Rheology: rheology and its terminology

Rheology is concerned with the flow and deformation of materials experiencing an applied force. Two extremes of rheological behaviour are:

ELASTIC behaviour - e.g. perfectly rigid solids - where any deformation reverses spontaneously when an applied force is removed. Energy is stored by the system, then released.

VISCOUS (or PLASTIC) behaviour - e.g. ideal Newtonian liquids - where any deformation ceases when the applied force is removed. Energy performs work on the material.

In between elastic and viscous behaviour lies the real world of most substances, which are viscoelastic materials. The rheology described on these net-pages is for slurries and emphasises the underlying chemical effects. We continue with some definitions and common terminology used in rheology.

Newtonian (or viscous) behaviour.

For ideal viscous materials, the rate of deformation is in proportion to the force applied. Deformation ceases when the applied force is removed. The apparent viscosity is constant with changing shear rates. This behaviour is typical of simple liquids such as water.

Simple Newtonian behaviour

Thixotropy and Rheopexy (work hardening)

A thixotropic material becomes more fluid with increasing time of applied force. The applied force could be stirring, pumping or shaking. This effect is sometimes called ‘work softening’. It is often reversible, so that if left undisturbed for some time a thixotropic slurry regains its viscosity. Quicksand is an example of a thixotropic material.

A rheopectic material becomes more viscous with increasing time of applied force. This effect is the opposite of thixotropy, and is sometimes called ‘work hardening’.

Dilatancy (shear thickening)

A dilatant material resists deformation more than in proportion to the applied force. For example, the more effort you put into stirring a dilatant material, the more resistant it becomes to stirring. This is usually an indication that the applied force is causing the material to adopt a more ordered structure. A thick slurry of wet beach sand is often dilatant.

Plastic and Pseudoplastic (shear thinning)

PLASTIC materials initially resist deformation, until a yield stress is reached. When that stress is exceeded, the shear rate becomes measurable. Further stress leads finally to linear (Newtonian) behaviour.

PSEUDOPLASTIC materials exhibit shear thinning without the initial resistance to deformation. Like plastic materials, they also show linear (Newtonian) behaviour at the highest levels of stress and shear rate.

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