Monte Carlo simulations of polydisperse ferrofluids: Cluster formation and field-dependent microstructure

We report on a Monte Carlo investigation of the microstructure of ferrofluids. The simulation parameters were chosen so that they reflect as close as possible the real ferrofluid samples that have been characterized experimentally. The consistency of the resulting equilibrium configurations with small-angle scattering data on the original samples allows us to consider the simulation a good description of the actual microstructure. We observe agglomerates consisting mainly of a small number of particles. The agglomeration probability is higher the larger the particle radius is. But also small particles (radius smaller than 5 nm) take part in the cluster formation. On applying an external magnetic field the agglomerates become elongated due to a rearrangement of the particles and orient themselves parallel to the field lines. We present a statistical evaluation of agglomeration degree, cluster sizes, and cluster composition (sizes of clustered particles) as well as of the field-dependent change of microstructure (orientation and shape anisometry of clusters).