Why is it possible to run across a pool filled with a liquid appearing suspension, such as cornstarch in water? Current explanations predict a thickening of the suspension but cannot account for the large forces required to keep a grown person from sinking in. We show that the remarkable impact resistance is produced by a different mechanism, where each step turns the liquid underfoot into a fast growing solid. The resistance emerges from a jamming process similar to what happens when a plow is pushed into loose snow.

Suspensions are model systems for the investigation of non-Newtonian flow behavior and at the same time important for a wide range of applications. One of the most striking suspension phenomena is that they can produce very large normal stresses during impact. As a result, dense suspensions are finding new uses in the design of impact absorbing and stab-resistant materials. We discovered that this impact resistance is due to local compression of the particle matrix, forcing it across the jamming transition and precipitating a rapidly growing solid mass. This is the first observation of such jamming front and it links nonlinear suspension dynamics in a new way to the jamming phase transition.

1. •Scott Waitukaitis and Heinrich M. Jaeger, “Impact-activated solidification of dense suspensions via dynamic jamming fronts”, Nature 487, 205-209, 2012. pdf
   

2. •see articles about this research in UChicago News, Science Mag’s ScienceNow, Discover Magazine, Science News, BBC Science News, Int’l Science Times, Wired, Popular Mechanics
   

The panel below compares the measured impact dynamics with a model we developed that takes as its ingredients the (time-dependent) depth of the jammed solid plug and size of the liquid region affected by it (this is treated as an “added mass”, similar to modeling the motion of solid bodies through liquids):