Title:
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The squeeze flow rheometry of foods
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Food pastes and suspensions are complex materials which are difficult to characterise in
conventional rotational rheometry due to their complex microstructure and their usual
tendency to slip at the wall. Squeeze flow, in which material flows between two circular
plates as they are brought together, has traditionally been used as a way of overcoming these
problems. Two squeeze flow rheometers were set up incorporating two novel pressuresensing
devices, a strip sensor and a grid sensor. The strip sensor which measures the
pressure distribution in the material along a radius, was found to be too sensitive to test fluids.
The grid sensor which measures the whole two-dimensional pressure distribution in the
material, was more robust and was validat~d using Newtonian silicone oils. Excellent
agreement was found between the pressure sensor data and the well-known Stefan's theory,
over the range of dimensionless times 0.4 to 0.8. On extension to non-Newtonian fluids,
reasonable agreement was found between the well-known Scott's theory and the pressure data
for a model pseudoplastic fluid, over an equivalent range of dimensionless times. The results
for a paste material, however, did not agree well with Scott's theory. The flow regime in
squeeze flow is different to that found in the simple shear flow of rotational rheometers. In
squeeze flow the flow pattern changes from elongational to shear flow as the experiment
progresses, which has two consequences: (i) it is unlikely that a single rheological equation
could describe the flow over the whole experiment and (ii) the different flow regimes mean
that comparison with rheological parameters obtained from conventional rheometry is
unlikely to be successful.
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