Hop On Pop, Jump On Oobleck

Even amateur cooks know that cornstarch is clutch when it comes to thickening Thanksgiving gravy or homemade soup. But as amateur (geeky) thrill-seekers have discovered, filling a swimming pool with cornstarch and water leads to a strange and fun phenomenon: You can run and jump across the sort-of-solid surface. Stop for a second, though, and you’ll sink like a stone. But the abnormal behavior of this “oobleck” (yes, it’s named after the Dr. Seuss book) doesn’t stop there. Vibrate a thin sheet of it on a loudspeaker at 20 Hz and “cornstarch monsters” will bounce upward as a solid and fall back down as a liquid. Despite being a pourable liquid, oobleck momentarily acts like a solid when an external force is applied, making it surprisingly difficult to scoop up with a spoon. Researchers have long thought that these wacky behaviors were a result of shear thickening, or the rapid increase of viscosity that occurs when fluid layers slide past each other.  A common example of shear thickening in action is the wet-sand effect—when stepping on wet sand along a beach causes sand to dry directly underfoot. But now a study in Nature suggests that it’s not shear force but compression that causes this counterintuitive phenomenon (DOI: 10.1038/nature11187). To come to this conclusion, Scott R. Waitukaitis and Heinrich M. Jaeger of the University of Chicago modeled a runner's legs pounding on an oobleck surface by plunging a rod repeatedly into containers filled with the ooey-gooey stuff. As an accompanying Nature news article explains, the researchers found that, below the rod’s impact site, a rapidly growing solidification front formed. It abided by the same principles that apply when dry particles are packed together: jamming and conservation of mass. The particles become so densely packed that they become a solid (jamming) at a rate proportional to the speed a runner’s feet are hitting the surface (conservation of mass). So the runner is essentially creating transient cornstarch shoes to keep afloat, for as long as he or she is moving. The results may have several implications in rheology (the study of the flow of matter), and the authors suggest it might be necessary to revisit other examples of impact resistance that have previously been explained by shear thickening. But in the short term, the rest of us can take some joy in demonstrating to friends that although we can't walk on water, we can certainly run on oobleck.

Author: Sophia Cai

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