Most natural and industrial fluids are not pure and contain particles of various nature and size. On may cite, for example, rivers, oil extraction muds, food products, cosmetic or pharmaceutical emulsions, paint, blood, polymer solutions, etc. The list is probably infinite. Such fluids which have a microstructure formed by inclusions are called complex fluids. Their flow behaviour does not usually satisfy Newton’s constitutive law (1.12) and often exhibits one or more non-Newtonian effects such as
A viscosity which depends on the shear rate: Many liquids, termed ‘pseudoplastic’, have a viscosity which decreases as the shear rate increases. This decrease can be very significant. On the other hand, the viscosity of starch suspensions increases with the shear rate. These phenomena must obviously be accounted for in the design of transport systems for such liquids.
Unequal normal stress under simple shear flow: This effect is observed when raw egg white climbs on the whip as we rotate it, instead of moving away from the whip as water would (so-called Weissenberg effect).