Chemistry Reference
In-Depth Information
2.3.1
Phenomenological rheology or macrorheology
This domain of rheology is concerned with real bodies on a macroscopic
level, and disregards the molecular nature of the material. Therefore,
the equations describing the observed phenomena contain coefficients
called rheological parameters, which have to be determined experimen-
tally. Phenomenological rheology tries to explain the complex phenom-
ena happening during the deformation of real bodies, and to formulate
rheological equations of state that define these.
2.3.2
Structural rheology or microrheology
Structural rheology is concerned with the connections between the bulk
rheological properties of a substance and its actual microscopic struc-
ture. It tries to predict the properties of macromolecular compounds
based on mathematical models of structure and dynamics. In microrhe-
ology, the rheological behaviour of two-phase and multiphase systems
is derived from the known rheological behaviour of their elements.
However, the actual composition of dispersed systems makes a mathe-
matical approach unworkable outside the simplest of cases. In these, a
mechanical model replaces the unknown structure and is assumed to be-
have in an analogous fashion. Such models consist of different elements
such as elastic springs (Hookean springs), viscous dashpots (Newtonian
dashpots) or a combination of these two, but in general these have no
exact counterpart within the real material. Structural rheology has been
successful in describing the behaviour of solid elastomers and dilute
polymer solutions. However, it has been hardly applicable to concen-
trated solutions.
2.3.3
Rheometry
Rheometry is concerned with the quantitative determination of rheo-
logical properties of the investigated system in an experimental way.
There are five principal types of rheometers: (i) the concentric cylin-
der or coaxial cylinder rotary viscometer, (ii) the rotating cylinder in
an 'infinite' medium, (iii) the cone-and-plate (and plate-plate) rotary
rheometer, (iv) the vane rheometer (v) and the capillary tube rheometer.
The first four rheometers are usually applied to obtain rheological prop-
erties of liquids or fairly soft pastes (Nielsen, 1977). The last can be used
to measure the rheology of stiffer (more rigid) pastes. In theory, funda-
mental rheological properties should be independent of the instruments
on which they are measured, so different instruments should yield the
same results. However, this is an ideal concept and different instruments
rarely give identical results. Therefore, it is important to distinguish the
