy input. Their final stirrer shapes are anything but circular and often move in jerks and fits to help shed vortices that do the actual job of mixing. (Image and research credit: M. Eggl and P. Schmid; via APS Physics)
Those of us who live in urban environments have experienced the clear, pollution-free air that comes after a rainstorm. But how exactly does rain clean the air? Air pollution typically has both gaseous and particulate components to it. As a raindrop falls, it experiences collision after collision with those particles. Depending on the particle’s surface characteristics — is it hydrophilic or hydrophobic? — and its momentum during impact, it can get trapped in the raindrop, skip off, or even pass through entirely. The physics, it turns out, are identical to those of a rock falling into or skipping off a lake — even though the raindrop and particle might be 1000 times smaller! (Image and video credit: N. Speirs et al.)
Australian photographer Ray Collins captures some of the most impressively dynamic photographs of ocean waves I’ve ever seen. The textures of the water range from glassy smooth to scaled to violent sprays of droplets. You can easily get lost in every image. For more, check out his website and Instagram. (Image credits: R. Collins; via Colossal)
Keeping wounds safe and clean is hard when bandages are so prone to coming off. A team of researchers may have found a solution, though, using ultrasound to enhance adhesion. For their technique, they applied a layer of adhesive primer to the skin and covered it with a hydrogel bandage. Then they used an ultrasound transducer to generate cavitation bubbles in the primer. As the bubbles grew and collapsed, the primer and hydrogel pulled toward the tissue, creating adhesive bonds up to 100 times greater than without ultrasound. The extra adhesion had staying power, too, with between two and ten times more fatigue resistance than the bandage and adhesive alone. The researchers hope their technique will aid tissue repair, wound management, and attaching wearable electronics. (Image and research credit: Z. Ma et al.; via Physics World)
Two vertical fibers, with a gap left between them, form a playground for flow in this Gallery of Fluid Motion video. If the fiber spacing is small enough, the flow will form a stable liquid sheet that runs the full length of the fibers. With a little more distance, though, the fluid forms intermittent bridges, whose spacing depends on flow rate. And when the fibers are not perfectly vertical, even more complex flows are possible. I love how a seemingly simple situation begets such complexity! (Image and video credit: C. Gabbard and J. Bostwick)
One enduring mystery of the solar wind — a stream of high-energy particles expelled from the sun — is how the particles get accelerated in the first place. The sun frequently belches out spurts of plasma, but without further momentum, that material simply falls back to the sun’s surface under the star’s gravity. Mechanisms like shock waves can further accelerate particles that are already moving quickly, but they cannot explain how the particles get going in the first place.
A recent study used supercomputers to tackle this challenging problem in turbulent plasma physics. Each simulation tracked nearly 200 billion particles, requiring tens of thousands of processors. The results showed that turbulence itself provides the necessary initial acceleration and serves as the first step to getting particles moving fast enough to escape the sun. (Image credit: NASA SDO; research credit: L. Comisso and L. Sironi; via Physics World)