Review of agglomeration in ferrofluids

Ideally, magnetic colloids have sufficient surfactant and thermal agitation to prevent the agglomeration of the particles. However, agglomeration has been observed in light scattering, light microscope observations, pulsed magnetization measurements, and gravitational settling experiments. Agglomerate sizes have ranged from a chain of a few particles to ellipsoidal clumps of more than 10¹⁰particles. The agglomeration appears to be reversible and strongly dependent upon the applied magnetic field and the particular ferrofluid. Theoretical approaches are based either on equilibrium statistical mechanics (for small agglomerates) or on a) minimizing the energy per particle and b) calculating a critical hydrodynamic shear stress (for large agglomerates under gravitational settling). We review the experiments, the theories, and some areas for future research.