|The corn-starch experiment. Figure from|
van Hecke's summary referenced below.
In an article* in the July 12, 2012 issue of Nature, two physicists from the University of Chicago explain the physics behind this phenomenon, and take issue with older theories that it is caused by "shear thickening." This work has implications for our understanding of blood flow, cement and clays, all of which can exhibit shear thickening. Waitukatis and Jaeger propose that the shear thickening is not due to shear, but to compression.
In a series of experiments in which they plunged a rod into vats of cornstarch in water and water mixed with glycerol, and observed the process with fast video, X-ray imaging and other sensors, they found that below the point of impact of the rod, a cone of solidification developed. In this zone, the material had the properties of a solid...briefly. As you will see from the YouTube video above when both a bowling ball and a man sink, the solid state only lasts a fraction of a second, or as long as the material is agitated.
The proposed mechanism for this thickening relies on particle jamming, and the size and dynamics of the zone depend on conservation of mass. Jamming occurs when the particles get close enough together to become densely packed and form a solid. Conservation of mass leads to predictions of the size (depth) of the solidified zone. The solidified zone absorbs the momentum of the impacting rod, providing the force that stops its motion. Interestingly, they were able to exclude some other effects with clever experiments. By adding a 1-cm deep layer of water to the surface of the suspension, they reduced the surface tension to zero, and ruled out liquid-air interface effects. By adding glycerin to the water, they increased its viscosity by more than an order of magnitude, finding little effect on the parameters during compression of the water, but a strong effect on the sinking of the rod into the mixture after the impact.
The work has practical applications, from preventing cement from bulking up and breaking pipes when poured to design of sports gear or body armor to absorb impacts and vibrations.
A 22 second video is available from this article by Devin Powell in Science News.
*Waitukaitis, S.R. and Jaeger, H.M., "Impact-activated solidification of dense suspensions via dynamic jamming fronts." Nature, 487, 205-209, 2012, with a very nice summary by Martin van Hecke of Leiden University, Netherlands, on pages 174-175.