Quicksand and other non-Newtonian fluids share properties with both liquids and solids. Non-Newtonian fluids consist of tiny grains suspended in liquid, with the appearance of a solid or gel. Stand on quicksand and you will sink (though not as rapidly as movies and cartoons suggest). But strike it quickly and it will briefly harden. Previous explanations of quicksand behavior relied on the presence of containment walls and effects like grain dilation under stress. However, a new experimental study challenges prior assumptions, showing that new concepts may be needed to explain non-Newtonian fluids.

Scott R. Waitukaitis and Heinrich M. Jaeger at the University of Chicago created a quicksand-like substance called "oobleck" out of cornflour and water, which they then struck with an aluminum rod. By measuring the position, speed, and acceleration of the rod as it interacted with the oobleck, they determined that its solidification arises from compression that propagates away from the impact point. By using a huge amount of fluid (25 liters), the researchers showed the bizarre non-Newtonian effects were independent of the size of the container, so the presence of confining walls is irrelevant.

Through X-ray imaging, they discovered a nearly cylindrical solid region forms directly below the impact point. The detailed analysis led the authors to develop a simple model for the impact, which bears striking similarity to models for objects falling into liquids, but produces very different effects.



A Newtonian fluid (named for Sir Isaac Newton) is always a liquid, no matter what forces are exerted on it. Water is a typical example: it doesn't thicken or become more viscous, and poking it with a stick doesn't leave a hole behind. Non-Newtonian fluids, as emphasized previously, behave very differently depending on the forces they experience. Their viscosity—resistance to flow—can change drastically under different circumstances, depending on what kind of fluid is involved, as anyone who has made pudding from scratch can attest.



This may have been the most carefully monitored bowl of starch ever devised. In the experiment, the researchers mounted the aluminum rod using guide rails to make sure it impacted along a single axis. For different trials, they either dropped the rod (free fall) or used a slingshot to drive it more quickly downward. The rod was fixed with an accelerometer, and the whole process was recorded on high-speed video to measure the instantaneous position, speed, and acceleration.

The grains of the cornflour in the oobleck are irregularly shaped and range in size from 5 to 20 microns (0.005 to 0.02 millimeters), which is typical of quicksand and other non-Newtonian fluids. Additionally, the suspension contained tracer particles that could be imaged with X-rays; motion within the oobleck could be tracked with the tracers. The authors positioned a force sensor directly below the rod at the bottom of the container to examine how the impact distributed itself through the fluid. They also used a laser line across the surface to determine how its shape changed.

To measure the effect of container size, the researchers tested fluid containers ranging from 8.5 cm to 20.5 in depth. They found that the rod experienced a rapid deceleration upon impact with the surface at the same point in time, regardless of the container depth. However, shallower containers experienced a rebound effect: after a time, the rod began accelerating upward again.

Additionally, X-ray images showed the tracer particles didn't spread much to the sides in the region immediately below the rod. Instead, they moved as a nearly cylindrical unit, acting almost like a second rod within the suspension. This plug of material was surrounded by a conical region where the suspension flowed outward and upward in response, lifting the surface slightly beyond the impact zone (as shown in the image above). After some time, the plug "melted," restoring the suspension to its usual quasi-liquid state.

Combining their data, the researchers constructed a simple model for the suspension, including the size of the solid-like plug and the conical displaced mass. The equation bore some similarities to ordinary fluid displacement models, again demonstrating the hybrid nature of suspensions. It also contrasts greatly with the usual approach to non-Newtonian fluids, where the walls of the container play a role and, instead of generating a nearly cylindrical plug, the force distributes itself along angles.

The physical picture of the process is clear: momentum from the impact was carried directly downward, and rebounded when it hit the bottom of the container (if it had sufficient time to do so before melting). While the study used cornflour for simplicity and cost-effectiveness, the authors argued the similarity in grain size and shape should make their model applicable to other suspensions.

Nature, 2012. DOI: 10.1038/nature11187 (About DOIs).