Dilatant

A dilatant material is one in which viscosity increases with the rate of shear. Such a shear thickening fluid, also known by the acronym STF, is an example of a smart material, a class of materials that respond to changes in the environment.

The dilatant effect can readily be seen with a mixture of cornstarch and water (sometimes called oobleck), which acts in counter-intuitive ways when struck or thrown against a surface.

The opposite of a dilatant material is a pseudoplastic.

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Applications

Traction control

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 road surfacing, 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 viscosity of the dilatant fluid limits the amount of torque that can be passed across the coupling.

Body armor

Various corporate and government entities are researching the application of shear thickening fluids for use as body armor. Such a system could allow the wearer flexibility for a normal range of movement, yet provide rigidity to resist piercing by bullets, stabbing knife blows, and similar attacks. The principle is similar to that of chainmail armor, though body armor using a dilatant would be much lighter and could be cut like cloth. The dilatant fluid would disperse the force of a sudden blow over a wider area of the user's body, reducing the blunt force trauma; against slow attacks, such as a slow but forceful stab, the dilatant would not provide any additional protection.

One current example is d3o, designed for protection against falls.

See also

• d3o
• Non-Newtonian fluid
• Power-law fluid
• Pseudoplastic
• Rheology
• Silly Putty
• Thixotropy