A dilatant material is one in which viscosity increases with the rate of shear (also termed shear thickening). The classic example is Silly Putty, which can be stretched slowly but snaps and fractures if pulled quickly. The dilatant effect can be seen more readily with a mixture of corn starch and water (sometimes called oobleck), which acts in counter-intuitive ways when struck or thrown against a surface.
Industrial uses
Dilatant materials have certain industrial uses due to their shear thickening behavior. For example, some all wheel drive systems use a torque converter full of dilatant fluid to provide power transfer between front and rear wheels. On high traction pavement, the relative motion between primary and secondary drive wheels is the same, so the shear is low and little power is transferred. When the primary drive wheels start to slip, the shear increases, causing the fluid to thicken. As the fluid thickens, the torque transferred to the secondary drive wheels increases proportionally, until the maximum amount of power possible in the fully thickened state is transferred. See also: limited slip differential, some types of which operate on the same principle.
To the operator, this system is entirely passive, engaging all four wheels to drive when needed, and dropping back to two wheel drive once the need has passed. This system is generally used for on-road vehicles rather than off-road vehicles, since the maximum viscocity of the dilatant fluid determines the maximum amount of torque that can be passed across the coupling.
The opposite of a dilatant material is a pseudo-plastic.
See also