Hye-Jin Ahn  Hoa-Youn Kuk  Ki-Won Song

Vol. 54,  No. 5, pp. 351-364, Oct.  2017
10.12772/TSE.2017.54.351

Xanthan gum large amplitude oscillatory shear (LAOS) Fourier transform (FT) rheology nonlinear viscoelastic function higher harmonic contribution Phase angle stress waveform

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The present study has been performed to quantitatively characterize the nonlinear rheological behavior of concentrated xanthan gum systems in large amplitude oscillatory shear (LAOS) flow fields by means of the methodology of Fourier transform (FT) rheology. Using an Advanced Rheometric Expansion System (ARES), the dynamic viscoelastic behavior of aqueous xanthan gum solutions with different concentrations has been experimentally investigated in LAOS flow conditions with a various combination of several fixed strain amplitudes and constant angular frequencies. The nonlinear viscoelastic functions were derived from the Fourier spectra of stress responses, and then the fundamental and higher harmonic contributions were calculated from discrete Fourier transform (DFT). The nonlinear viscoelastic behavior was interpreted by displaying 3D plots, and then the effects of strain amplitude and angular frequency on the nonlinear behavior were discussed in depth. A comparison of the experimentally obtained stress response with the stress waves calculated from higher harmonic nonlinear viscoelastic functions was made to clarify the role of higher harmonic contributions. The main findings obtained from this study are summarized as follows : (1) When the strain amplitude becomes larger than a critical value of 50%, the higher nonlinear viscoelastic functions must be considered to explain the nonlinear viscoelastic behavior. However, the fundamental stress amplitude is dominantly larger than those of higher harmonics. (2) As the strain amplitude is increased, the fundamental stress amplitude is steeply increased within the linear viscoelastic region beyond which followed by an approximately constant magnitude at large deformations. This strain dependence may be explained as a distinction between linear and nonlinear viscoelastic behavior. (3) When the strain amplitude exceeds a critical value of 50%, the higher stress amplitudes are sharply increased with an increase in strain amplitude at large+L5 deformations, indicating that the higher stress harmonics exert an influence on the nonlinear viscoelastic behavior. (4) In order to interpret the complicated nonlinear viscoelastic behavior occurring at large deformations, the fundamental terms as well as the higher harmonics of phase angles defined at the odd terms should be deliberated for a quantitatively advanced analysis. (5) An overall shape of the experimentally obtained stress response is strongly affected by the higher harmonic contributions. As the angular frequency is decreased, a more distorted and a sharper stress waveform is observed at LAOS deformations. (6) A distorted stress waveform is qualitatively inclined to the left side with respect to a sinusoidal curve. In order to explain the nonlinear viscoelastic behavior of concentrated xanthan gum systems in LAOS flow fields, the nonlinear viscoelastic functions from the first to at least seventh harmonics should be considered for an accurate analysis.