Rocky, isolated islands disturb the atmosphere, sending air swirling off one side of the island and then the other. The effects are not always visible to the naked eye, but, as they do here, they can show up in satellite imagery as whirling von Karman vortex streets. The eddies of this image are due to the Canary Islands, and if you follow the line of swirls backward, you’ll find their originating islands. Note that the cloudy swirls don’t appear immediately behind the islands. That’s because there wasn’t enough moisture in the air for clouds to condense yet; the same swirls that you see in the downstream clouds exist in the clear air closer to the islands. (Image credit: A. Nussbaum; via NASA Earth Observatory)
Tag: physics
“The Beauty in Creation”
Volcanoes are endlessly fascinating to watch, especially in this era of drone photography. Joey Helms’s short film “The Beauty in Creation” shows the Fagradalsfjall eruption in Iceland. Sail over rivers of lava, watch fountains spurting, and even see lava dripping back into the caldera. They’re views that no human gets to witness directly, but they certainly do highlight the peculiar collision of destruction and creation inherent in volcanic eruption. (Video credit: J. Helms)
Shouting Into the Wind is Easier Than You Think
“Shouting into the wind” usually means a failure to communicate, but it turns out that shouting into the wind doesn’t work the way people usually think. In fact, it’s easy for people upstream to hear your shouting, thanks to an acoustical effect called convective amplification. You’ve likely experienced it firsthand as an ambulance approaches. With its sirens blaring, the ambulance sounds louder as it comes toward you and quieter after it’s past. (This is separate from the Doppler effect, which changes the pitch of the approaching and receding vehicle.)
So why does shouting into the wind seem so hard? It’s because your ears are downstream of your mouth. Like the ambulance that’s already gone by, your voice comes from ahead of your ears and therefore sounds quieter to you than it does to your audience upstream. (Image credit: I. Huhtakallio; research credit: V. Pulkki et al.; via Science News; submitted by Kam-Yung Soh)
The Epic Migration of Plankton
Zooplankton are tiny creatures found throughout Earth’s oceans. During the daytime, they linger in the twilight depths, where they are harder for predators to spot. But once the sun sets, zooplankton migrate hundreds of meters upward to reach the abundant food near the surface. When sunrise comes, they migrate back downward. Given their size, this feat is astounding; equivalent to a human running two 10-kilometer races a day at Olympic marathon speeds. And, despite their tiny size, these motions leave a mark; researchers have shown that the collective action of all these tiny swimmers is large-scale turbulence with serious mixing potential. (Video and image credit: Be Smart)
Fixing Reverse Osmosis
Desalination and water treatment plants both rely on reverse osmosis to generate clean water for human use. The standard theory behind reverse osmosis for the last half century suggested that the membranes separated water and other chemicals by forcing water molecules, driven by chemical gradients, to travel one-by-one through a dense membrane forest. But over the years, researchers saw signs that this theory didn’t hold up; for one, the membranes water travels through have pores in them that are larger than individual water molecules.
A new study examines the underlying assumptions of the prevailing model and finds instead that water moves through reverse osmosis membranes by pore flow. Instead of individual molecules pushed by concentration, flow takes place through pores and is driven by a pressure gradient. The difference is important because it enables engineers to design more efficient membranes according to real-world physics. By understanding the underlying mechanism, designers can tweak the pore size, density, and other features of reverse osmosis membranes to better filter unwanted chemicals and to remove salt from water with less energy input. (Image credit: Florida Water Daily; research credit: L. Wang et al.; via Wired; submitted by Kam-Yung Soh)
Liquid Lens Rupture
A blob of sunflower oil floating on soapy water forms a disk known as a liquid lens. But add some dyed ethanol and things take a turn. The lens rapidly expands and distorts as the ethanol and soapy water meet. These surface flows are driven by the imbalance of surface tension between the different liquids. The liquid lens deforms and abruptly ruptures, releasing dye and ethanol before rebounding into a stable lens again. Adding more ethanol to the lens will repeat the cycle. (Image credit: C. Kalelkar and P. Dey; research credit: D. Maity et al.)
Marshy Veins
From above, the salt marshes of Alviso Marina County Park look like veins and capillaries in this photo from Tayfun Coskun. The waterways curve and branch, forming fractal patterns only apparent from the air. Although the mechanisms that form these dendritic patterns vary, they are very common in fluids, appearing over and over at many scales. (Image credit: T. Coskun; via Gizmodo)
Bubble Trails – Straight or Wonky?
Watch the bubbles rising in a glass of champagne and you’ll see them form tiny straight lines, with each bubble following its predecessor. But in a carbonated soda, the bubbles rise all over the place, each following its own zig-zaggy line. Why the difference? A recent study points out the culprits: bubble size and surfactants.
As bubble size increases from left to right, the bubble trail straightens. Looking at a variety of beverage scenarios, researchers found that both a bubble’s size and its surfactant concentration affected what sort of path it followed. For clean (surfactant-free) bubbles, small bubbles take a winding path, but bigger ones move in a straight line. Simulations show that bubbles can only form a straight path if they produce enough vorticity on their surface. Small bubbles just can’t deform enough to do that.
For bubbles of the same size, increasing the surfactants on the bubbles straightens their path. When surfactants get added, though, the story changes. For bubbles of a set size, adding surfactants made their paths straighter. This was due, the team found, to a bump in vorticity provided by the stabilizing effect of the surfactants. Champagne, they concluded, has straight bubble paths despite its tiny bubbles because of the drink’s high number of flavorful surfactants. (Image credit: top – D. Cook, experiments – O. Atasi et al.; research credit: O. Atasi et al.; via APS Physics)
Wave Clouds From Space
An astronaut snapped this image of wave clouds formed around the Crozet Islands, which lie between South Africa and Antarctica. Clouds like these form when warm, moist air gets pushed up and over a mountain. As it rises, the air cools and its pressure decreases, causing condensation. Pushed out of equilibrium, gravity then pulls the air back downward in the wake of the mountain. That warms the air, causing evaporation. Like a mass bouncing on a spring, the air continues to yo-yo up and down, forming cloudy stripes and clear ones until the energy from its mountain climb is spent. (Image credit: NASA; via NASA Earth Observatory)
Lanes in Crowds
In nature — from atoms to human crowds — two groups moving in opposite directions often spontaneously organize into interwoven lanes flowing in their respective directions. Now researchers have built a mathematical model for this behavior, building on Einstein’s observations of Brownian motion.
To test their model, the researchers performed numerical simulations and experiments with pedestrians. Intriguingly, they found that introducing rules like “always pass on the right” created unexpected results, such as tilted lanes. With their model verified — at least for low-density crowds — the group hope to uncover other hidden patterns within crowds. (Image and research credit: K. Bacik et al.; via Physics World)
In their validation experiments, the researchers filmed groups of pedestrians walking past one another under different conditions. Note the lanes that form as the two groups interleave.
