Particle Tracking as a Probe of Microrheology in Food Colloids - Food Colloids: Self-Assembly and Material Science - page 311

10

1

0.1

0.01

1E-5 1E-4 1E-3 0.01 0.1 1 10 100 1000

Shear rate(s -1 )
Figure 3 Apparent viscosity against shear rate at 201C for oil-in-water emulsions (30
vol.% oil, 1.4 wt.% sodium caseinate, pH 6.8) containing different concentra-
tions of xanthan in the aqueous phase: D , none; ' , 0.03 wt.%; & , 0.04 wt.%;
K , 0.05 wt.%; J , 0.06 wt.%; m , 0.07 wt.%

and increasingly high viscosities at low shear rates for increasing polysaccha-
ride concentrations. Figure 4 shows the limiting zero shear rate viscosity of the
emulsions (left ordinate axis) plotted against the aqueous phase xanthan
concentration. Also plotted on a different scale in Figure 4 (right ordinate
axis) are corresponding data for 0.03-0.07 wt.% aqueous xanthan solutions.
We note the enormous difference in absolute viscosity values for the emulsions
and the solutions. That is, the viscosities of the xanthan-containing emulsions
are of the order of a thousand times larger than those of the equivalent
xanthan-containing solutions.
Figure 5 shows particle tracking data for the caseinate-stabilized emulsion
(30 vol.% oil, 1.4 wt.% protein) without any added xanthan. The two sets of
data refer to two sizes of microspheres (0.5 and 0.89 mm). To allow direct
comparison of results from the two kinds of probe particles, the quantity
a h Dr 2 ( t ) i (i.e. particle radius MSD) is plotted against lag time t on a log-log
scale. In each case the slope of the best linear fit to the data is close to unity
indicating that the particle motion is predominantly diffusive ( a 4 0.9). The
combined data can be fitted by the Stokes-Einstein relation [Equation (2)] with
an effective viscosity Z ¼ (6.5 1.5) mPa s. This value is in satisfactory
agreement with the value Z ¼ (9.5 2) Pa s measured at low shear rate in the
rheometer (see Figure 3).
As has been reported previously, 14 the emulsions containing 4 0.03 wt.%
xanthan appear heterogeneous and structurally time dependent under the
confocal microscope. In the polysaccharide concentration range 0.03-0.05
wt.%, we observed time-dependent microscopic separation into oil-rich and
oil-depleted regions. These regions exhibited relaxation, coarsening and coa-
lescence phenomena. In particular, after stirring stopped, the interfacial tension