Time-dependent clustering and magnetization in magnetic colloidal suspensions

This work analyzes the influence of the time-dependent clustering aggregation process on the transient and equilibrium magnetization of a monodisperse magnetic colloidal suspension under a uniform magnetic field via Brownian dynamics simulations. The clustering aggregation process is characterized by microstructural properties, such as the nucleation-growth factor, 〈n_c(t)〉, mean cluster size, 〈N_c(t)〉, kinetic exponent, z, effective radius, 〈R_eff〉, and radial distribution function, g(r). These are analyzed in terms of the volume fraction, ϕ, the dipolar coupling parameter, λ, and the Langevin parameter, α. Here, λ and α measure the magnetic dipole-dipole interaction energy and the magnetic field-dipole interaction energy relative to the thermal energy, respectively. The magnetization in transient and equilibrium regimes is analyzed relative to these microstructural properties for different values of ϕ, λ, and α. The analysis of the microstructural properties reveals a reduction in the dipolar chain growth at higher λ and ϕ values in the range of 1 < α < 10, which contrasts with the increase observed for low values of the same parameters. This reduction is caused by the lateral interactions between the chains formed. For higher ϕ and λ values, these interactions lead to side-by-side coupling of the long-dipolar chains that enhances the transient and equilibrium magnetization. The equilibrium magnetization values have been compared with some predictive models, showing a significant discrepancy at 0.01 ≤ α ≤ 10, which involves the aforementioned range of α. Also, the Langevin magnetic susceptibility, χ_L, used in these models provides a way to characterize dilute suspensions with strong magnetic interparticle interactions (χ_L ≥ 0.09). These results may contribute to formulating more accurate models to predict magnetization of these suspensions.