Aggregation of water-based magnetic liquids observed with the polarising microscope

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Abstract
Aggregation phenomena in water-based magnetic liquids have been observed using a polarising microscope fitted with a phase compensator. Macrochains formed in an external magnetic field are shown to have a greater birefringence than the surrounding 'unchained' liquid. A simple model of the colloidal suspension allows the optical data to be interpreted and hence a numerical estimate of the birefringence of the macrochains to be made. This indicates that for fluid diluted five-fold, the chain birefringence is some four times greater than the initial bulk birefringence. It is also found that the bulk fluid birefringence remains essentially constant for times up to 2 h, independent of the formation of macrochains.