Chemistry Reference
In-Depth Information
Torque
σ
()
r
≤
σ
σ
()
r
≤
σ
y
y
Fig. 2.9
Schematic diagram of a vane rheometer with pseudo-infinite vessel arrangement
for a suspension with a yield stress (
σ
y
); i.e. the shear stress falls below the yield stress before
the wall, which prevents slippage at the outer boundary of the rheometer.
shear field varies with radial position; therefore, the reported shear rate
is an average value. This geometry is also susceptible to sedimentation.
2
.
8
.
1
.
3
Concentric cylinder rheometry
Concentric cylinder geometry (Fig. 2.9) has a well-defined flow, with
small annular gaps, and accurate interpretation is possible. It does not
expel suspension even at high shear rates, and it is more sensitive to
low viscous material than the cone-and-plate and parallel plate systems.
However, its narrow gaps are inappropriate for solid particles, as they
may compact particles and jam the instrument; settling and slippage of
solids may occur (Bongenaar
et al.
, 1973).
2
.
8
.
1
.
4
Vane rheometry
This approach to rheological measurements has been employed by a
number of authors, and has been suggested as a reliable and largely
reproducible method for characterising biological broths (Tucker and
Thomas, 1993; Riley
et al
., 2000). The success and attraction of this
method lies in the use of a turbine as the moving rotor so that the sus-
pension is forced to shear rather than slip at the moving surface (Barnes
and Nguyen, 2001). For a suspension exhibiting a yield stress, or which
is very shear thinning, the turbine can be contained in a vessel large
enough so that the shear stress falls to below the yield stress before the
wall (Fig. 2.9). Thus, it is a so-called pseudo-infinite vessel arrangement,
which also prevents slippage at the outer boundary of the rheometer. This
equipment has a significant disadvantage of having a poorly defined flow
