MRI of Couette experiments in a newly developed shear device - suitable for pastes and concentrated suspensions?

Nuclear magnetic resonance (NMR) imaging (MRI) was used to investigate the flow behaviour of PTFE pastes and other highly viscous fluids in a new Coue tte shear device, developed especially for pastes. The mean moisture conten ts of the PTFE pastes were between 35 and 55%, the normal stress varied bet ween 0 and 5 bar. By means of MRI the local moisture in the material, the d eformation and velocity profiles of the investigated specimens can be deter mined non-destructively and contact-free. The results are in part contrary to known ideas and cannot be satisfactorily explained. We believe that the application of the shear device, and perhaps of shear experiments in genera l, is problematic in the use of disperse, to phase-separation tending syste ms. Velocity profiles and, hence, in combination with the torque necessary for rotation of the inner shaft, flow functions and possibly wall slip func tions can in principle be determined for highly viscous quasi-homogeneous m aterials by means of MRI. The main purpose of the investigations, however, is not to determine flow functions with a new technique. But rather unexpec ted, often disturbing phenomena of disperse solid-fluid systems in Couette devices shall be presented. (C) 2001 Elsevier Science B.V. All rights reser ved.