If you’ve ever clapped near a wall with a corrugated surface, you may have noticed some strange echoes. Surfaces like these can cause a chirping sound to observers. The reason, as Nick Moore explains in the video above, is that the original sound reflects off the corrugations at different times and travels back to the observer such that the first reflections to arrive are closely spaced (and thus higher pitched) while the later reflections are spread further out. This creates a chirp that starts at a high pitch and then falls to lower ones. Have you ever come across structures that do this? (Video credit: N. Moore)
Category: Phenomena
Watching Radiation
We’re used to radiation being invisible. With a Geiger counter, it gets turned into audible clicks. What you see above, though, is radiation’s effects made visible in a cloud chamber. In the center hangs a chunk of radioactive uranium, spitting out alpha and beta particles. The chamber also has a reservoir of alcohol and a floor cooled to -40 degrees Celsius. This generates a supersaturated cloud of alcohol vapor. When the uranium spits out a particle, it zips through the vapor, colliding with atoms and ionizing them. Those now-charged ions serve as nuclei for the vapor, which condenses into droplets that reveal the path of the particle. The characteristics of the trails are distinct to the type of decay particle that created them. In fact, both the positron and muon were first discovered in cloud chambers! (Image credit: Cloudylabs, source)
Glacial Remains
The high walls of this alpine canyon were cut by flowing glacial ice. This type of amphitheater-shaped valley is known as a cirque. The photo shows one of the Chicago Lakes on Mount Evans in the Colorado Rockies. The glacier that once sat here carved the steep walls you see in the background but also hollowed out a series of depressions like the ones shown in the figure below. When temperatures warmed and the glacier melted, it left behind a series of three small lakes, or tarns, like the one in the photo above. Cirques are found throughout the mountain ranges of the world. (Image credit: Mt. Evans – J. Shoer; cirque formation – DooFi)
Tendrils of Fog
Fog snakes its way from the ocean into the Strait of Juan de Fuca in this animation constructed from satellite imagery. The strait lies between Vancouver Island and the Olympic Peninsula in the Pacific Northwest. Fogs like this form when skies are clearer and heat from the surface is able to escape upward. The surface air then cools and condenses into fog. Steady winds pushed fog into the strait over the course of about 9 hours. There’s a remarkable level of detail in the satellite images, taken by the new GOES-16 satellite that launched in late 2016. Notice the ragged wave front as the fog stretches eastward and the shock-wave-like lines behind it in the strait. Both result from interactions between the fog cloud and the shape of the land masses it’s encountered. (Image credit: NASA Earth Observatory)
Cavitating
Cavitation happens when the local pressure in a liquid drops below its vapor pressure. A low-pressure bubble forms, typically very briefly, when this occurs. These bubbles are spherical unless they form near a surface. In that case, the bubbles take on a flatter, oblong shape. As they collapse, the bubbles form a jet, like the one seen inside the bubble above. The jet extends through the bubble and stretches into a funnel shaped protrusion on the bubble’s far side. Eventually, the whole shape becomes unstable and breaks into many smaller bubbles. Shock waves can be generated in the collapse, too; often the jet generates at least two in addition to the ones created when the bubble reaches its minimum size. This is part of why cavitation can be so destructive near a surface. (Image credit: L. Crum)
Shadows of Flow
In the latest Veritasium video, Derek demonstrates how to see gas motions that are normally invisible using a schlieren photography set-up. Schlieren techniques have been important in fluid dynamics for well over a century, and Derek’s set-up is one of the two most common ways to set up the technique. (The other method uses two collimating mirrors instead of a single spherical or parabolic one.) As explained in the video, the schlieren optical set-up is sensitive to small changes in the refractive index, making density changes or differences in a gas visible. This makes it possible to distinguish gases of different temperatures or compositions and even lets you see shock waves in supersonic flows. (Video and image credit: Veritasium; submitted by Paul)
Perijove
The Juno spacecraft continues to send back incredible photos of Jupiter’s atmosphere. This video animates images from the sixth close pass of Jupiter to give you a sense of what Juno sees as it swoops by our system’s largest planet. The trajectory passes from the north pole to the south, showing Jupiter’s whitish zones, dark belts, and massive storms. Up close Jupiter looks like an Impressionist painting, all vortices and shear instabilities. The large white spots you see are enormous counterclockwise rotating vortices known as anticyclones – many of them larger than our entire planet. (Video credit: NASA / SwRI / MSSS / G. Eichstädt / S. Doran)
Capillary Action in Microgravity
On Earth, gravity dominates over many fluid effects, but in microgravity a different picture emerges. This animation shows a two-channel apparatus partially filled with silicone oil being dropped. While in free-fall, the liquid experiences microgravity conditions and the height of the fluid in the two connected channels changes. The oil meniscus climbs up the walls of the tubes thanks to capillary action. This is the result of intermolecular forces between the liquid and solid walls. Capillary action is most effective in narrow tubes where surface tension and the adhesion between the liquid and solid can actually propel liquid up the walls, as seen here. On Earth we mostly ignore capillary action, except in very small spaces, but for space systems, it is a major force to reckon with in designing flows. (Image credit: NASA Glenn Research Center, source)
Building Labs on a Chip
In their second video on microfluidics, the Lutetium Project takes viewers inside the process of creating microfluidic circuits, also known as labs-on-a-chip. When you want to build pipes only a few microns across, you need to use special techniques. As the video shows, manufacturing starts with photolithography, a process used to selectively mask parts of the substrate which are then etched away chemically. This creates a mold that’s later covered in a polymer solution. Once hardened, the polymer is removed from the mold, treated and attached to a glass slide. The result is a tiny fluid circuit that’s only a few square centimeters in total size. To really appreciate the process, check out the video, which helpfully takes you inside the clean room to see the chip manufacturing process firsthand. (Video and image credit: The Lutetium Project)
Breaking Waves in the Sky
Under the right atmospheric conditions, clouds can form in a distinctive but short-lived breaking wave pattern known as a Kelvin-Helmholtz cloud. The animation above shows the formation and breakdown of such a cloud over the course of 9 minutes early one morning in Colorado’s Front Range region. Kelvin-Helmholtz instabilities occur when fluid layers with different velocities and/or densities move past one another. Friction between the two layers moving past creates shear and causes the curling rolls seen above.
In the background, you can also see a foehn wall cloud low to the horizon. This type of cloud forms downwind of the Rocky Mountains after warm, moist Chinook winds are forced up over the mountains, cool, and then condense and sink in the mountains’ wake. (Image credit and submission: J. Straccia, more info)
