Category: Phenomena

Popping
Popcorn’s explosive pop looks pretty cool in high-speed video, but just watching it with a regular camera doesn’t show everything that’s going on. If we take a look at it through schlieren optics, the kernel’s pop looks even more extraordinary:
The schlieren technique reveals density differences in the gases around the corn–effectively allowing us to see what is invisible to the naked eye. The popcorn kernel acts like a pressure vessel until the expansion of steam inside causes its shell to rupture. The first hints of escaping steam send droplets of oil shooting upward. The kernel may hop as steam pours out the rupture point, causing the turbulent billowing seen in the animation above. As the heat causes legs of starch to expand out of the kernel, they can push off the ground and propel the popcorn higher. As for the eponymous popping sound, that is the result of escaping water vapor, not the actual rupture or rebound of the kernel! See more of the invisible world surrounding a popping kernel in the video below. (Image credits: Warped Perception, source; Bell Labs Ireland, source; WP video via Gizmodo; BLI video submitted by Kevin)
https://youtu.be/Mnf5HgM292s
January 31, 2017
As the Dust Blows In
This towering cloud of dust is known as a haboob, and while it appears apocalyptic, it is a relatively common occurrence in parts of the world, including the U.S. southwest and the Middle East. Haboobs often form when a collapsing thunderstorm releases a downburst of cold air. That wind picks up loose dust along the ground and creates a wall of sediment that may be as much as 100 kilometers wide and several kilometers tall. Inside the haboob, winds can reach speeds as high as 100 kph and visibility can be reduced to nearly zero. Because of this, the storms can be quite dangerous, especially to anyone who attempts to drive during one. (Image credit: D. Bryant)
January 26, 2017
Wrapping Up
It’s often at the intersection of topics that we can learn something new and fascinating. The latest video from The Lutetium Project shows examples of this at the intersection of solid mechanics and fluid dynamics with a look at elastocapillarity. Breaking that word down, that’s where elasticity – that stretchy quality associated with solids – meets capillarity – the surface-tension-dominated behavior of a fluid. In particular, they explore some of the mind-boggling and surprising interactions that happen between drops, bubbles, and thin flexible fibers smaller than the width of a human hair. Check out the full video below. (Images credit: K. Dalnoki-Veress et al.; video credit: The Lutetium Project)
January 25, 2017
Vibrated to Bits
Sound and vibration can be powerful tools for controlling liquids. In this animation, a water/glycerin drop violently bursts into a cloud of droplets when it is vibrated vertically 1000 times per second by a piezoelectric actuator. This vibration shakes the drop with accelerations of 150 g. Initially, the amplitude is small enough to simply create ripples around the drop’s circumference. As it increases, the drop deforms more at the edges and starts to eject droplets there. When the vibration hits a critical amplitude, the entire drop explodes into droplets. The technique is called vibration-induced droplet bursting, and its near-instantaneous ability to atomize drops makes it a candidate for applications like spray cooling microprocessors or spray coating a solid surface. (Video credit: B. Vukasinovic, source)
January 24, 2017
Turbine Wakes in the Sea
What we we build always has an impact on the environment around us. The white dots you see in the image above are an array of offshore wind turbines, standing in waters 20 to 25 meters deep. The brownish lines extending from each turbine show the underwater wakes of the turbines, colored by the sediment they’ve picked up. As with trees in a snowstorm, the currents flowing past the base of the turbine likely form a horseshoe vortex that lifts up the sediment into the wake. Because the tides in this area reverse direction every six hours, these sediment plumes can appear quite dynamic in satellite imagery, frequently changing strength and direction. (Image credit: NASA Earth Observatory)
January 18, 2017
Pascal’s Barrel
Pascal’s Law tells us that pressure in a fluid depends on the height and density of the fluid. This is something that you’ve experienced firsthand if you’ve ever tried to dive in deep water. The deeper into the water you swim, the greater the pressure you feel, especially in your ears. Go deep enough and the pressure difference between your inner ear and the water becomes outright painful.
In the video demonstration above, you’ll see how a tall, thin tube containing only 1 liter of water is able to shatter a 50-liter container of water. Not only does this show just how powerful height is in creating pressure in a fluid, but it shows how a fluid can be used to transmit pressure over a distance – one of the fundamental principles of hydraulics! (Video credit: K. Visnjic et al.; submitted by Frederik B.)
January 16, 2017
Washington Ice Disk
Winter weather in northern latitudes sometimes brings with it unusual phenomena like this ice disk spinning in the Middle Fork Snoqualmie River in Washington state. Photographer Kaylyn Messer ventured out to capture photos and videos of the event over the weekend. There are a couple theories as to how such disks form, but swirling river eddies are a key ingredient. One theory posits that chunks of ice forming on the river get caught up by the spinning eddy and slowly freeze together to form the disk. Another theory proposes that the disks occur when an existing chunk of ice breaks away, gets caught in the spinning eddy and slowly has its edges ground down into a circle. Personally, I lean toward the former explanation, though there is likely grinding at the edges either way. See more about this ice circle over at Messer’s blog. (Image credit: K. Messer; GIF by @itscolossal; via Colossal)
January 11, 2017
Sedimentary Swirls
Sediment swirls in Bear Lake caught the eye of an astronaut aboard the International Space Station last year. Bear Lake is situated in the Rocky Mountains, on the Idaho-Utah border. The eddies in the center of the lake are each about 3 km across and are likely the result of inflow from the lake’s tributaries. Silt and sediment picked up by the rivers and streams gets deposited into Bear Lake, revealing the turbulent mixing of tributary waters with those already in the lake. (Image credit: NASA; via NASA Earth Observatory)
January 3, 2017
Why Ice is Slippery
Ice is slippery. This is a fundamental fact we humans have dealt with so often that we rarely take the time to ask why. Other solids aren’t inherently slippery, so what is it that makes ice so? Remarkably, scientists only began to ask this question and propose theories within the past couple hundred years. One common suggestion is that the high pressure of an ice skate on ice locally melts the ice, creating a thin liquid layer a skater glides across. But this does not explain why ice is slippery for shoes or tires, nor why it’s possible to ice skate at more than a few degrees below freezing. Several other effects may be in play, such as frictional heating or the peculiar molecular forces between water molecules. Current research suggests that ice has a thin liquid layer tens or hundreds of nanometers thick that causes its slippery nature. For a great review of the subject, see Robert Rosenberg’s Physics Today article. (Video credit: SciShow)
December 30, 2016
Superhydrophobic Coatings
Superhydrophobic–or water repellent–materials are much sought after. Their remarkable ability to shed water is actually mechanical in nature–not chemical. Surfaces with a highly textured microstructure, like a lotus leaf or a butterfly wing, shed water naturally because air trapped between the high points prevents the water from contacting most of the solid surface. The result is that a drop sitting on the surface will have a very high contact angle and be nearly spherical. Instead of wetting the surface and spreading out, it can slide right off, as seen in the animations above. Here researchers have treated the coins and the right half of the cardboard with a spray-on coating that creates superhydrophobic microscale roughness. Similar coatings are commercially available, but such coatings are delicate and lose their hydrophobicity over time as the microstructure breaks down. (Image credits: Australian National University, source)
December 27, 2016