92168.pdf

Rheology 245

years considerable advances have been made towards understanding

rheological behaviour and putting it on to a quantitative basis^120.

For convenience, this chapter has been divided into three sections

in which the viscosity of dilute solutions and dispersions, non-

Newtonian flow, and the viscoelastic properties of semi-solid systems

are discussed.

Viscosity

Newtonian viscosity

The viscosity of a liquid is a measure of the internal resistance offered
to the relative motion of different parts of the liquid. Viscosity is
described as Newtonian when the shearing force per unit area cr
between two parallel planes of liquid in relative motion is proportional
to the velocity gradient D between the planes - i.e.

o-=7?D (9.1)

where TJ is the coefficient of viscosity. The dimension of 17 is,
therefore, (mass) (length)"^1 (time)"^1.
For most pure liquids and for many solutions and dispersions, j\ is a
well-defined quantity for a given temperature and pressure which is
independent of a and D, provided that the flow is streamlined (i.e.
laminar). For many other solutions and dispersions, especially if
concentrated and if the particles are asymmetric and/or aggregated
deviations from Newtonian flow are observed. The main causes of
non-Newtonian flow are the formation of a structure throughout the
system and orientation of asymmetric particles caused by the velocity
gradient.

Measurement of viscosity^121

Capillary flow methods

The most frequently employed methods for measuring viscosities are
based on flow through a capillary tube. The pressure under which the
liquid flows furnishes the shearing stress.
The relative viscosities of two liquids can be determined by using a