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Oscillatory
tests
During Oscillatory tests the measuring geometry
oscillates around the rotation axis instead of moving constantly
in one direction. Therefore the measured values show not only
the viscous behaviour but also the stiffness (elasticity) of
the sample.
Although oscillatory tests can supply more information compared
to rotational tests, they are rarely used for characterization
in industry.
This has several reasons:
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1)
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Oscillatory tests introduce unusual physical values, for which no
experience exists. |
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2)
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To perform oscillatory tests an air bearing rheometer is required,
which is not always available. |
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3)
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Rheological textbooks often describe oscillatory tests very theoretically.
In industrial labs it is not necessary to deal with the rheological
theory. Normally the measurements are only comparative between "good"
and "bad" samples, whereupon the differentiation has to
be reliable. Thus the demand of reproducibility is very high. |
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4)
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For some samples, like suspensions with big particles or bad homogeneity,
the reproducibility of oscillatory tests is worse than for rotational
tests. |
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5)
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Oscillatory tests normally describe only the static condition at
very low deformation, thus without disturbance of the quiescent
structure. |
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Nevertheless
it is reasonable to perform oscillatory tests for industrial
labs. In many cases they give more information than rotational
tests. Meanwhile rheological textbooks exist, which explain
oscillatory measurement techniques and other modern rheological
measuring techniques related to practical applications. Advisable
is e.g. "The Rheology Handbook" written by T. Mezger,
Vincentz Network Hannover, 2nd ed. 2006.
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