Chemistry Reference
In-Depth Information
fluid gels produced at a single polymer concentration, but at different
cooling rates, can behave quite differently in terms of their rheology.
Samples produced at the lower cooling rates have greater interaction
when the sample is at rest and thus show the highest yield stress. How-
ever, once the samples are flowing, the fluid gels produced at the higher
cooling rates have the highest viscosity. This is probably because the
particles are larger and have a more irregular shape.
As this example shows, there are many parameters that can be
changed in the formation of the gel particles. These not only change
the shape and size of the particle, but also manipulate the extent of
bridging between them and hence parameters like yield stress. Other
parameters which need to be considered and will be discussed later are
the type and concentration of the hydrocolloid used.
So having obtained sheared gel particles and having developed an
understanding of their material properties, the question becomes, how
do they behave in the mouth? As can be seen from Fig. 10.13, parameters
like yield stress and viscosity at a shear rate of 100 per second can be
manipulated. However, if we are to consider the use of these gel particles
as replacement for fat in an emulsion, we need to consider how they
behave in terms of lubrication in the mouth. Here again we turn our
attention to the thin film behaviour. Investigations of the tribology of
fluid gels are just beginning (Gabriele
et al.
, 2010), but are showing
some very interesting results.
Fig. 10.14 shows the tribological response from two concentrations
of agarose. At 1% agarose, there is a hysteresis between the ramp up
and the ramp down. It seems that on the ramp up, the particles are
initially excluded from the gap, but as the speed is increased, particles
0.5
0.5
4% Agarose ramp up
4% Agarose ramp down
1% Agarose ramp up
1% Agarose ramp down
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
(a)
20
40
60
80
100
0
20
40
60
80
100
(b)
Disk speed (mm/second)
Disk speed (mm/second)
Fig. 10.14
Difference in the friction ramps for different fluid gels with different agarose
concentrations. The load W is constant to 4 N for all the experiments (adapted from
Gabriele et al., 2010).
