View Full Version : A striking experiment shows how you can run on quicksand

07-11-12, 02:20 PM
http://cdn.arstechnica.net/wp-content/uploads/2012/07/quicksand_rod_strike-640x312.jpg A metal rod strikes the surface of a suspension of cornflour in water. Unlike normal (Newtonian) fluid, the surface of the suspension acts nearly like a solid.
Scott Waitukaitis

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.

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