Chemistry Reference
In-Depth Information
Controlled stress
Controlled strain
Drag-cup motor
Position sensor | Load cell
Air bearing
Measurement
geometry
(a)
Cone-and-plate
Parallel plate
Concentric
Concentric
Double gap
(b)
Fig. 2.8
Schematic of two types of rotary rheometers and related geometries (reproduced
from Goodwin and Hughes, 2008): (a) controlled stress versus control strain and (b) cross
sections of common measuring geometries.
in the cone-and-plate rheometry, the size of any particles present within
the sample should be limited to 10-20% of the truncation height of this
geometry; i.e. particles should typically be smaller than 5-20 µminsize
to avoid jamming the gap and irregularities in the data. Care must be
taken when measuring at high shear rates as material can 'escape' from
between the plates. Furthermore, this geometry is very susceptible to
sedimentation.
2
.
8
.
1
.
2
Parallel plate rheometry
Similar to the cone-and-plate, parallel plate geometry (Fig. 2.8) has a
well-defined flow, and accurate interpretation is, in principle, possible.
It requires only a small sample volume, and it is easy to clean. The main
advantage of this geometry is that the user can define the gap which can
be set to values considerably larger than those of typical particles in the
material to be tested. This gap can also be varied to probe wall slip effects
(Barnes, 1995). The parallel plate geometry has the disadvantage that the
