An explanation for negative parallel superposition moduli

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Abstract
Parallel superposition flows have been examined in the rheology literature since 1965; they consist of simultaneously applied steady shear and oscillatory flows. One interesting result found from the study of parallel superposition flows has been reportedly negative values of the dynamic moduli. This apparently unphysical result has led to some speculation regarding its origin. In this work, we provide mathematical and experimental evidence showing that negative moduli are potentially due to the oscillatory signal being distorted by inclusion of the steady shear component in the Fourier analysis. However, the moduli can be easily corrected to restore physical meaning.

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