Month: July 2020

“Oooh !! My Delicious Coffee”
I’m not a coffee person, but Thomas Blanchard’s “Oooh !! My Delicious Coffee” manages to capture my favorite part of the beverage – watching cream and coffee mix. From feathery flows driven by surface tension to droplets floating like miniature cappuccinos, the short film features many of the fantastical landscapes we find when staring into a coffee cup. But don’t get too eager to drink it; Blanchard used a combination of coffee, oil, and paint to achieve those effects! (Image and video credit: T. Blanchard)
July 17, 2020
Crocodilian-Inspired Aerodynamics
Inspired by crocodilians, young scientist Angela Rofail designed attachments to reduce wind loads on high-rise buildings. When crocodilians swim, the ridges on their back help hide their motion from observation above the surface. Rofail wondered whether similar ridges would reduce the wind-induced swaying of high-rise buildings. Using a scale-model and crocodile-inspired knobs, the Year 10 student (read “high-school freshman” for U.S. readers) conducted wind tunnel tests that showed her modifications reduced drag on the model and kept it from moving in windy conditions. (Image credit: H. Roettger; video credit: CSIRO; via CSIRO; submitted by Kam-Yung Soh)

July 16, 2020
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)
July 15, 2020
Searching For Solar Neutrinos
An experiment in Italy has reported new findings confirming a long-standing theory of nuclear fusion in our Sun. The researchers were able to detect neutrinos released by the relatively rare fusion of carbon and nitrogen. But catching those neutrinos took an impressive fluid dynamical feat.
The Borexino solar-neutrino detector is essentially an enormous nylon balloon, filled with liquid hydrocarbons, immersed in water, and buried beneath a kilometer of rock. Most neutrinos fly through this milieu unhindered, but a few collide with hydrocarbon molecules, creating streaks of light picked up by the detector.
The challenge in distinguishing solar carbon-nitrogen neutrinos comes from an isotope in the balloon’s nylon lining, which slowly leaks into the detector. The noise caused by the leaking isotope is easily confused with the true solar signal. To tamp down on that noise level, the researchers took elaborate steps to ensure that all 278 tonnes of liquid in the detector remained at exactly the same temperature, thereby eliminating convection in the detector. With only molecular diffusion to move the noisy isotopes, the researchers held the liquid incredibly still. One team member described the fluid as moving only tenths of a centimeter a month! (Image credit: NASA SDO; via Nature; submitted by Kam-Yung Soh)
July 14, 2020
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.)
July 13, 2020
Alien Eggs? A Virus?
Nope, they’re hydrogel beads! The team at Chemical Bouillon seem to have once again coated them in something like paint before placing them in water. As the gel beads absorb water, they expand, tearing through their coating. The result is weirdly mesmerizing and kind of creepy. It’s no wonder that special effects artists have historically turned to fluids for sci-fi films! (Image and video credit: Chemical bouillon)
July 10, 2020
How Well Do Masks Work?
Many mixed messages have been spread about the efficacy of masks in preventing transmission of COVID-19. Nevertheless, there is good evidence that they help, as discussed in this video from It’s Okay to Be Smart. Much of the video shows schlieren imaging of a (healthy) individual engaging in regular activities – like talking, breathing, and coughing — with and without a cloth mask.
Now, it’s important to note that what you see in these images is airflow, not the droplets that can carry the virus. However, research has shown that these airflows play a significant role in transporting droplets. It follows that disrupting those airflows can disrupt transmission of diseases passed via droplet. This is one of the key reasons to wear a mask.
Notice how far jets and plumes of air fly from a maskless person’s mouth and nose. We cannot even observe how far momentum carries that air because the area visualized in this schlieren set-up is smaller than the full distance the air moves! But wearing a mask breaks up that flow structure. It reduces the air’s momentum, and it forces any air that does escape to move in smaller, less efficient structures. Even without considering any filtering effects or the fact that masks catch large droplets coming out of the wearer’s mouth, it’s clear that mask-wearing keeps others nearby safer. (Video and image credit: It’s Okay to Be Smart; references)
July 9, 2020
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)
July 8, 2020
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.)
July 7, 2020
Ejecting Water from a Smartwatch
Making electronics water-resistant can be a challenge, but as this Slow Mo Guys video demonstrates, engineers have some clever ways to deal with unwanted liquids. The Apple Watch, for example, uses its speakers to eject water that gets into the watch during immersion. As seen above, the vibration of the speakers ejects most of the water as tiny droplets. Occasionally, surface tension makes this tough and drops instead coalesce on the watch’s surface. To counter this tendency, the speakers sometimes pause, allowing water to collect before they begin vibrating again. (Video and image credit: The Slow Mo Guys)
July 6, 2020