Particle Tracking as a Probe of Microrheology in Food Colloids - Food Colloids: Self-Assembly and Material Science - page 314

120

100

A C

80

B

60

40

20

0

100 150 200 250 300 350

Figure 7 Some trajectories 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) trajectory in xanthan-rich region; (B) trajectory in oil-
droplet-rich region; (C) trajectory passing across boundary between micro-
phases. Axis dimensions are in pixels (1 pixel ¼ 0.183 m m)

some care was taken to ensure that such trajectories were omitted as much as
possible from our statistical analysis, their existence conveniently demonstrates
the ability of probe particles to move easily across the interface between phase-
separating regions of contrasting composition and microstructure.
Figure 8 shows a log-log plot of MSD versus lag time t for microspheres of
diameter 0.21 mm in the xanthan-rich regions of the emulsions containing
different concentrations of xanthan. Slopes of the straight line fits were close to
unity ( a > 0.9) indicating a predominantly viscous response. The inferred
viscosities in the xanthan-rich regions are quite low ( o 5mPa), being slightly
higher than those inferred from particle tracking in the corresponding simple
xanthan solutions, 14 and fairly consistent with the sort of values measured by
bulk viscometry (Figure 4).
Figure 9 shows a log-log plot of MSD versus lag time t from the tracking of
microspheres of diameter 0.5 mm in the oil-droplet-rich regions of emulsions of
different xanthan contents. Predominantly diffusive motion (slope 4 0.9) is
obtained only for emulsions containing 0.03 or 0.04 wt.% xanthan; at higher
concentrations the microsphere mobility appears substantially hindered.
Nevertheless, at these higher polysaccharide contents, the MSD is still reason-
ably linear in t for t 4 2 s, and so for these relatively long-time processes in the
viscous regime, effective viscosities could be approximately estimated.
Figure 10 shows that the inferred viscosity of the flocculated oil droplet region
of the emulsions increases dramatically with xanthan concentration c.Also
indicated on the same graph are the apparent viscosities in the xanthan-rich
regions of the same emulsions, as determined from the particle tracking data in
Figure 8. A large contrast in absolute values can be seen: the inferred viscosities
in the oil-droplet-rich regions are some 10 2 -10 3 times larger than those in the
xanthan-rich regions. Hence, while the viscosity of the xanthan-containing
phase does increase with increasing biopolymer concentration, the main