Published on November 24th, 2015 | By: April Gocha0
Snap, crackle, pop: Rice Krispies reveal compaction bands wave through crushed porous, brittle materialsPublished on November 24th, 2015 | By: April Gocha
[Image above] Credit: Wil C. Fry; Flickr CC BY-NC-ND 2.0
Beyond being thankful, Thanksgiving means one thing—food. And lots of it.
With the average holiday meal heaping up on your plate at a whopping 4,500 total calories, food has a serious position at the center of our holiday celebrations. (Anyone else thinking about smaller portions this week?)
So with our cultural obsession with food aplenty, it’s good to see another use for grocery store goods aside from the dinner table this week. Researchers at San Diego State University and beyond are to the rescue—they’re using food as an experimental medium to uncover phenomena of materials science.
The team’s experiments to characterize deformation in highly porous, brittle materials turned to the morning meal for a cheap and accessible material to test—puffed rice cereal.
To characterize how these granular materials break in response to compression, the team used a piston-equipped acrylic tube to crush a column of Rice Krispies cereal. Microphones at either end of the tube recorded the crushing noises of the puffed rice pieces to acoustically map their response to compression.
Using knowledge from classic friction experiments, SDSU researcher Julio Valdes predicted that “as the piston compressed the cereal, the top of the pack would compact and the bottom would not, given that the cereal would transfer force to the cylinder’s sidewalls via friction,” according to an SDSU press release.
Valdes’ former graduate student Johan Gallay performed the experiments, but the results weren’t quite as expected. “I said, ‘Johan, you’ve clearly made some kind of mistake. Go run it again,’” Valdes says in the release.
Running the experiment again didn’t change the results, however.
Instead, listening to the snap-crackle-pop as the cereal was compacted, microphones mapped an unusual pattern of cereal destruction—an alternating wave of deformation that moved through the column from top to bottom. The scientists observed a similar pattern when they visually watched the snapping cereal.
“As the cereal compacted, the researchers could see a rising band in the tube, indicating where the material was being crushed, or deformed, in the materials science lingo,” according to press release.
Watch the experiment below to see for yourself.
Credit: Particles and Grains Laboratory; YouTube
Testing various crushing velocities, the team found different patterns of deformation in the column of Krispies.
According to the release, “At very low velocities, the cereal exhibited an erratic deformation pattern, crushing at various points within the tube. At very high velocities, it all crushed down fairly uniformly. And at in-between velocities, the researchers saw their rising propagating compaction bands.”
The phenomena wasn’t restricted to just Rice Krispies, however. Compression tests with other breakfast-worthy granular materials—Cocoa Puffs and Cocoa Krispies—showed similar deformation patterns, although at different velocities because of those materials’ chocolate coating, the researchers say.
In addition to helping build a phase diagram “using two dimensionless groups that represent fabric collapse and external dissipation” (à la the paper’s abstract), the team’s results provide some insight into the complex mechanics of porous, brittle materials.
“The findings could have applications in manufacturing, in the pharmaceutical industry, for example, as well as in assessing the stability of snowpack after an avalanche,” according to the release.
The paper, published in Nature Physics, is “Dynamic patterns of compaction in brittle porous media” (DOI: 10.1038/nphys3424).
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