Chemistry Reference
In-Depth Information
In a tube of radius R, we use the volumetric flow rate, Q, to calculate a mean
velocity along the tube and we have:
Qr
pRZ
R
e
E
ð
1
:
21
Þ
It is important that we know at what Reynolds number our instrumental
configurations give turbulent flow and work below this figure or we will think
that shear thickening is occurring! A figure of R
e
o
3000 to 10 000 is usually
satisfactory for cone and plates or capillary viscometers, but values as low as
300 may be the maximum for some cup and bob units.
1.4 MACROMOLECULAR AND COLLOIDAL SYSTEMS
The range of diffusional timescales for dilute systems that are shown in Figure 1.9
and were calculated using eqn (1.13) immediately shows us that macromolecules
and colloidal particles have diffusional timescales that are within the range
covered by our laboratory instrumentation. Moreover, as soon as we concentrate
these systems, the motion of each unit is slowed by the interaction with its
neighbours and timescales become much longer. Hence, quite slow motions may
be in the region of large Deborah or Pe´ clet numbers. It is for this reason that
polymers and clay particles are often added to formulations as ''thickeners'' or
''viscosity modifiers''. We will deal with these systems in considerable detail in
subsequent chapters and we shall see that it is their chemical nature, and the
chemical environment that we put them in, which gives such a rich pattern of
behaviour.
mols.
macromols.
colloids
10
4
10
2
10
0
10
-2
10
-4
10
-6
10
-8
10
-10
10
-12
10
-1
10
0
10
1
10
2
10
3
10
4
radius, a/nm
Figure 1.9
Diffusional timescale from eqn (1.13).
