Enlarge / Brown University scientists used two 3D-printed plastic disks to explore the Cheerios effect. (credit: A. Hooshanginejad et al., 2023)

Scientific research often produces striking visuals, and this year's winners of the Gallery of Soft Matter Physics are no exception. Selected during the American Physical Society March Meeting last week in Las Vegas, Nevada, the winning video entries featured the Cheerios effect, the physics of clogs, and exploiting the physics behind wine tears to make bubbles last longer. Submissions were judged on the basis of both striking visual qualities and scientific interest. The gallery contest was first established last year, inspired in part by the society's hugely successful annual Gallery of Fluid Motion . All five of this year's winners will have the chance to present their work at next year's March meeting in Minneapolis, Minnesota.

Mermaid Cereal

As we've previously reported , the " Cheerios effect " describes the physics behind why those last few tasty little "O"s of cereal tend to clump together in the bowl: either drifting to the center or to the outer edge. The effect can also be found in grains of pollen (or mosquito eggs) floating on top of a pond or small coins floating in a bowl of water. The culprit is a combination of buoyancy, surface tension, and the so-called " meniscus effect." It all adds up to a type of capillary action . Basically, the mass of the Cheerios is insufficient to break the milk's surface tension. But it's enough to put a tiny dent in the surface of the milk in the bowl, such that if two Cheerios are sufficiently close, they will naturally drift toward each other. The "dents" merge and the "O"s clump together. Add another Cheerio into the mix, and it, too, will follow the curvature in the milk to drift toward its fellow "O"s.

Measuring the actual forces at play on such a small scale is daunting, since they're on about the same scale as the weight of a mosquito. Typically, this is done by placing sensors on objects and setting them afloat in a container, using the sensors to deflect the natural motion. But Cheerios are small enough that this was not a feasible approach. So Brown University postdoc Alireza Hooshanginejad and cohorts used two 3D-printed plastic disks , roughly the size of a Cheerio, and placed a small magnet in one of them. Then they set the disks afloat in a small tub of water, surrounded by electric coils, and let them drift together (attraction). The coils in turn produced magnetic fields, pulling the magnetized disk away from its non-magnetized partner (repulsion).

Enlarge / Look at this lovely cube-shaped piece of poo, courtesy of the Australian bare-nosed wombat. (credit: Patricia J. Yang et al., 2021)

Scientists have been puzzling for decades over how the Australian bare-nosed wombat poops out neat little cubes of feces instead of tapered cylinders like pretty much all other animals. According to a new paper published in the journal Soft Matter, the secret lies in their intestines, which have varying stiff and soft regions that serve to shape the poo during the digestive process. Earlier preliminary findings by the same group won the 2019 Ig Nobel Physics Prize.

"Bare-nosed wombats are renowned for producing distinctive, cube-shaped poos. This ability to form relatively uniform, clean cut feces is unique in the animal kingdom," said University of Tasmania wildlife ecologist Scott Carver , a co-author on the paper. "They place these feces at prominent points in their home range, such as around a rock or a log, to communicate with each other. Our research found that these cubes are formed within the last sections of the intestine—and finally proves that you really can fit a square peg through a round hole."

Zoologist Eric Guiler first noted the unusual shape of wombat droppings in 1960, and to date, wombats are the only known animals to produce six-sided cube-shaped poo. It's one of several examples of naturally occurring pattern formation, such as the columns of Ireland's Giant's Causeway (formed by cooling lava), or how vibrating membranes can make grains of sand form " Chladni figures ." But naturally occurring cube shapes are extremely rare. The Australian bare-nosed wombat ( Vombatus ursinus ) can pump out as many as 100 cube-shaped droppings a day.

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