Chemistry Reference
In-Depth Information
with
λ
having a value of 1 at
t
=
0 and overall described by a first-order
decay equation:
d
d
t
=−
k
1
(
λ
−
λ
e
) r
λ>λ
e
(2.8)
where
e
is the final value for complete breakdown of the structure and
k
1
is the rate constant which depends on the shear rate.
λ
2.4.2
Wall slip
Slip happens during the flow of multiphase systems by the displacement
of the disperse phase away from solid boundaries and walls. As a result,
a thin layer adjacent to the wall will be rich in continuous phase, which
is usually less viscous in comparison to the bulk viscosity of the system.
Consequently, the fluid will flow (slip) much easier near the wall(s), as
the formed thin layer will induce some lubrication effect (Barnes, 1995;
Bertola
et al.
, 2003).
In terms of rheometry, if a system exhibits wall slip, then its viscosity
becomes a function of the size of the measuring geometry, the gap
size used as well as the magnitude of the applied stress (Meeker
et al
.,
2004). A sudden 'break' in the flow curve can be an indication of slip.
The presence of large particles in the fluid coupled with smooth walls
and flow of small dimensions can increase the risk of slip (Barnes, 1995).
Fig. 2.3 shows photographs of a sample that slips during a rheometry
test. In order to see the slip effect, the sample and both upper and lower
plates were marked before any shear applied (Fig. 2.3a). Fig. 2.3b shows
the same sample after a few seconds of applying shear and what can be
Upper plate
Upper plate
Paste
Paste
Lower plate
Lower plate
(a)
(b)
Fig. 2.3
Depiction of wall slippage by tracing a mark in rheological measurement of
mycoprotein paste at a shear rate of 20 per second, using 4 cm roughened parallel plates
(Miri, 2003).
