Dilatant
This behavior is only one type of deviation from Newton's law of viscosity, and it is controlled by such factors as particle size, shape, and distribution.
The properties of these suspensions depend on Hamaker theory and Van der Waals forces and can be stabilized electrostatically or sterically.
A large portion of the properties of these systems are due to the surface chemistry of particles in dispersion, known as colloids.
This 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.
In addition to these parameters, all shear thickening fluids are stabilized suspensions and have a volume fraction of solid that is relatively high.
These solutions are different from a Colloid in that they are unstable; the solid particles in dispersion are sufficiently large for sedimentation, causing them to eventually settle.
In an unstable suspension, the dispersed, particulate phase will come out of solution in response to forces acting upon the particles, such as gravity or Hamaker attraction.
Shear thickening behavior is typically observed in suspensions of small, solid particulates, indicating that the particle-particle Hamaker attraction is the dominant force.
Similar to Van der Waals forces, Hamaker theory describes the magnitude of the particle-particle interaction as inversely proportional to the square of the distance.
However, at short distances, the Hamaker attraction dominates, causing the particulates to coagulate and fall out of solution.
Suspensions of similarly charged particles dispersed in a liquid electrolyte are stabilized through an effect described by the Helmholtz double layer model.
The first layer is the charged surface of the particle, which creates an electrostatic field that affects the ions in the electrolyte.
The following equation provides the energy between two colloids as a result of the Hamaker interactions and electrostatic repulsion.
However, once shear forces dominate, particles enter a state of flocculation and are no longer held in suspension; they begin to behave like a solid.
Shear thickening behavior is highly dependent upon the volume fraction of solid particulate suspended within the liquid.
In theory the particles have extremely small interparticle gaps, rendering this momentary, transient hydrocluster as incompressible.
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.
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.
Researchers demonstrated that high-strength fabrics such as Kevlar can be made more bulletproof and stab-resistant when impregnated with the fluid.
[12][13] The goal of the “liquid armor” technology is to create a new material that is low-cost and lightweight while still offering equivalent or superior ballistic properties compared to current Kevlar fabric.
[15] The company D3O invented a non-Newtonian–based material that has seen wide adaptation across a broad range of standard and custom applications, including motorcycle and extreme-sports protective gear, industrial work wear, military applications, and impact protection for electronics.
