Particle Tracking as a Probe of Microrheology in Food Colloids - Food Colloids: Self-Assembly and Material Science - page 313

Figure 6 Confocal images of 0.21 m m probe particles in phase-separating emulsion (30
vol.% oil, 1.4 wt.% sodium caseinate, pH 6.8) with 0.05 wt.% xanthan in the
aqueous phase: (a) shortly after stopping stirring and (b) after xanthan-rich
blobs had relaxed to circular (spherical) scale bar ¼ 10 m m

Image (b) was recorded 1-2 h later: following coarsening, a large phase-
separated xanthan-rich blob has relaxed into a circular (spherical) shape.
The probe microspheres were quite evenly distributed within the phase-
separated emulsion regions. Nevertheless there appeared to be a gradual
migration of particles from the oil-droplet-rich microphase to the xanthan-rich
microphase. That is, there seemed to be a thermodynamic preference for probe
particles to be located in the xanthan-rich (water-rich) phase. Presumably this
can be attributed to unfavourable repulsion (due to excluded volume interac-
tions) between the charged microspheres and the protein-coated emulsion
droplets in the highly concentrated oil-droplet-rich phase. For instance, at
the composition illustrated in Figure 6, there was initially observed to be an
excess particle number density of 9% in oil-droplet-rich regions (compared

with the random distribution), whereas after around 3 h there was a slight
excess particle density (

B

4%) in the xanthan-rich regions. Migration of probe
particles from the oil-droplet-rich phase to the xanthan-rich phase occurred
much more slowly than the timescale of individual particle Brownian move-
ment. So, fortunately, it did not affect the interpretation of the diffusion
coefficients derived from the particle tracking experiments.
Some representative particle trajectories in the emulsion with 0.05 wt.%
xanthan are shown in Figure 7. We can clearly see that, during the same time
interval (10 s), a probe particle located in the xanthan-rich microphase moves
much further (trajectory A) than one located in the oil-droplet-rich microphase
(trajectory B). Also illustrated in Figure 7 is the path of a particle that starts in
the xanthan-rich phase but ends up in the oil-droplet phase (trajectory C). While

B