Imaging of particle shear migration with electrical impedance tomography

Electrical impedance tomography (EIT) is used to investigate the net migration of particles in a suspension undergoing pressure-driven flow through a tube at low Reynolds number. A low frequency electrical current is applied to the flowing suspension by two flush-mounted electrodes on the pipe wall to create a potential field which is sampled by other pairs of flush-mounted electrodes. A numerical inversion of the data, which takes into account the geometry and symmetry of the problem, gives the conductivity variation within the flow. An image of the suspension particle volume fraction in the tube is then formed using a relationship between the local conductivity and local suspension concentration. Consistent with particle shear migration of a concentrated suspension at low Reynolds number, the images from experimental data show a net migration of particles toward the centerline of the tube. The images of a 0.25 volume fraction suspension at the lowest Reynolds number examined compare favorably to an existing continuum theory of particle shear migration. Other images from experiments at higher, but still small Reynolds numbers, and at volume fractions of 0.25 and 0.40 are also presented. The particle migration measured with the relatively inexpensive EIT at the latter condition is in very good agreement with the particle distribution measured with magnetic resonance imaging by Hampton et al. [J. Rheol. 41, 621 (1997)]. (C) 1999 American Institute of Physics. [S1070-6631(99)03208-0].