Sneezing can be a major factor in the spread of some illnesses. Not only does sneezing spew out a cloud of tiny pathogen-bearing droplets, but it also releases a warm, moist jet of air. Flows like this that combine both liquid and gas phases are called multiphase flows, and they can be a challenge to study because of the interactions between the phases. For example, the buoyancy of the air jet helps keep smaller droplets aloft, allowing them to travel further or even get picked up and spread by environmental systems. Researchers hope that studying the fluid dynamics and mathematics of these turbulent multiphase clouds will help predict and control the spread of pathogens. Check out the Bourouiba research group for more. (Video credit: Science Friday)
Tag: physics
“Smoke”
Ethereal forms shift and swirl in photographer Thomas Herbich’s series “Smoke”. The cigarette smoke in the images is a buoyant plume. As it rises, the smoke is sheared and shaped by its passage through the ambient air. What begins as a laminar plume is quickly disturbed, rolling up into vortices shaped like the scroll on the end of a violin. The vortices are a precursor to the turbulence that follows, mixing the smoke and ambient air so effectively that the smoke diffuses into invisibility. To see the full series, see Herbich’s website. (Image credits: T. Herbich; via Colossal; submitted by @jchawner, @__pj, and Larry B)
P.S. – FYFD now has a page listing all entries by topic, which should make it easier for everyone to find specific topics of interest. Check it out!
Bioluminescence
In the dark of the ocean, some animals have evolved to use bioluminescence as a defense. In the animation above, an ostracod, one of the tiny crustaceans seen flitting near the top of the tank, has just been swallowed by a cardinal fish. When threatened, the ostracod ejects two chemicals, luciferin and luciferase, which, when combined, emit light. Because the glow would draw undesirable attention to the cardinal fish, it spits out the ostracod and the glowing liquid and flees. Check out the full video clip over at BBC News. Other crustaceans, including several species of shrimp, also spit out bioluminescent fluids defensively. (Image credit: BBC, source video; via @amyleerobinson)
The Churning of Corals
Corals may appear static, but near the surface the tiny hair-like cilia of these polyps are churning the water. Although it has been known for some time that corals have cilia, scientists had previously assumed they only moved water parallel to the coral’s surface. Instead recent flow visualizations show that the cilia’s movements generate larger-scale vortical flows near the coral that can help draw fresh nutrients in as well as flush waste away. This means that, instead of being reliant on currents and tides, corals can exert some control on their environment in order to get what they need. This insight into coral cilia may shed some light on the micro- and macroscopic flows generated by other cilia, like those in our lungs. For a similar example of seemingly-passive organisms generating their own flows, check out how mushrooms create air currents to spread their spores. (Image credits: O. Shapiro et al. and MIT News; source video; h/t to Katie B)
Soap Film Physics
Soap films consist predominantly of water, yet their thin, virtually two-dimensional nature is impossible for water alone to achieve. The small amount of added soap acts as a surfactant, lowering the surface tension of the fluid and preventing it from bursting into droplets. When forming a film, the soap molecules align themselves along the outer surfaces of the film, with their hydrophilic heads among the water molecules and their hydrophobic tails oriented outward. For the most part, the water molecules stay sandwiched between the surfactant layers, forming a film only about as thick as the wavelength of visible light. In fact, the psychedelic colors of a soap film are directly related to the film’s thickness with the black regions being the thinnest. The video above shows a horizontal soap film at the microscopic scale and some of the dynamics exist therein. (Video credit: J. Hart)
ALS Ice Bucket Challenge
When fluid dynamicists get into the ALS ice bucket challenge, they give it a good fluidsy twist. Here are some selections, including lots of high speed video and an infrared video. Check out all those liquid sheets breaking up. Links to the full videos are below. (Image credits: Ewoldt Research Group, source video; TAMU NAL, source video; BYU Splash Lab, source videos 1, 2, 3, 4)
Breaking Drops with Vibration
Atomization is the process of breaking a liquid into a spray of fine droplets. There are many methods to accomplish this, including jet impingement, pressure-driven nozzles, and ultrasonic excitement. In the images above, a drop has been atomized through vibration of the surface on which it rests. Check out the full video. As the amplitude of the surface’s vibration increases, the droplet shifts from rippling capillary waves to ejecting tiny droplets. With the right vibrational forcing, the entire droplet bursts into a fine spray, as seen in the photo above. The process is extremely quick, taking less than 0.4 seconds to atomize a 0.1 ml drop of water. (Photo and video credit: B. Vukasinovic et al.; source video)
Death Valley’s Roaming Rocks
The mystery of the roaming rocks of Death Valley’s Racetrack Playa may be at an end. Since their discovery in the 1940s, researchers have speculated about what conditions on the playa could cause 15+ kg rocks to slide tens or hundreds of meters across the dry lakebed. But the rare nature of the movement and the remoteness of the location had prevented direct observation of the phenomenon until last December when a research team caught the rocks in motion (see the timelapse animation above or the source video). Winter rain and snow had created a shallow ice-encrusted pond across the playa by the time the researchers arrived to check their previously installed equipment. Late one sunny morning, the melting ice, only millimeters thick, cracked into plates tens of meters wide and began to move under the light breeze (~4-5 m/s). Despite its windowpane-like thickness, the ice pushed GPS-instrumented rocks up to hundreds of meters at speeds of 2-5 m/min. It took just the right mix of conditions–sun, wind, snow, and water–but the two ice-shoving instances the team observed go a long way toward explaining the sailing rocks. (Image credits: R. Norris et al.; J. Norris, source video; NASA Goddard; via Discover and SciAm)
Seahorse Hunting
Those who have observed the languid pace of seahorses or seadragons swimming might think these fish only hunt slow prey. In fact, the tiny crustaceans on which they feed are extremely quick, capable of velocities over 500 body lengths per second. Instead of speed, the seahorse relies on stealth to capture its prey, as shown in the holographic video above. Seahorses use a pivot method to feed, simultaneously shifting their snouts up and sucking water in to catch their target. But this method of feeding only works for distances of about 1 mm. To get that close in the first place, the seahorse must approach its prey without alerting it. Researchers found that both the seahorse’s head shape and its natural posture create a hydrodynamic quiet zone just off the seahorse’s snout, directly in its strike zone. Fluid velocity and deformation rates in this region are significantly lower than those around the rest of the seahorse’s face when it moves, allowing the fish to sneak up on its prey. These adaptations are remarkably effective, too; the researchers observed that the seahorses were able to position themselves within 1mm of their prey without alerting them 84% of the time. (Video credit: B. Gemmell et al.; via Discover)
Frisbee Physics, Part 2
Yesterday we discussed some of the basic mechanics of a frisbee in flight. Although frisbees do generate lift similarly to a wing, they do have some unique features. You’ve probably noticed, for example, that the top surface of a frisbee has several raised concentric rings. These are not simply decoration! Instead the rings disrupt airflow at the surface of the frisbee. This actually creates a narrow region of separated flow, visible in region B on the left oil-flow image. Airflow reattaches to the frisbee in the image after the second black arc, and the boundary layer along region C remains turbulent and attached for the remaining length of the frisbee. Keeping the boundary layer attached over the top surface ensures low pressure so that the disk has plenty of lift and remains aerodynamically stable in flight. A smooth frisbee would be much harder to throw accurately because its flight would be very sensitive to angle of attack and likely to stall. (Image credits: J. Potts and W. Crowther; recommended papers by: V. Morrison and R. Lorentz)
