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Oobleck is basically a mixture of cornstarch and water. It has the interesting property of going hard when pressure is applied. For example, it feels like a liquid when you swirl your fingers through it, but it becomes hard for a second if you punch it.

The common explanation for this is that the starch-water mixture is a suspension, and the starch particles "lock" when pressure is applied.

What exactly is the mechanism of this "locking"?

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Wikipedia already offers a decent summary of two theories of shear thickening behavior - namely the generation of transient 'defects' in a high-density suspension that either resist further growth or are incompressible in the direction of shear.

A potential macroscale analogue of shear thickening is the dramatic increase in viscosity of large particle-air mixtures upon pneumatic removal of the interparticle fluid, as is exploited in jamming skin enabled locomotion. (paper here) This should be roughly comparable to the transient decrease in particle spacing in a shear thickening fluid that is being compressed beyond its relaxation rate.

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