Aggregation dynamics of nonmagnetic particles in a ferrofluid.

Nonmagnetic microspheres confined in a ferrofluid layer are denoted by magnetic holes. They form aggregates due to dipolar interactions when an external magnetic field is exerted. Their cluster-cluster aggregation was studied for various magnetic fields using optical microscopy, both for small spheres of diameters, d=1.9 and 4 microm, for which Brownian motion was important and for large spheres of diameter, d=14 microm, for which Brownian motion was not important. The results for the two smaller sizes were in agreement with standard dynamic scaling theory and the dynamic scaling exponent z for the average cluster length S(t) approximately t(z) was found to be slightly smaller than 0.5, while for the largest spheres the z exponent showed a strong dependence on the magnetic-field strength.