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Anne Kowalczyk, Bernhard Hochstein, Philipp Stahle, Norbert Willenbacher
Characterization of complex fluids at very low frequency: experimental verification of the strain rate-frequency superposition (SRFS) method
Appl. Rheol. 20:5 (2010) 52340 (10 pages)
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Strain rate frequency superposition (SRFS) has been suggested as new method to extend the frequency range for assessment
of the complex storage modulus G* of soft glassy materials to lower frequencies. The basic idea is that relaxation processes
in such fluids are accelerated by an external shear field, analogous to the effect of a temperature shift in polymer melts and
solutions. Master curves for G' and G'' are constructed from the apparent modulus data determined from non-linear oscillatory
shear experiments. Here we validate the SRFS principle for the first time by independent experiments and also demonstrate
its limitations.We compare SRFS results to directly measured G', G'' at frequencies down to 10-3 rad/s and creep experiments
lasting up to 104 s for a variety of gel-like fluids, including polymeric thickener solutions, a highly concentrated w/oemulsion,
and wormlike micellar surfactant solutions, as well as a weakly viscoelastic non-Brownian suspension of glass
beads. Good agreement between SRFS data and directly measured G', G'' values for the thickener solutions, the emulsion as
well as the suspension. Apparent viscosity data obtained from creep experiments and absolute values of the complex viscosity
in the low frequency limit agree fairly well for these fluids. But the method fails for the wormlike micellar solutions and
this could be due to non-uniform flow or due to flow-induced structural changes. Finally,we demonstrate that the combination
of SRFS, rotational rheometry, and advanced high frequency rheology methods allows for a broad bandwidth characterization
of complex fluids spanning an unprecedented frequency range of about eleven decades.
► Cite this publication as follows:
Kowalczyk A, Hochstein B, Stahle P, Willenbacher N: Characterization of complex fluids at very low frequency: experimental verification of the strain rate-frequency superposition (SRFS) method, Appl. Rheol. 20 (2010) 52340.
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