NUCLEAR-MAGNETIC-RESONANCE IMAGING INVESTIGATION OF SEDIMENTATION OF CONCENTRATED SUSPENSIONS IN NON-NEWTONIAN FLUIDS

Nuclear magnetic resonance imaging is used to study rates of sedimenta tion in several non-Newtonian fluids. It is shown that either shear th inning or elasticity in a fluid can result in a time-dependent sedimen tation rate, as measured by the motion of the suspension-supernatant i nterface. Sedimentation is most rapid at the beginning of the process, and grows steadily slower in time. Two-dimensional images of vertical sections of suspension show clearly the development of a nonhomogeneo us microstructure during sedimentation in a viscoelastic fluid. Elonga ted columns of particles form in the direction of gravity, and these c olumns are separated by comparably sized regions of pure fluid. Such s tructures are not present in sedimentation in Newtonian fluids, where the suspension microstructure is homogeneous for the duration of the p rocess. (C) 1998 The Society of Rheology. [S0148-6055(98)00306-X].