Imagine poking a liquid to create holes that persist like those in swiss cheese. Incredible as that might sound, a group of scientists has done it.
Although the bizarre effect showed up only under extreme laboratory conditions, similar effects might be present in some industrial and geologic processes that involve fluids, says physicist Robert D. Deegan of the University of Texas at Austin. He and his colleagues describe their hole-riddled fluids in the May 7 Physical Review Letters.
The Texas team produced the enduring holes in curious fluids that are already well known to chefs: concentrated suspensions of cornstarch in water. Such fluids undergo what’s called shear thickening, which means they resist stirring by becoming more and more viscous the more vigorously they are stirred.
Deegan and his coworkers tried a new way to agitate the cornstarch suspensions. They poured a layer of the stuff into a shallow dish and then vibrated it up and down with a range of accelerations. Some exceeded twice the maximum acceleration endured by a fighter pilot flying tricky maneuvers.
Although scientists have subjected fluids to up-and-down vibrations since the 1800s, “the idea of subjecting a shear-thickening fluid to [such] oscillations is new and original,” comments Tomas Bohr of the Technical University of Denmark in Lyngby.
The first long-lived hole appeared when Deegan touched the fluid with his finger. “I tried pushing on it to see if anything would happen, and this is what happened,” he recalls.
Since then, he and his colleagues have demonstrated that, within a certain range of vibration speeds and intensities, holes made using puffs from high-pressure air jets last as long as the vibrations continue. At the highest accelerations, the holes morph into a riot of ever-changing blobs and voids.
“It’s the wildest thing I ever saw,” comments Thomas A. Witten of the University of Chicago.
The results using cornstarch suspensions were surprising enough, but the researchers observed similarly persistent holes in fluids that consist of water and microscopic glass beads. That result suggests the same effect “could probably occur in many different suspensions of particles,” says Harry L. Swinney, a coauthor of the report and the head of the lab where the experiments took place.
There is some precedent for the vibration-induced formation of long-lived, discrete structures in a material that usually flows. About 8 years ago, a different team in Swinney’s lab discovered that shaking a bed of minute bronze spheres up and down could produce isolated towers of beads. Called an oscillon, such a tower periodically gives way to a complementary form—a dent in the bed of beads (SN: 8/31/96, p. 135).
