MAGNETOHYDRODYNAMIC AGGREGATION OF CHOLESTEROL AND POLYSTYRENE LATEX SUSPENSIONS

The aggregation state of flowing (0.49 m s(-1) linear velocity) colloi dal dispersions of polystyrene latex microspheres (certified particle diameter of 156 +/- 6 nm; measured diameter, 156 +/- 3 nm at pH 5.3 in 50 mM NaCl) in NaCl solution and cholesterol (measured diameter, 533 +/- 9 mn at pH 5.3), stabilized in NaCl solution by sodium taurodeoxyc holate, was studied using photon correlation spectroscopy. For cholest erol suspensions having electrolyte concentrations close to the critic al coagulation concentration (50 mM NaCl), pronounced aggregation was observed after 15 to 30 min of recirculation in the presence of an ort hogonally applied magnetic field (2.0, 1.0 and 0.15 T). In all experim ents with cholesterol, aggregation was followed by a period of deaggre gation, after which aggregation again occurred. Comparable effects wer e not observed when cholesterol suspensions were recirculated in the a bsence of the magnetic field or when the suspensions were exposed to a n equivalent magnetic field in the absence of flow. For cholesterol su spensions, the increase in particle size was most pronounced at 0.15 a nd 1.0 T rather than at 2.0 T. Aggregation effects were also observed when suspensions of polystyrene latex in 200 mM NaCl were made to flow through a 1.0-T field. In both systems, the magnetic aggregation does not appear to involve direct interaction between the field and the so lid phase, but is interpreted in terms of orthokinetic effects involvi ng magnetohydrodynamic changes in the flow profile resulting from the presence of the transverse field. (C) 1996 Academic Press, Inc.