Page 269 - Introduction to Colloid and Surface Chemistry

258 Rheology

For Hookean elastic solids the stress and strain are in phase, whereas
for purely viscous liquids the strain lags 90° behind the applied stress.

Time-dependent deformation and structural characteristics

The deformation of a material when subjected to a constant stress is,
6
as discussed, usually time-dependent. At times of c. 10~ s and less all
materials, including liquids, have shear compliances (i.e shear/shear
11 9 2
stress) of c. 10~~ to 10~~ m N~*. This is because there is only
sufficient time available for an alteration of interatomic distances and
bending of bond angles to take place, and the response of all
materials is of the same order of magnitude in this respect. The time
required for the various structural units of a material to move into
new positions relative to one another depends on the size and shape
of the units and the strength of the bonds between them.
The molecules of a liquid start to move relative to one another and
the shear compliance increases rapidly after the shearing force has
6
been applied for only c. 10~ s. On the other hand, hard solids, such
as diamond, sodium chloride crystals and materials at a low enough
temperature to be in the glassy state, show only the above rapid
elastic deformation, even after the shearing stress has been applied
for a considerable time.

10 Liquid

810-
c
.2
~OL
E
o
U Crystalline or
glass-like solid
TO-"
10-6 10*
Time/s
Figure 9.7 The time-dependence of deformations under constant stress