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What do these viscosity terms mean?

- Newtonian fluids exhibit a constant viscosity independent of deformation rate.
- Non-Newtonian fluids exhibit a viscosity function that is dependent on shear rate or shear stress.
- Non-Newtonian fluids may be structured or simple fluids.
- Non-Newtonian systems may also exhibit time dependent shear effects such as thixotropy or rheopexy.

- Pseudoplasticity represents a class of non-Newtonian fluids that exhibit decreasing viscosity with increasing shear rate.
- One example of pseudoplasticity is a Power Law fluid.
- Over a wide range of shear rates, pseudoplasticity can occur over a specified range of shear rates.
- Some fluids exhibit rheopectic behavior where the vsicosity increases with increasing shear rate. This can occur over a long or short shear rate range.

Shear thinning is an ambiguous term since it can refer to either shear rate or shear stress viscosity dependence or time of shear, as shown in the expression below:

fddt, t

where

= shear stress

ddt = shear rate which is the

time rate of change of strain

strain

t = time of shear

Many fluids and soft solids are thixotropic, meaning that at a constant rate of deformation, the viscosity is a decreasing function of time of shear. For example, if we have a fluid in a Brookfield viscometer and monitor the torque as the spindle rotates at a constant rpm, the torque will begin to decrease as the spindle continues to rotate, eventually reaching a steady state value torque or shear stress value.

At certain shear rates, rheopexy can occur whereby the viscosity at a constant shear rate will increase with time of shear. This is less common but does occur. Actually this probably occurs more frequently than one would suspect.

The best method to determine time dependent shear effects is to run constant shear rate or shear stress measurements over an appropriate range of experimental conditions of time of shear and rate of deformation. A common test method that combines the effect of shear rate and time of shear is the thixotropic loop whereby the shear rate is ramped from rest to a maximum value and then decelerated back to rest within a prescribed time interval.

As with many rheological properties, these properties may also be temperature dependent.

Plesae refer to our technical bulletins for more information on characterizing these fluid systems.

Technical Bulletin 01: Newtonian and non_Newtonian Fluids

Technical Bulletin 02: Time Dependent Shear Effects