single particles by a factor of about 1.8N^1 1/D. The sedimentation also
enhances aggregation, since the larger aggregates tend to overtake the
smaller ones, whereby larger aggregates are formed, etc. An example is
creaming of cold milk. Raw cows’ milk contains a cryoglobulin, a large
associate of protein molecules, that adsorbs onto and forms bridges between
the milk fat globules at low temperature. Upon cooling it takes a while
before aggregation starts, but then creaming becomes very fast, to such an
extent that formation of a cream layer in a tank of 10 m height occurs almost
as fast as in a 10 cm beaker.
Figure 13.11c shows creaming profiles of an emulsion to which a little
xanthan gum has been added to induce depletion aggregation (see Section
12.3.3). After aggregates have formed, creaming becomes quite rapid, and a
sharp front is formed between an all but empty subnatant and the rest of the
liquid, which then ‘‘shrinks’’ to form a sharply defined cream layer.
Afterwards, the cream layer becomes somewhat more compacted. Notice
that in Figure 13.11b the jvalue in the cream is about 0.7, roughly
corresponding to dense packing; in frame cjequals about 0.5, due to the
fractal aggregates having an internaljvalue well below unity. If the volume
fraction of particles in the liquid is much smaller than the value 0.2 in the
figure, thejvalue in the cream layer may be as small as 0.25. Altogether,
aggregation leads to a greatly enhanced sedimentation rate and to a
sediment or cream layer that is far less densely packed than for
sedimentation of single particles.
The aggregation in the system of Figure 13.11c must have been partly
reversible. If more xanthan gum is added, the ‘‘bonds’’ formed between the
particles are virtually irreversible, and the whole system tends to gel before
significant creaming can occur.
Preventing or Retarding Sedimentation. In many cases,
settling or creaming of particles during storage is undesirable, be it in
the final product or at some stage during processing. It may not be the
sedimentation as such that causes the problem, since it can often be simply
undone, for instance by inverting the closed vessel a couple of times.
However, the particles in a sediment or cream layer are closely packed and
this may lead to gradual formation of strong bonds between them (see
Section 12.4, bond strengthening). This then gives rise to a coherent mass
that cannot be easily dispersed again. Emulsion droplets in a cream layer
may possibly coalesce.
Complete prevention of sedimentation is often impossible, but slowing
down to a specified level may then suffice. The following measures may be
considered.