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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:

  • Oscillatory tests introduce unusual physical values, for which no experience exists.

  • To perform oscillatory tests an air bearing rheometer is required, which is not always available.

  • 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.

  • For some samples, like suspensions with big particles of bad homogeneity, the reproducibility of oscillatory tests is worse than for rotational tests.

  • Oscillatory test normally describe only the static condition at very low deformation, thus without disturbance of the quiescent structure.

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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