Structure and magnetic properties of dilute ferrofluids suspended in gels

When dispersed inside a gel matrix a ferrofluid loses its translational degree of freedom, but the particles remain free to rotate and locally deform the surrounding polymer matrix. Small angle X-ray scattering methods were used to investigate the structure of dilute suspensions of two different ferrofluids dispersed in soft polyacrylamide hydrogels. The particles both in the free fluid and in the gel are shown to be fractal aggregates composed of smaller elementary units of radius ca. 5 nm. The volume fractal dimension D of the aggregates is strongly sample dependent, taking the value 1.7 in one case and 2.9 in the second. In a magnetic field the aggregates tend to orient along the direction of the field, but owing to the translational constraint are unable to form linear arrays as in the free fluid. At high magnetic field strengths (ca. 1 T), the magnetization of the elementary particles aligns along the field and the aggregates tend to stretch in that direction. The effect of the gel elasticity is treated as a potential barrier to the orientation process.