creaming was chosen so that Eq. (13.24) predicts the same amount of
creaming for both series of emulsions. As seen, this was indeed the case for
j!0. For gravity creaming, Eq. (13.26) was obeyed withn¼8.6. The
Pe ́clet number is calculated at about 0.4, implying that group sedimentation
would have been negligible. Centrifugal creaming occurred at 200?g, hence
Pe&80, implying that considerable group sedimentation must have
occurred. This is borne out by the results aroundj¼0.03. For higherj
values, mutual hindrance must have more than offset the effect of group
rising. For gravity sedimentation of large and dense particles, hence high Pe
values, an increase of settling rate over the Stokes velocity has also been
observed (up to a factor of about 2.5) atj-values around 0.02.
Aggregating Particles. Repulsive colloidal interaction forces
between particles hardly affect sedimentation, but the effect of attractive
forces can be very strong. Aggregates naturally sediment faster than single
particles. Fractal aggregates containingNparticles tend to move faster than
FIGURE 13.12 Creaming rate relative to the Stokes velocity v/vSof O–W
emulsions of various volume fractionjand of average droplet sized 53 ¼1.4mm.
Creaming under gravity (.) or in a centrifuge (*). (Results from the author’s
laboratory.)