pensions (flow rates up to 2.2 cm 3 min 01 ) (1), stationary The aggregation state of flowing (0.49 m s 01 linear velocity) suspensions (1-8), and slowly settling suspensions (9)(10)(11). colloidal dispersions of polystyrene latex microspheres (certified For the case of slowly settling and stationary suspensions, particle diameter of 156 { 6 nm; measured diameter, 156 { 3 Svoboda and co-workers (3-5) and Parker and co-workers nm at pH 5.3 in 50 m M NaCl) in NaCl solution and cholesterol (9-11) have modified classical DLVO theory by adding a (measured diameter, 533 { 9 nm at pH 5.3), stabilized in NaCl new term, V M , to allow for the magnetic interaction between solution by sodium taurodeoxycholate, was studied using photon particles in the applied field. According to this theory, V M correlation spectroscopy. For cholesterol suspensions having elecis given by trolyte concentrations close to the critical coagulation concentration (50 m M NaCl), pronounced aggregation was observed after 15 to 30 min of recirculation in the presence of an orthogonally applied magnetic field (2.0, 1.0 and 0.15 T). In all experiments
with cholesterol, aggregation was followed by a period of deaggregation, after which aggregation again occurred. Comparable effects were not observed when cholesterol suspensions were recircuwhere a is the particle radius, r is the interparticle distance lated in the absence of the magnetic field or when the suspensions (between centers), m 0 is the magnetic permeability of a vacwere exposed to an equivalent magnetic field in the absence of uum, x is the magnetic susceptibility of the solid phase, and flow. For cholesterol suspensions, the increase in particle size was B 0 is the magnetic induction. The introduction of the V M most pronounced at 0.15 and 1.0 T rather than at 2.0 T. Aggregaterm into the DLVO expression produces a secondary minition effects were also observed when suspensions of polystyrene mum in the potential curve at relatively large interparticle latex in 200 mM NaCl were made to flow through a 1.0-T field.
distances. If a threshold (3-5) or critical (9-11) magnetic
In both systems, the magnetic aggregation does not appear to field is applied, the colloid becomes unstable and flocculates involve direct interaction between the field and the solid phase, but is interpreted in terms of orthokinetic effects involving magne-as loose aggregates in this secondary minimum. tohydrodynamic changes in the flow profile resulting from the In the case of flowing suspensions, Tomba ´cz et al. (12) presence of the transverse field. ᭧ 1996 Academic Press, Inc.
and Ma (13) have studied the effect of applied magnetic Key Words: magnetohydrodynamics; colloidal cholesterol; colfields on monodisperse hematite sols. These workers found loidal latex; aggregation state; orthokinetic effects; photon correlathat application of a 0.15-T magnetic field orthogonal to the tion spectroscopy.
direction of fluid flow (0.826 m s 01 linear velocity) resulted in increased aggregation of the hematite sols compared to flow in the absence of the magnetic field.