A dilatant (also termed shear thickening) 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 non-Newtonian fluid.
The dilatant effect can readily be seen with a mixture of cornstarch and water (sometimes called oobleck), which acts in counterintuitive ways when struck or thrown against a surface.
- Rheopecty is a similar property in which viscosity increases with cumulative stress or agitation over time.
- The opposite of a dilatant material is a pseudoplastic.
Dilatant materials have certain industrial uses due to their shear thickening behavior. For example, some all wheel drive
systems use a viscous coupling unit
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.
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.
Two current examples of dilatant materials being used in personal protective equipment are d3o, and Active Protection System, manufactured by Dow Corning.