The size ratio effect on the microstructure and magnetization of a bidisperse magnetic colloidal suspension

Abstract

This research examines how the size ratio influences the microstructure and time-dependent magnetization in a bidisperse magnetic colloidal suspension under a uniform magnetic field. Two types of particles model the bidisperse suspension: the small particles of radius R_s and the large particles of radius R_l. The size ratio, ξ = R_l/R_s, defines the particle size difference. The total volume fraction of the suspension, ϕ, is obtained from ϕ = ϕ_s + ϕ_l, where ϕ_s and ϕ_l are the volume fractions of the small and large particles, respectively. The magnetic dipole–dipole interaction among the small particles and the large ones is characterized by the dipolar coupling parameters λ_s and λ_l, respectively. The interactions among the applied magnetic field and the magnetic dipoles of the small and large particles are measured by the Langevin parameters α_s and α_l, respectively. This study performs Brownian dynamics (BD) simulations of a bidisperse suspension comprising N = 1000 particles, with ϕ = 10⁻³ and ϕ_s = ϕ_l = 5 × 10⁻⁴. Also, α_s ranges from 0 to 1000, and λ_s from 5 to 30. The size ratio, ξ, takes values of 1, 2 and 3. The values of λ_l and α_l are computed by the parameters aforementioned by assuming that all particles exhibit the same saturation magnetization. Our results show a rich variability in the microstructure as ξ increases. As the large particles increase in size, they exhibit a greater magnetic dipole moment, which induces a non-uniform local magnetic field around them. The surrounding small particles then aggregate with the large ones, driven by this local magnetic field. Small α_s values lead to the formation of flux-closure structures such as rings of small and large particles as well as shell-like structures, which consist of small particles surrounding the large ones. The formation of these microstructures directly affects time-dependent magnetization of the suspension, which exhibits a decay with time in the limit of long times. These findings have important implications for synthesizing magnetic colloidal suspensions with enhanced properties.