Montgomery T. Shaw, Zhizhong Z. Liu
Single-point Determination of Nonlinear Rheological Data from Parallel-Plate Torsional Flow
Appl. Rheol. 16:2 (2006) 70-79Abstract: Of the torsional drag-flow experiments, the hands-down winner for simplicity and ease of use is that using parallel- plate fixtures. This geometry is highly flexible, allowing custom modification of plate size and material, and is easily adaptable for optical use and the application of electric fields. However, its nonuniform flow is a major encumbrance for measuring nonlinear response. In 1987, Cross and Kaye offered a simple and clever solution for this problem, which essentially states that one assumes the sample is Newtonian, but the shear rate assigned to the observed ''Newtonian'' viscosity is 3/4ths of the rim shear rate . This shift factor arises from the use of Gaussian integration over radius of the nonlinear stress profile. Recent re-examination of the Cross-Kaye rule indicates that there may be a more accurate rule of thumb with the shift factor being 0.8 instead of 0.75 (4/5 instead of 3/4). However, for complex materials, the real question is how much useful information is covered up by this approach vs. the traditional differentiation of the integral to account for the stress profile. We have attempted to answer this question using a selection of nonlinear measurements on an AB block copolymer solution that is rheologically complex. © 2006 Applied Rheology.
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Appl Rheol 16 (2006) issues:
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