A drop sliding down a fiber can do so asymmetrically or symmetrically. The asymmetric configuration is unstable and will spontaneously shift to a symmetric one. Adding a second, parallel fiber stabilizes an asymmetric drop, letting it slide without shifting. And twisting the two fibers together gives even more control, allowing researchers to tweak drop shape, speed, and orientation independent of properties like the drop’s volume or viscosity. (Image and video credit: V. Kern and A. Carlson)
Our Sun‘s visible surface, the photosphere, is about 5800 Kelvin, but the temperature of the wispy corona is far hotter, reaching a million Kelvin in some places. Why the corona is so hot remains something of a mystery. Scientists have theorized multiple culprits for the extreme temperatures found in the corona, but the full details of the phenomenon are still unclear.
Recent solar missions and observations are increasingly identifying small but widespread solar activities, like the nanoflares shown above. Unlike the monstrous coronal loops researchers focused on previously, these flares are tiny and occur in regions without discernible solar flare activity. The nanoflares are brief but they can reach temperatures above a million Kelvin. Since nano- and even picoflares have been observed across the full Sun, they likely play a significant role in the overall picture of coronal heating. (Image credit: ISAS/JAXA; see also L. Sigalotti and F. Cruz)
Even adding a small amount of polymers to a fluid can drastically change its behavior. Often polymer-doped fluids act more like soft solids, able to hold their shape like your toothpaste does when squeezed onto your toothpaste. Under a little stress, though, the fluids still flow; that’s why your toothpaste gets less viscous as you scrub.
To study the changes polymers make, this research team collides two jets of fluid to create a liquid sheet. Depending on the flow rate and the added polymers, the break-up pattern of the sheet changes. By observing changes in the sheet thickness and the holes that form, they can draw conclusions about what the polymers are doing. (Video credit: C. Galvin et al.)
Rivers are systems in a constant state of change, balancing flow speeds, path length, sediment deposition, and erosion, as seen in this previous Practical Engineering video. The next video in this mini-series considers what human interventions do to rivers. As convenient as it is for humanity to force a river into a straight and constant course, the long-term effects can be incredibly destructive both upstream and downstream.
In this video, Grady takes a look at several types of interventions: stream straightening, dams, river crossings, and more. With the help of a stream table, he demonstrates just how these efforts shift the river’s balance and what effects — in terms of erosion, deposition, and flooding — each can cause. These disadvantages, along with habitat destruction, are part of why stream remediation projects are on the rise. (Video and image credit: Practical Engineering)
Blooms of the algae Karenia brevis — known as a red tide — bring havoc to Gulf Coast shores. The algae can kill fish and other marine life, and it causes skin irritation and even respiratory problems for humans. But in spite of the moniker, these algae can be hard to spot; they can add a green, brown, red, or black hue to the water.
The false-color image above uses a new image processing technique that reveals the bloom. Using satellite images taken over multiple days, scientists can track and study the red tide in unprecedented detail. The new technique will be a boon to those trying to monitor and understand red tides. (Image credit: Y. Yao/USF/Planet Labs/L. Dauphin; via NASA Earth Observatory)
Inspired by a simulation, Steve Mould asks a great question in this video: can water solve a maze? Yes — with some caveats. Steve makes two different maze patterns — a simple and a complex path — in two different sizes. With the small, simple-path version, the water immediately follows the correct path without taking any wrong turns. What keeps it on the right path seems to be a combination of air pressure and surface tension. In the dead-end passages, the air has nowhere to go in order to allow the water in. So the pressure of the trapped air and the narrowness of the passages (which allows surface tension to help hold the water in place) keeps the water out of the false paths.
With the larger mazes, the water is able to take some false turns as it seeks the lowest possible path. But after awhile the incorrect region fills and the water takes the next lowest path available, which eventually leads it to the outlet.
Toward the end of the video, Steve notes that the large mazes sometimes stop flowing, even though water is still in the reservoir. I’ll quibble slightly here with his explanation, though; I don’t think surface tension is playing as much of a role in this stoppage as friction. The water is basically being driven through a long, narrow pipe, which means quite a lot of friction between it and the walls. Just as you need a certain driving pressure to keep water in a pipe flowing, the maze needs a high enough driving pressure to keep the water going. The point at which drainage stops is the point where the upstream pressure (caused by the depth of the reservoir above the maze) is equal to the pressure lost due to friction in the pipe. All in all, it’s a very cool experiment and a video well-worth watching! (Video and image credit: S. Mould)
Without surfactants to stabilize them, bubbles don’t last long at room temperature. But adding a little heat changes the picture. When heated, the bubbles get stabilized by a thermal gradient that lifts fluid toward the bubble’s peak, where it cools and gathers. Eventually, the cold fluid grows heavy enough to sink down the side of the bubble (in either a constant stream or occasional drips); with warm fluid getting pulled up to replace it (via the Marangoni effect), the process repeats and the bubble lives on. (Video credit: S. Nath et al.; see also)
Acoustic cameras use arrays of microphones to isolate where sounds are coming from. As Steve Mould shows in this video, they have some incredibly cool properties. They can show engineers which part of a device is producing particular sound frequencies, which is handy, for example, when trying to quiet a vacuum cleaner or learn which wheels on a train need maintenance. They can also show how sound moves around a room; near the end of the video, you can see the echo from a clap flashing around a room. Check out the full video for more! (Video credit: S. Mould)
Droplets with particles in them can leave complex stains when they dry — just look at coffee rings and whiskey marks! Here, researchers look at the patterns left on glass by small droplets that evaporated and left behind their nanoparticles. As evaporation takes place, the droplet’s shape changes, adding stress to the growing layer of nanoparticle residue. Cracking is one way to relieve that stress. Another method is delamination — peeling up from the surface. On the leftmost drop, the outer rim of nanoparticles delaminated — as seen from the circular fringes — which released stress without cracking. The rightmost drop, which had a smaller contact angle with the surface, couldn’t delaminate and instead cracked throughout. (Image credit: M. Ibrahim et al.)
In the Cheerios effect, floating objects can fall into one another due to capillary attraction — just like Cheerios link up in a cereal bowl. Here researchers play with that effect by adding repulsive magnets to their “cereal” pieces. They find that their so-called mermaid cereal falls into preferential spacing, with pieces pairing up but never touching. Adding lots of these pieces in a confined space creates interesting crystalline and striped patterns, as seen later in the video. (Video credit: A. Hooshanginejad et al.)