Modeling the Brownian relaxation of nanoparticle ferrofluids: Comparison with experiment

Abstract

Purpose:The authors investigate the ability of current models for magnetic nanoparticles immersed in dilute ferrofluids and external sinusoidal magnetic fields to explain recent experiments in which the relaxation effects are dominated by viscous damping. Methods: The Fokker-Planck (FP) equation appropriate for the nanoparticle magnetic moment distribution corresponding to the underlying stochastic Langevin model is numerically studied and solutions compared to experimental results. The FP equation is solved using an expansion in Legendre polynomials. The polydisperse properties of the ferrofluids are incorporated into the analysis. Results: By using a FP approach that includes polydispersion, the authors obtain good agreement with recent experimental results using ferrofluids containing nanoparticles with average hydrodynamic diameters in the 40-120 nm range. Conclusions: For nanoparticles used in recent magnetic spectroscopy experiments, the FP approach can be used to accurately model experimental data in the situation where Brownian relaxation effects are dominant and the ferrofluids are dilute.

Comparisons of characteristic timescales and approximate models for Brownian magnetic nanoparticle rotations

“…(16). The values of the characteristic time (with respect to the applied field) from our simulations agree with both the form of s YE and the approximate characteristic time s MRS .…”

“…A typical way to discuss the dynamics is through the timescales of the nanoparticle rotations. [15][16][17] In particular, we often consider the relaxation time: the timescale for a sample of particles to return to equilibrium after some perturbation (e.g., alignment with a field). Conventional magnetic particles are understood to have two rotational mechanisms.…”

“…(2) with additional stochastic torques) can be completed 15,16,28 to examine the characteristic time and compare with the analytical expressions.…”

“…Several papers go through derivations for the equations of motion. 15,16,22,28 We choose a compact form for the equation…”

Temperature-dependent dynamic correlations in suspensions of magnetic nanoparticles in a broad range of concentrations: a combined experimental and theoretical study

“…The main conclusion of these works is that the dipolar correlations slow down the dynamics and result in larger relaxation time-scales [61][62][63] . The role of magnetic dipolar interactions in the specific absorption rate was actively studied both theoretically and experimentally [65][66][67][68] . In these studies, it was shown that the interparticle correlations might lead to the decrease, as well as to the increase, in the hypothermia efficiency depending on the magnetic particle size.…”