Reversible, field induced agglomeration in magnetic colloids

Abstract

Reversible agglomeration of magnetic particles (20–300-Å diameter) in a colloidal suspension has been induced by applying various magnetic fields: nonuniform static fields and uniform ac or dc fields of magnitudes from 2.5 to 230 Oe. A sensitive Colpitts oscillator circuit monitored the spatial and temporal variations in magnetization in a vertical 11-cm column of magnetic fluid. The results are consistent with the hypothesis that spherical agglomerates of 10 7–10 9 particles are formed and settle gravitationally. The agglomeration was most pronounced in a commercially available water-base, 200-G ferrofluid; 20% of the particles formed agglomerates of ≥ 10 7 particles and settled out in 1 hr in a 230-Oe uniform ac or dc field. This effect was present to a lesser extent in other fluids (200-G ester base, 3% of the particles agglomerate in 1 hr; 200-G hydrocarbon base, ∼ 0.05% in 2 hr). Electron micrograph studies indicate that all sizes of particles participate in the agglomeration. Centrifuge experiments show that the agglomerates are not limited by close packing upon settling to the bottom of the tube. The Colpitts oscillator should be useful in determining the stability of magnetic fluids used in applications such as magnetic ink printers, metal separation, etc. It may also be useful in experimental investigation of agglomeration hypotheses because the extent of the agglomeration can be conveniently controlled via the applicatioa of magnetic fields.