Biomedical Engineering Reference
In-Depth Information
Fig. 2.12
Generalized flow
behavior of fluids
Time-Independent Non-Newtonian Fluids
Here too, two subdivisions have to be made:
Bingham
: differs only from Newtonian fluids by the
yield stress
i.e., the stress to
be exceeded before flow starts. Many common fluids approximate this type. A fluid
at rest contains a 3D structure which is sufficiently rigid to resist any stress less than
the yield stress (example: oil paints).
Pseudoplasticity
: no yield value but the shear stress decreases progressively with
shear rate. The molecules of a pseudo-plastic fluid align after starting shear so
quickly that the time effect cannot be detected.
Dilatancy
: similar to pseudoplasticity, showing no yield stress but the apparent
viscosity increases with increasing shear rate. It is observed for highly concentrated
suspensions. At rest the voids between the agglomerated particles is at a minimum
and the liquid sufficient to fill the voids. When sheared, the agglomerates are broken
up and the material 'dilates' because the liquid phase is not sufficient to fill up the
voids.
Time-Dependent Non-Newtonian Fluids
For these fluids, the relation shear stress/shear rate depends not only on shear rate
but also on the duration of shear. The behavior cannot be described by simple
rheological equations. Two classes are distinguished:
thixotropic fluids
for which shear stress or apparent viscosity is decreasing with
shear duration;
rheopectic fluids
for which shear stress is increasing with shear duration.
Many colloidal suspensions are thixotropic and exhibit structural hysteresis
[67, 68].
