Frictional Fingers

Air pushes into a thin gap filled with water and granular particles in the labyrinth-like image above. The encroaching air pushes grains like a bulldozer's blade, building up a compacted wall.  (Image and research credit: E. Flekkøy et al.) Read the full article

Electronic Friction

Years ago, physicists discovered that water flows with surprisingly little friction through narrow carbon nanotubes. To unravel the mystery, researchers had to delve into quantum mechanics.  (Image credit: cintersimone; research credit: N. Kavokine et al.; via SciAm; submitted by Kam-Yung Soh) Read the full article

Beijing 2022: Ice's Slideability

As scientists continue to unravel the peculiarities of ice, they've found that ice's friction depends both on the object sliding on it and the ice's hardness.  (Image credit: SHVETS Productions; research credit: R. Liefferink et al.; via APS Physics; submitted by Kam-Yung Soh) Read the full article

In an avalanche, grains spontaneously flow when a slope reaches a critical angle, and they continue flowing until they settle at a new, lower angle. Scientists have long debated why this angle mismatch occurs, and, in recent years, the general opinion was that the avalanche’s inertia kept it flowing long enough to settle at a lower angle. But a new experiment, using a slowly-rotating drum similar to the one above*, shows that friction, not inertia, is the key player. 

The researchers used silica beads suspended in water, which allowed them to cleverly control the interparticle friction. In water, silica beads build up negative electrostatic charges, which push the grains apart and eliminate friction. In that frictionless state, the researchers found that the beads tumbled smoothly; their starting and ending angles were always the same. 

By adding salt to the water, the researchers were able to eliminate some of the electrostatic charge and thereby tune the friction. When they did that, the difference between starting and stopping angles came back and grew more substantial as the friction increased. All in all, the results indicate that friction between particles is what makes an avalanche avalanche. (Image credit: J. Gray and V. Chugunov, source; research credit: H. Perrin et al.; via APS Physics; submitted by Kam-Yung Soh)

* If you’re curious about the patterns in the image, I explain them in this previous post.

Granular materials are complicated beasts. When packed, forces between grains create a network (above) that shifts as force is applied. And, while grains can stick and resist that force, push a little further and they may slip and avalanche. A new study of this stick-slip behavior monitors disks similar to those above by listening for changes leading up to the slip. Researchers found that vibrations inside a granular material changed measurably before the grains slipped. The scientists hope this will one day allow for monitoring of landslide and avalanche-prone areas. While the changes are not enough to definitively predict when a slide will occur, they may provide valuable estimates of when one is likely. (Research credit: T. Brzinski and K. Daniels; image credit: OIST, source; via J. Ouellette)

As a child, I loved to ride in the car while it was raining. The raindrops on the window slid around in ways that fascinated and confused me. The idea that the raindrops ran up the window when the car moved made sense if the wind was pushing them, but why didn’t they just fly off instantly? I could not understand why they moved so slowly. I did not know it at the time, but this was my early introduction to boundary layers, the area of flow near a wall. Here, friction is a major force, causing the flow velocity to be zero at the wall and much faster -- in this case roughly equal to the car’s speed -- just a few millimeters away. This pushes different parts of large droplets unevenly. Notice how the thicker parts of the droplets move faster and more unsteadily than those right on the window. This is because the wind speed felt by the taller parts of the droplet is larger. Gravity and the water’s willingness to stick to the window surface help oppose the push of the wind, but at least with large drops at highway speeds, the wind’s force eventually wins out. (Image credit: A. Davidhazy, source; via Flow Viz)

To wrap up our look at Olympic physics, we bring you a wintry mix of interviews with researchers, courtesy of JFM and FYFD. Learn about the research that helped French biathlete Martin Fourcade leave PyeongChang with 3 gold medals, the physics of avalanches, and how bubbles freeze. 

If you missed any of our previous Olympic coverage, you can find our previous entries on the themed series page, and for more great interviews with fluids researchers, check out our previous collab video. (Video credit: T. Crawford and N. Sharp; image credits: GettyImages, T. Crawford and N. Sharp)

A labyrinthine pattern forms in this timelapse video of a multiphase flow in a Hele-Shaw cell. Initially glass beads are suspended in a glycerol-water solution between parallel glass plates with a central hole. Then the fluid is slowly drained over the course of 3 days at a rate so slow that viscous forces in the fluid are negligible. As the fluid drains, fingers of air invade the disk, pushing the beads together. The system is governed by competition between two main forces: surface tension and friction. Narrow fingers gather fewer grains and therefore encounter less friction, but the higher curvature at their tips produces larger capillary forces. The opposite is true of broader fingers. Also interesting to note is the similarity of the final pattern to those seen in confined ferrofluids.  (Video credit and submission: B. Sandnes et al. For more, see B. Sandes et al.)

Not long ago, researchers showed that cats use friction to their advantage when drawing liquids into their mouths. New research shows that dogs rely on the same mechanism--they're just far less efficient with it. The dog touches its backwards-curled tongue to the surface of the water; when it draws the tongue back, friction causes a column of fluid to follow. The dog then closes its jaws around the water. Some water also gets picked up by the back of the tongue, but since dogs have no cheeks, it spills out the sides, creating a mess familiar to dog owners. #

While humans use suction and dogs scoop water using their tongues*, cats use a dainty fluid mechanism to drink. Researchers used high-speed video to find that cats drink by touching the surface of their tongue to the water and drawing their tongue rapidly back into their mouth. Friction between their tongue and the water creates a fluid column about which the cat closes its jaw before gravity breaks off the column. They also built an artificial tongue to test different frequencies and found an optimal lapping frequency dependent upon the mass of the feline.

*ETA: More recent research show that dogs actually use the same technique as cats, not a scooping method.

In the Science Storms section of the Chicago Museum of Science and Industry, you'll find the mesmerizing sight of an avalanche disk. This 20ft disk spins at a variable rate and angle, and, from the video, you can see that the glass beads simulating an avalanche on the disk move very much like a fluid even though they are not. This is what's called a granular flow and it's driven by gravity and friction between particles.