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Posts Tagged → rheology

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). Continue reading →