Rheometry and detection of apparent wall slip for Poiseuille flow of polymer solutions and particulate dispersions by nuclear magnetic resonance velocimetry

Abstract

A flexible and robust approach to nuclear magnetic resonance (NMR) based capillary rheometry has been developed. The precision of the technique has been explored, with particular regard to the capability for measurement of apparent wall slip velocities. The practical implications of alternative NMR rheometry protocols are discussed with regard to robustness and speed. NMR rheometry results are presented for a Newtonian fluid (aqueous 50.4% sucrose), a shear thinning solution (0.2% aqueous xanthan gum), and a particulate system composed of 5–50 μm irregular, soft agar gel particles. In all cases, fully developed Poiseuille flow was studied in a 4‐mm‐i.d., glass capillary. Radial velocity profiles were measured by NMR velocimetry; radial differentiation provides shear rate values, which have been scaled by the associated radial positions and measured pressure drops to determine viscosity as a function of shear rate. Agreement with cone‐and‐plate or parallel plate rheometry has been established for each system. The xanthan solution shows power‐law behavior, and no evidence of significant apparent wall slip. The particulate gel system exhibits power‐law behavior, but with a pronounced apparent wall slip. Apparent slip velocities determined by extrapolation of the NMR velocimetry data to the wall surface and by a Mooney analysis of flow through four different diameter capillaries agree.