Category: Phenomena

Self-Stopping Leaks
A leak can actually stop itself, as shown in this video. To demonstrate, the team used a tube pierced with a small hole. When filled, water initially shoots out the hole in a jet. The pressure driving the jet comes from the weight of the fluid sitting above the hole. As the water level drops, the pressure drops, causing the jet to sag and eventually form a rivulet that wets the side of the tube. As the water level and driving pressure continue to fall, the rivulet breaks up into discrete droplets, whose exact behavior depends on how hydrophobic the tube is. Eventually, a final droplet forms a cap over the hole and the leak stops. At this point, the flow’s driving pressure is smaller than the pressure formed by the curvature of the capping droplet. (Image and video credit: C. Tally et al.)
May 30, 2022
Inside Viscous Fingers
Sandwich a viscous fluid between two transparent plates and then inject a second, less viscous fluid. This is the classic set-up for the Saffman-Taylor instability, a well-studied flow in which the interface between the two fluids forms a wavy edge that develops into fingers. Despite its long history, though, there is still more to learn, as shown in this video. Here, researchers alternately injected a dyed and undyed version of the less viscous fluid. The result (Image 3) is a set of concentric dye rings that show how the fluid moves far from the fingers along the edge. Notice that the waviness of the fingers appears in the flowing fluid well before it approaches the interface. (Image and video credit: S. Gowan et al.)
May 25, 2022
Blowing Up Euler
The mathematics of fluid dynamics still have many unknowns, which makes them an attractive playground for mathematicians of all stripes. One perennial area of interest is the Euler equations, which describe an ideal (i.e., zero viscosity), incompressible fluid. Mathematicians suspect that these equations may produce impossible answers — vortices with infinite velocities, for example — under just the right circumstances, but so far no one has been able to prove the existence of such singularities.
A recent Quanta article delves into this issue and the race between researchers using traditional methods and those using new deep learning techniques. Will the singularities be found and who will get there first? It’s well worth a read, whether theoretical mathematics is your thing or not. (Image credit: S. Wilkinson; see also Quanta; submitted by Jo V.)
May 23, 2022
Turquoise Eddies
During the summer months, the Barents Sea between Norway and Russia is streaked with blue and teal swirls. These beautiful patterns are the result of a phytoplankton bloom, as viewed by earth-observing satellites (with a little color enhancement). Although each cell in the bloom is only nanometers across, their collective presence is visible from space! They also act as tracers in the water, revealing the swirling flow patterns present there. (Image credit: J. Stevens/NASA Earth Observatory)
May 18, 2022
Double Diffusive Flow
Diffusion is the tendency for differences in a fluid — in density, temperature, or concentration — to even out over time. Think about a drop of food coloring in a glass of water. Even without stirring, that dye will eventually disperse throughout the glass through diffusion. But when there is more than one factor controlling diffusion — like temperature and salinity — things get more complicated. In the ocean, for example, this double-diffusion causes salt fingers like those shown in the first image.
But what happens when the two diffusing fluid layers are flowing? That’s the question at the heart of this video, which explores the intricate mixing that takes place between doubly-diffusing liquids in a channel. (Video and image credit: A. Mizev et al.)
May 16, 2022
Brilliant Auroras
Glowing auroras billow across Canada in this satellite image from a recent geomagnetic storm. As our sun enters a more active part of its solar cycle, we can expect more space weather as the high-energy particles of the solar wind interact with our planet’s magnetic field. The auroras themselves are light released by energetically excited atoms of oxygen and nitrogen high in the upper atmosphere.
Earth is not the only place in the solar system to experience these light shows. With their strong magnetic fields, Jupiter and Saturn have auroras that make Earth’s look paltry in comparison. (Image credit: J. Stevens; via NASA Earth Observatory)
May 11, 2022
Deciphering Krakatau
In 1883, the eruption of Krakatau (also called Krakatoa) shook the world, sending shock waves and tsunamis ricocheting across the globe. Some of the smaller waves hit shorelines in the Atlantic and Pacific that were entire continents and ocean basins away from the original explosion. At the time, scientists were so perplexed by the phenomenon that they blamed coincidental earthquakes for the wave action.
Only when Tonga experienced a similarly devastating volcanic eruption earlier this year were scientists able to verify what they’d long suspected: these smaller tsunamis were not caused by solid material displacing water; instead they are the result of atmospheric pressure waves coupling to the ocean. Follow the full story over at Quanta. (Image credit: M. Barlow; via Quanta; submitted by Kam-Yung Soh)
May 9, 2022
Re-Entry For X-Wings
Fans of sci-fi and fantasy have a long-standing tradition of exploring the physics and/or practicality of creations in their fandom, and Star Wars fans are no exception. Here engineers ask whether Luke Skywalker’s X-wing fighter could survive the descent through Dagobah’s atmosphere as he searched for Master Yoda. Their results are based on a numerical simulation, with some assumptions about the spacecraft’s descent path and design as well as the planet’s atmosphere. Fans of the Jedi will be glad to hear that the X-wing can survive its supersonic descent intact, delivering the last Jedi safely to his mentor. (Image credit: Y. Ling et al.)
May 4, 2022
Ant Bridge
As red ants scout their way to food, the terrain can sometimes get in the way. Here a leading scout has made their body into a bridge that their fellows can use to cross the watery gap. Take a close look at the water’s surface and you’ll see that the meniscus curves up to meet the rocks. That’s a clue that this image is really very small! For water on Earth, that curvature only occurs at lengths below a couple of millimeters, where surface tension has the power to overcome gravity’s efforts to flatten the surface. The ants’ bridge is only possible because the red ant is small enough and light enough for surface tension to support it. Learn more about the amazing interactions of ants and water in some of my previous posts. (Image credit: Chin Leong Teo; via Colossal)
May 3, 2022
Explaining the Roaming Rocks
For nearly a century, the long meandering tracks etched into Death Valley’s Racetrack Playa remained a mystery. Clearly, some force was pushing the heavy rocks there and leaving behind these grooves. But with the remoteness of the location, it took investigators years to catch the rocks in action and solve the puzzle. For those who haven’t watched the video yet, I’ll refrain from revealing the answer here (though you can find it in previous FYFD entries)! I’ll just say that it requires all the right conditions to come together. (Image and video credit: Physics Girl; for related research see here)
April 28, 2022