A comparison of molecular dynamics and analytic calculation of correlations in an aligned ferrofluid

Structure and dynamics in suspensions of magnetic platelets

In this research, we employ Brownian dynamics simulations, density functional theory, and mean-field theory to explore the profound influence of shape anisotropy of magnetic nanoplatelets on suspension magnetic response. Each platelet is modelled as an oblate cylinder with a longitudinal point dipole, with an emphasis on strong dipolar interactions conducive to self-assembly. We investigate static structural and magnetic properties, characterising the system through pair distribution function, static structure factor, and cluster-size distribution. The findings demonstrate that shape-specific interactions and clustering lead to significant changes in reorientational relaxation times. Under zero field, distinctive modes in the dynamic magnetic susceptibility identify individual particles and particle clusters. In the presence of an applied field, the characteristic relaxation time of clusters increases, while that of single particles decreases. This research provides insights into the intricate interplay between shape anisotropy, clustering, and magnetic response in platelet suspensions, offering valuable perspectives for recent experimental observations.

Theory and simulation of anisotropic pair correlations in ferrofluids in magnetic fields

Anisotropic pair correlations in ferrofluids exposed to magnetic fields are studied using a combination of statistical-mechanical theory and computer simulations. A simple dipolar hard-sphere model of the magnetic colloidal particles is studied in detail. A virial-expansion theory is constructed for the pair distribution function (PDF) which depends not only on the length of the pair separation vector, but also on its orientation with respect to the field. A detailed comparison is made between the theoretical predictions and accurate simulation data, and it is found that the theory works well for realistic values of the dipolar coupling constant (λ = 1), volume fraction (φ ≤ 0.1), and magnetic field strength. The structure factor is computed for wavevectors either parallel or perpendicular to the field. The comparison between theory and simulation is generally very good with realistic ferrofluid parameters. For both the PDF and the structure factor, there are some deviations between theory and simulation at uncommonly high dipolar coupling constants, and with very strong magnetic fields. In particular, the theory is less successful at predicting the behavior of the structure factors at very low wavevectors, and perpendicular Gaussian density fluctuations arising from strongly correlated pairs of magnetic particles. Overall, though, the theory provides reliable predictions for the nature and degree of pair correlations in ferrofluids in magnetic fields, and hence should be of use in the design of functional magnetic materials.

Computer simulations of the structure of colloidal ferrofluids

The structure of a ferrofluid under the influence of an external magnetic field is expected to become anisotropic due to the alignment of the dipoles into the direction of the external field, and subsequently to the formation of particle chains due to the attractive head to tail orientations of the ferrofluid particles. Knowledge about the structure of a colloidal ferrofluid can be inferred from scattering data via the measurement of structure factors. We have used molecular-dynamics simulations to investigate the structure of both monodispersed and polydispersed ferrofluids. The results for the isotropic structure factor for monodispersed samples are similar to previous data by Camp and Patey that were obtained using an alternative Monte Carlo simulation technique, but in a different parameter region. Here we look in addition at bidispersed samples and compute the anisotropic structure factor by projecting the q vector onto the XY and XZ planes separately, when the magnetic field was applied along the z axis. We observe that the XY-plane structure factor as well as the pair distribution functions are quite different from those obtained for the XZ plane. Further, the two-dimensional structure factor patterns are investigated for both monodispersed and bidispersed samples under different conditions. In addition, we look at the scaling exponents of structure factors. Our results should be of value to interpret scattering data on ferrofluids obtained under the influence of an external field.

Magnetic field dependent ordering in ferrofluids at SiO2 interfaces

We report pronounced smecticlike ordering in a ferrofluid adjacent to a SiO2 wall. In the presence of small magnetic fields perpendicular to the interface, ordered layers of magnetite nanoparticles form that can extend up to 30 layers. We also show that short ranged ordered structures emerge when the magnetic field direction is parallel to the interface; however, the layering is strongly perturbed. These results have been obtained by in situ neutron reflectometry which gives a detailed microscopic picture of these ordering phenomena. They also reveal the formation of a wetting double-layer which forms the magnetic template for the observed ordering sheets. The implications of these findings are discussed.

Field-induced pseudocrystalline ordering in concentrated ferrofluids

Concentrated surfactant stabilized cobalt ferrofluids up to 6 vol % Co have been studied by small-angle scattering using polarized neutrons and synchrotron x rays. The combination of these techniques allowed the magnetic and nuclear form factors to be reliably separated from the structure factors. Above 1 vol % Co, inter particle interactions are induced by an applied external magnetic field that gives rise to pseudocrystalline ordering of cobalt core-shell particles. Particles are arranged in hexagonal planes, with the magnetic moments aligned parallel to the [110] direction. Two types of equivalent textures were found to be present simultaneously, corresponding to a stacking of the hexagonal planes in horizontal and vertical direction. The in-plane nearest-neighbor distance is almost independent of the concentration and temperatures, whereas the distance between the neighboring planes, c, strongly varies from sample to sample. In addition, segments of chains of particles with parallel moments are aligned along the magnetic field and frozen-in when the carrier liquid is solidified. The field induced pseudocrystalline lamellar hexagonal particle arrangement, observed experimentally in colloidal magnetic liquids, confirms predictions from molecular-dynamics and Monte Carlo simulations.

Thermophysical properties of gases, liquids, and solids composed of particles interacting with a short-range attractive potential

A short-range polynomial interaction potential is introduced which has both a repulsive core and an attractive part. It is cut off smoothly such that its first and second derivatives vanish at the cutoff distance. The potential therefore enables efficient simulation studies of a model material that exhibits similarities to a full (but computationally expensive) classical Lennard-Jones system. Thermophysical properties of the model are calculated by (nonequilibrium) molecular dynamics computer simulations and compared with analytical results. Among the quantities studied is the pressure as a function of the density for various temperatures. Equations of state for the fluid and the solid are tested. The coexistence of gaseous, (metastable) liquid, and fcc solid phases is found for a range of temperatures. Bulk and shear moduli are computed. The response of the system to a shear deformation with a constant shear rate is analyzed. The liquid shows viscoelastic behavior that can be described with a Maxwell model. The solid behaves as an elastic medium up to a finite deformation and then undergoes a transition to plastic flow, which is stick-slip-like at small shear rates and continuous at higher ones.

Pressure of fluids and solids composed of particles interacting with a short-range repulsive potential

A simple short range repulsive potential (SR), with an even smoother cut off than the Weeks-Chandler-Andersen (WCA)-Lennard-Jones potential, yields practically the same pressure, both in the fluid state and for the fcc solid, when the potential parameters are chosen such that the forces are the same at the distance where the the two potential curves are equal to k(B)T. The comparison of the pressure for the SR and the WCA systems is based on molecular dynamics computer simulations. The fluid branch of the equation of state is rather well described by a modified Carnahan-Starling expression.