makesysomewhat higher, since a yield stress tends to be smaller for longer
time scales (see Section 5.1.3).
A yield stress of 1 Pa or less generally goes unnoticed: 1 Pa corresponds
to the stress exerted by a column of water of height 0.1 mm. This also
implies that slight agitation, e.g., caused by pouring liquid out of the vessel,
will overcome the forces holding the network together, whereby the yield
stress disappears. For the dispersion to be stable, the network should then
be restored before the particles can show substantial sedimentation. It is
observed that a resting time of about 15 min often suffices to restore fully
the value of sy. Taking the same example as just given, and assuming,
moreover, that the effective value ofZcequals 0:1Pa?s, we calculate that
vS&55 nm?s^1 , which then leads to sedimentation over at most 50mmin
15 min. This will generally be acceptable.
The particles that would sediment can also themselves form or
participate in forming a weak, generally fractal, network. A good example is
soya milk, made by wet milling of soaked (whole or dehulled) soya beans.
The soya milk so obtained contains small oil droplets and a variety of solid
particles, including cell fragments, whole cells and fragments of the hulls.
Some of these particles are severalmm in size and would certainly sediment
in a Newtonian aqueous liquid. However, soya milk tends to be quite stable.
Figure 13.13 shows some flow curves, and it is seen that all samples exhibit a
yield stress (the intercepts of the curves on the stress axis). It is seen that the
magnitude ofsydepends on pretreatment. Comparing soya milks from
whole or dehulled beans, the former contains larger particles, which would
demand a higher yield stress, as is indeed the case.
Finally, a gel network containing entrapped particles, or consisting of
particles, may still exert a gravitational force onto the liquid. This can result
in slow compaction of the network, starting at the bottom or at the top,
according toDrbeing negative or positive, respectively. The only way in
which this is prevented for very weak gels is by a network thatsticks to the
wall of the vessel, whereby a stress at the wall can counterbalance the
gravitational force. It depends on the material and the cleanliness of the
vessel whether sticking occurs.
Question
A food company produces a number of different oil-in-water emulsions. It is decided
to apply a centrifuge test to predict how fast creaming will occur in the stored
emulsions. In the test the percentage of the oil that reaches the cream layer under
specified conditions is taken as the criterion. (a) What should the test conditions (like