Hye-Jin Ahn  Geun-Seok Choi  Yong-Seok Kim  Ki-Won Song

Vol. 56,  No. 6, pp. 386-401, Dec.  2019
10.12772/TSE.2019.56.386

Xanthan gum transient rheology start-up shear flow Wagner constitutive equation stress overshoot stress decay time-strain separability damping function

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The present study has been designed to theoretically predict the transient rheological behavior of concentrated xanthan gum systems in start-up shear flow fields using the Wagner constitutive equation. Using an Advanced Rheometric Expansion System (ARES), a number of constant shear rates were suddenly imposed to aqueous xanthan gum solutions with different concentrations and then the resultant shear stress responses were detected with time. The linear and nonlinear stress relaxation moduli at various deformation magnitudes were also measured to determine the damping function. The linear relaxation modulus was characterized by a power-law expression to determine the memory function and a time-strain separability of the nonlinear relaxation moduli was employed to predict the nonlinear response. The experimentally obtained damping function was compared with the fitted results calculated from the two mathematical forms of the Wagner and Soskey- Winter equations in order to examine the effect of damping function on the predictive performance of the Wagner model. The overall applicability of the Wagner model for predicting the whole procedures of a transient rheological behavior at start-up of steady shear flow was discussed in depth. The main findings obtained from this study are summarized as follows : (1) The Wagner model has a predictive ability to qualitatively express the whole procedures of a transient rheological behavior of concentrated xanthan gum solutions for all shear rates imposed, regardless of selecting a damping function. (2) The values of the maximum reduced stress predicted by the Wagner model employing the Wagner damping function exhibit an almost equal magnitude, irrespective of the shear rates imposed, whereas those predicted by the Wagner model employing the Soskey- Winter damping function are gradually decreased with an increase in imposed shear rate. (3) For all shear rates applied, the Wagner model having the Wagner damping function has a fairly good ability to predict the time at which the maximum stress occurs, tmax, while the Wagner model having the Soskey-Winter damping function predicts a much faster value of tmax. (4) The Wagner model using both of the Wagner and Soskey-Winter damping functions has a weakness with respect to predicting a stress decay which always show a slower decrement than does the predicted results by the Wagner model using the two different forms of a damping function. (5) The Wagner model adopting the Wagner damping function exhibits a superior performance to the Wagner model adopting the Soskey-Winter damping function for predicting the whole steps of a transient rheological behavior.