JOURNAL OF COLLOID AND INTERFACE SCIENCE 183, 528–538 (1996) ARTICLE NO. 0576 Magnetohydrodynamic Aggregation of Cholesterol and Polystyrene Latex Suspensions KENNETH W. BUSCH, 1 SETHURAMAN GOPALAKRISHNAN, 2 MARIANNA A. BUSCH, AND ETELKA TOMBA ´ CZ 3 Department of Chemistry, Baylor University, Waco, Texas 76798-7348 Received January 22, 1996; accepted July 2, 1996 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 – 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 elec- is given by trolyte concentrations close to the critical coagulation concentra- tion (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 V M Å0 32p 2 x 2 a 6 B 2 0 9m 0 r 3 , [1] with cholesterol, aggregation was followed by a period of deaggre- gation, after which aggregation again occurred. Comparable ef- fects were not observed when cholesterol suspensions were recircu- where 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 vac- were 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 ratherthan at 2.0 T. Aggrega- term into the DLVO expression produces a secondary mini- tion effects were also observed when suspensions of polystyrene mum in the potential curve at relatively large interparticle latex in 200 m M 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; col- fields on monodisperse hematite sols. These workers found loidal latex; aggregation state; orthokinetic effects; photon correla- that 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. INTRODUCTION Since hematite is weakly ferromagnetic, it could be argued that the aggregation observed for these sols resulted from magnetization of the solid phase. However, particle size ( ini- The effect of applied magnetic fields on the stability of tial diameter approximately 90 nm, Ref. 12), magnetic sus- colloidal suspensions is of interest from a purely theoretical ceptibility of the solid, and the magnetic induction (0.15 T) as well as practical standpoint. To date, workers have studied employed in these studies (12, 13) were too small for the the effect of applied magnetic fields on gently flowing sus- V M term in Eq. [1] to significantly alter the DLVO potential energy curve. Moreover, no aggregation was observed when 1 To whom correspondence should be addressed. these hematite sols were exposed to the same magnetic field 2 Present address: Department of Chemistry, State University of New in the absence of flow. York at Buffalo, Buffalo, NY 14260-3000. Given that the magnetic aggregation of hematite (12, 13) 3 Present address: Department of Colloid Chemistry, Attila Jo ´ zsef Univer- sity, Szeged, Hungary. could not be explained by Eq. [1] and was observed only 528 0021-9797/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.