Shear Factor

By Davide Castelvecchi

Blow a puff of air through a straw on the surface of your favorite soft drink. You would expect to see a dip form and quickly disappear, wouldn’t you? It is perhaps the most intuitive fact about fluids that they tend to settle in their container, and to level out, forming a flat surface. But for a thick mixture of water and starch, subjected to rapid vibration, things don’t go quite that way: One can blow holes in the fluid that never close up, a team reports in the 7 May PRL. The researchers speculate that the shaking could produce an anomalous increase in the fluid’s thickness, until the fluid virtually behaves as a solid—though it goes back to normal as soon as the shaking stops. Understanding the behavior of such fluids—which no known theory can explain—could lead to improvements in a variety of industrial processes that involve mixing.

Starch does not completely dissolve in water. Instead, it stays in lumps that are microscopic but still very large compared to the size of molecules. There is no general theory regulating the behavior of such mixtures, which really are neither liquid nor solid, containing both water and solid particles. The same is true of “granular” materials, such as sand or coffee beans. Granular materials are important in industries such as food processing or the manufacturing of pharmaceuticals, where often tiny, but precise amounts of active ingredients need to be mixed in.

Robert Deegan and his colleagues at the University of Texas at Austin put a thin layer of cornstarch mixed with water on a round tray. The tray was attached to a motor that shook it up and down, with accelerations of up to 27g. At slow vibration rates, the fluid just formed regular wavy patterns, as the team expected. But at a frequency of 120 cycles per second, when the researchers blew holes that reached to the bottom of the plate, the holes stayed open indefinitely. Pairs of holes would engage in complicated dances, briefly merging to bounce off of each other again, as one can see in a video available on the Texas team’s Web page. The researchers observed similar effects when, instead of starch, they mixed in glass microspheres, less than one-hundredth of a millimeter wide.

Physicists have seen organized madness in vibrated fluids before, but this one took everyone by surprise. “It’s completely counterintuitive,” Deegan told New Scientist magazine. Even more baffling is what the team saw when they cranked up the vibrations to 150 cycles per second: The holes virtually took a life of their own, developing fingers from their rims. The fingers grew, waved, wandered, and multiplied, until the entire tray seemed to be teeming with some kind of morphing maggots.

The researchers believe that a phenomenon called shear thickening—the same one that turns quicksand into a deadly trap—could be at play. “Shear” is when different layers of a fluid flow at different speeds, as in rivers, where deeper layers flow slower than the superficial ones. Most fluids get thicker as the rate of shear increases. But in the Texas experiments, the lumps of starch or the glass spheres make the thickness of the mixtures grow anomalously fast.

Eventually, the goal would be to discover fundamental equations that can explain shear thickening and, more generally, predict the behavior of granular materials and mixtures, says Michael Cates of the University of Edinburgh, in Scotland. “We don’t know yet whether this new phenomenon will reveal any deep principle,” he says. “Until then, it’s fascinating data.”
For additional reading, see:

Persistent Holes in a Fluid,
Merkt et al., Phys. Rev. Lett. 92, 184501 (2004)

Vibrated shear thickening fluids
Web page maintained by the Center for Nonlinear Dynamics at the University of Texas at Austin

Persistent holes
(Physics News Update, May 6, 2004)

Mysteries of matter
(Science, May 21 2004)

Holes in liquid defy explanation
(New Scientist, 15 May 2004)

This result was named one of the top physics stories of 2004 by my colleagues at Physics News Update.

I wrote this article in November 2004 as an assignment I had for the selection process for an internship. This piece is unedited.

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