When filming, things don’t always go according to plan. Glasses break, splashes obscure your subject, and sometimes effects just don’t turn out the way you expect. But if you’re the Macro Room team, even those mistakes and outtakes are pretty darn fascinating to watch! I especially like some of the granular “splash” sequences here. (Image and video credit: Macro Room)
Here’s a neat bit of fluid dynamics derived from a day at the beach! Our experiment begins with well-mixed (and likely compacted) sand grains and sea water in a bottle. When flipped, the sand layer sits at the top of the bottle with the water layer beneath.
Very quickly new layers establish themselves in the bottle. The lower half of the bottle turns into a turbulent churn of water and sand, topped by a thin air bubble, then the thick sand layer, and finally, a layer of filtered water. That air bubble beneath the sand means that the sand layer is compacted enough that surface tension keeps the air from being able to squeeze through the grains. On the other hand, water is able to filter through, eventually making it into that upper region. The compact layer of sand is supported in the bottle by force chains running through the largest grains, which is why only fine sediment settles down through the turbulent layer at this point.
Eventually, the top sand layer erodes enough that it can no longer support its weight, and the sand collapses. As the grains settle out, we end up with fine sediment on the bottom (as previously discussed), followed by a layer of coarse sand from the erosion and collapse of the sand layer, topped with a layer of very fine grains that — due to their light weight — are the very last to settle out of the water. I love that such a simple seaside experiment contains such scientific depth! (Video and submission credit: M. Schich; special thanks to Nathalie V. for helpful input)
Flocks of birds and schools of fish swarm in complicated collective motions, but groups of non-living components can move collectively, too. In this Lutetium Project video, we learn about grains that, when vibrated, self-propel and form complex collective motions similar to those seen in groups of living organisms.
A key feature of the grains is their lack of symmetry. To be self-propelling, they must have a well-defined orientation, defined by a different front and back. The grains also have the freedom to move in a direction that is not the same as the direction they’re oriented in. This allows the grains to rotate, which enables them to perform the large-scale motions seen in the experiments. (Video and image credit: The Lutetium Project; research credit: G. Briand et al.)
An inquiring reader wants to know:
How does kinetic sand work to make it flow like a liquid? Thanks!
– 3 Year Olds Everywhere
I confess I don’t have any firsthand experience with Kinetic Sand, but it certainly looks fun. It’s a colorful, moldable sand toy that holds together far better than your typical pile of sand. From what I’ve been able to find, the secret ingredients are a little bit of polydimethylsiloxane (PDMS) — a type of silicon-based polymer — and olive oil, which coats the sand and keeps it from drying out.
PDMS is viscoelastic, which is what gives the Kinetic Sand its unique properties. When a force is applied quickly, the material reacts like a solid, which is why you can mold or cut the sand and have it maintain its shape. But when left alone for awhile under gravity’s influence, the sand will flow like a liquid. This combination of behaviors usually comes down to the polymers in the material. When forces try to stretch these long molecules quickly, they resist; that’s what creates the elasticity of the material. On the other hand, when a force is gradual, the complex molecules have the time to untangle and relax, allowing the material to flow. (Image credit: Kinetic Sand, source)
Antlion larvae catch prey by digging conical pits in sand. The steep walls of the trap are near the angle of repose, the largest angle a granular material can maintain before grains slide down. When a hapless ant wanders into the trap, the antlion throws sand from the center of the pit, triggering a sandslide that carries the ant downward. The act of flinging sand also helps the antlion maintain the pit, correcting any disruptions to the pit’s steep sides caused by its flailing prey. (Image and research credit: S. Büsse et al.; via Science)
Sand and other granular materials can flow, jam, and transmit forces in counterintuitive ways. This Lutetium Project video gives a nice overview of some of these bizarre properties.
Many of sand’s odd characteristics come from the way forces move through grains that touch. Around 5:20 there’s a demo of one of these effects: the Janssen effect. Using a scale, the video shows the mass of a bunch of grains. Then, the host pours those grains into a narrow cylinder. If you watch the scale, you’ll see that it shows a smaller mass than before. That’s not because of a difference in mass between the bowl and the cylinder; the scale is calibrated to only measure the mass of the grains. In the narrow cylinder the grains appear to weigh less because part of their weight is being supported by force chains that run to the container’s walls. (Image and video credit: The Lutetium Project)
Finger-like shapes often form on fluids injected between glass plates, but what happens when that injected fluid contains particles? That’s the situation in this recent study, where researchers sandwiched a fluid between two glass plates and then injected a second, similar fluid laced with particles.
Despite the differences from the traditional Saffman-Taylor set-up, the granular-filled fluid still forms fingers as long as there’s even a slight density difference between the original and injected fluids. It doesn’t even matter which of the two fluids has the greater density! (Image and research credit: A. Kudrolli et al.)
With planning for manned and unmanned missions to the Moon, Mars, and many asteroids underway, engineers are using numerical simulations to understand how spacecraft thrusters interact with planetary surfaces. Most practical data for this problem comes from the Apollo program and is of limited use for current missions. Recreating a Martian landing on Earth isn’t straightforward, either, given our higher gravity. Thus, supercomputers and numerical simulation are the best available tool for understanding and predicting how the plumes from a spacecraft’s thrusters will interact with a surface and what kind of blowback the spacecraft will need to withstand. (Video credit: U. Michigan Engineering; research credit: Y. Yao et al.; submission by Jesse C.)
When excited, a group of particles can behave much like a gas. These granular gases exhibit many similarities to molecular gases but contain one vital difference: without a constant input of energy, granular gases lose kinetic energy to collisions.
Over the years, scientists have developed a special theory to describe the behaviors of granular gases, but most of its predictions could only be tested numerically. A new study used a microgravity experiment aboard a sounding rocket to physically test the theory.
The experiment, shown above, consists of nearly 2800 magnetic particles, which the researchers could stir up using pairs of magnets. Once they shut off the magnets (which occurs at t=0 in the image above), the granular gas begins to “cool” as collisions sap away its energy. With this set-up, the researchers were able to confirm several key predictions of the granular gas theory. (Image and research credit: P. Yu et al.; via APS Physics)
The dynamics inside a shaken granular material, like sand, are fascinatingly complex. In this study, researchers used x-ray radiograms to peer inside a horizontally-shaken container of sand. They found that the sand soon formed bands of lower density (seen as yellow in the radiogram) near the center of the container. Because these bands show a lot of horizontal movement between grains, they’re known as shear bands.
The shear bands don’t simply stay still, though. One remains more or less stationary at the center, but others split and rise through the upper half of the container. The researchers suggest this migration happens due to gravity; because the shear band is less dense than the material above, it cannot support the weight. Sand sinks into the void, making the less dense region effectively migrate upward. They also suggest that these moving shear bands are responsible for the fluctuations in sand height seen at the surface. (Image credit: beach – RAMillu, radiogram – J. Kollmer et al.; research credit: J. Kollmer et al.)