THEORY OF LARGE AGGLOMERATES IN MAGNETIC COLLOIDS

Very large agglomerates in magnetic colloids have been observed experimentally in settling velocity experiments by Peterson and Krueger [J. Colloid Interface Sci. 62, 24 (1977)]. We investigate the size, shape, and settling velocities of the agglomerates as a function of applied magnetic field. The bases for our theory are: (a) the minimization of the energy, magnetic plus surface tension, and (b) the marginal stability of the trailing particle in the gravitationally settling agglomerate. The latter is a balance between the magnetic force holding the particle on the agglomerate and the hydrodynamic drag force pulling it away from the agglomerate. For 200-G waterbase ferrofluid, the settling velocities range from 4 x 10-4 to 4 x 10-3 cm/sec for applied fields from 2 to 230 Oe. The number of particles/agglomerate and the minor axis/major axis ratio vary from 108 to 1012 and 10-1 to 10-4, respectively. The theory reproduces the qualitative features of the settling velocity as a function of magnetic field. © 1979.