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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)

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.

Norbert Willenbacher
Rheology of colloidal systems

Appl. Rheol. 20:4 (2010) 254-254

Cite this publication as follows:
Willenbacher N: Rheology of colloidal systems, Appl. Rheol. 20 (2010) 254.

Katarzyna Niedzwiedz, Oliver Arnolds, Norbert Willenbacher, Rudiger Brummer
Capillary Breakup Extensional Rheometry of Yield Stress Fluids

Appl. Rheol. 19:4 (2009) 41969 (10 pages)

Filament breakup of high viscosity fluids with apparent yield stress has been investigated and strategies for an appropriate characterization of their behavior in CaBER experiments are discussed. Filament profiles of such fluids exhibit significant concave curvature. Accurate determination of filament shape is mandatory for understanding deformation behavior. Therefore, we have set up an optical train including high-speed camera, telecentric objective and telecentric back-light illumination with a blue light emitting diode (LED) providing high contrast filament shape imaging. Image analysis allows for diameter determination with an accuracy of 3.55 μm/pixel. In addition to the transient filament diameter at the neck we have extracted the curvature at this point as a function of time and the region of deformation, in order to characterize the extensional flow behavior.We have investigated the time evolution of filament shape as a function of various experimental parameters like stretching time, velocity profile during stretching, stretching ratio and initial sample volume at constant stretching ratio. Filament thinning is independent of stretching time, tsub>s and stretching velocity profile. But when the same stretching ratio is applied at different initial volume fraction, filament curvature increases strongly with decreasing sample volume leading to an increase of filament life time according to the negative contribution of its curvature to the Laplace pressure inside the fluid.

Cite this publication as follows:
Niedzwiedz K, Arnolds O, Willenbacher N, Brummer R: Capillary Breakup Extensional Rheometry of Yield Stress Fluids, Appl. Rheol. 19 (2009) 41969.

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