Structure formation in layered ferrofluid nanofilms

Chain Formation and Phase Separation in Ferrofluids: The Influence on Viscous Properties

Ferrofluids have attracted considerable interest from researchers and engineers due to their rich set of unique physical properties that are valuable for many industrial and biomedical applications. Many phenomena and features of ferrofluids' behavior are determined by internal structural transformations in the ensembles of particles, which occur due to the magnetic interaction between the particles. An applied magnetic field induces formations, such as linear chains and bulk columns, that become elongated along the field. In turn, these structures dramatically change the rheological and other physical properties of these fluids. A deep and clear understanding of the main features and laws of the transformations is necessary for the understanding and explanation of the macroscopic properties and behavior of ferrofluids. In this paper, we present an overview of experimental and theoretical works on the internal transformations in these systems, as well as on the effect of the internal structures on the rheological effects in the fluids.

Dynamical self-assembly of dipolar active Brownian particles in two dimensions

Based on Brownian Dynamics (BD) simulations, we study the dynamical self-assembly of active Brownian particles with dipole-dipole interactions, stemming from a permanent point dipole at the particle center. The propulsion direction of each particle is chosen to be parallel to its dipole moment. We explore a wide range of motilities and dipolar coupling strengths and characterize the corresponding behavior based on several order parameters. At low densities and low motilities, the most important structural phenomenon is the aggregation of the dipolar particles into chains. Upon increasing the particle motility, these chain-like structures break, and the system transforms into a weakly correlated isotropic fluid. At high densities, we observe that the motility-induced phase separation is strongly suppressed by the dipolar coupling. Once the dipolar coupling dominates the thermal energy, the phase separation disappears, and the system rather displays a flocking state, where particles form giant clusters and move collective along one direction. We provide arguments for the emergence of the flocking behavior, which is absent in the passive dipolar system.

Effective Landau theory of ferronematics

An effective Landau-like description of ferronematics, i.e., suspensions of magnetic colloidal particles in a nematic liquid crystal (NLC), is developed in terms of the corresponding magnetization and nematic director fields. The study is based on a microscopic model and on classical density functional theory. Ferronematics are susceptible to weak magnetic fields and they can exhibit a ferromagnetic phase, which has been predicted several decades ago and has recently been found experimentally. Within the proposed effective Landau theory of ferronematics, one has quantitative access, e.g., to the coupling between the magnetization of the magnetic colloids and the nematic director of the NLC. On mesoscopic length scales, this generates complex response patterns.

Electrode polarization and unusual magnetodielectric effect in a transformer oil-based magnetic nanofluid thin layer

In the present paper, we provide low-frequency dielectric spectra for a thin layer of a nanofluid based on transformer oil and iron oxide nanoparticles stabilized by oleic acid. The complex dielectric permittivity measured in the frequency range from 1 mHz to 200 kHz shows an obvious electrode polarization effect and a Debye-like dielectric relaxation process. Both effects stem from the presence of space charge in the oil due to impurity ions, and in the nanofluid represented predominantly by a residual surfactant and uncompensated particle surface charge. It is shown that the spectra, which were measured in the temperature range from 298 K to 358 K, can be well represented by a fitting function consisted of one Havriliak-Negami term and the Jonscher's power law. In the investigated magnetic nanofluid layer, we found that the onset of the electrode polarization is suppressed to lower frequencies by the application of an external magnetic field (300 mT). This phenomenon is explained by a slowed-down migration of the space charge due to the Lorentz force and by a hindering effect of the formed magnetic nanoparticle aggregates. Surprisingly, a moderate decrease in the whole permittivity spectrum was observed for both parallel and perpendicular orientations of the electric and magnetic fields. This is in contradiction with the usual magnetodielectric anisotropy effect. Based on our qualitative analysis, we discuss potential reasons accountable for the observed effect.

Effects of magnetic field gradients on the aggregation dynamics of colloidal magnetic nanoparticles

We have used low-field (1)H nuclear-magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) to investigate the aggregation dynamics of magnetic particles in ionic ferrofluids (IFFs) in the presence of magnetic field gradients. At the beginning of the experiments, the measured NMR spectra were broad and asymmetric, exhibiting two features attributed to different dynamical environments of water protons, depending on the local strength of the field gradients. Hence, the spatial redistribution of the magnetic particles in the ferrofluid caused by the presence of an external magnetic field in a time scale of minutes can be monitored in real time, following the changes in the features of the NMR spectra during a period of about an hour. As previously reported [Heinrich et al., Phys. Rev. Lett., 2011, 106, 208301], in the homogeneous magnetic field of a NMR spectrometer, the aggregation of the particles of the IFF proceeds in two stages. The first stage corresponds to the gradual aggregation of monomers prior to and during the formation of chain-like structures. The second stage proceeds after the chains have reached a critical average length, favoring lateral association of the strings into hexagonal zipped-chain superstructures or bundles. In this work, we focus on the influence of a strongly inhomogeneous magnetic field on the aforementioned aggregation dynamics. The main observation is that, as the sample is immersed in a certain magnetic field gradient and kept there for a time τinh, magnetophoresis rapidly converts the ferrofluid into an aggregation state which finds its correspondence to a state on the evolution curve of the pristine sample in a homogeneous field. From the degree of aggregation reached at the time τinh, the IFF sample just evolves thereafter in the homogeneous field of the NMR spectrometer in exactly the same way as the pristine sample. The final equilibrium state always consists of a colloidal suspension of zipped-chain bundles with the chain axes aligned along the magnetic field direction.

Spontaneous ordering of magnetic particles in liquid crystals: From chains to biaxial lamellae

Using Monte Carlo computer simulations we explore the self-assembly and ordering behavior of a hybrid, soft magnetic system consisting of small magnetic nanospheres in a liquid-crystalline (LC) matrix. Inspired by recent experiments with colloidal rod matrices, we focus on conditions where the sphere and rod diameters are comparable. Already in the absence of a magnetic field, the nematic ordering of the LC can stabilize the formation of magnetic chains along the nematic or smectic director, yielding a state with local (yet no macroscopic) magnetic order. The chains, in turn, increase the overall nematic order, reflecting the complex interplay of the structure formation of the two components. When increasing the sphere diameter, the spontaneous uniaxial ordering is replaced by biaxial lamellar morphologies characterized by alternating layers of rods and magnetic chains oriented perpendicular to the rod's director. These ordering scenarios at zero field suggest a complex response of the resulting hybrid to external stimuli, such as magnetic fields and shear forces.

Magnetization of multicomponent ferrofluids

The solution of the mean spherical approximation (MSA) integral equation for isotropic multicomponent dipolar hard sphere fluids without external fields is used to construct a density functional theory (DFT), which includes external fields, in order to obtain an analytical expression for the external field dependence of the magnetization of ferrofluidic mixtures. This DFT is based on a second-order Taylor series expansion of the free energy density functional of the anisotropic system around the corresponding isotropic MSA reference system. The ensuing results for the magnetic properties are in quantitative agreement with our canonical ensemble Monte Carlo simulation data presented here.

Nanodrop of an Ising magnetic fluid on a solid surface

The density functional theory of inhomogeneous simple fluids is extended to an Ising magnetic fluid in contact with a solid surface, which is subjected to an external uniform or nonuniform magnetic field. The system is described by two coupled integral equations regarding the magnetic moment and fluid density distributions. The dependence of the contact angle that a nanodrop makes with the solid surface on the parameters involved in the magnetic interactions between the molecules of fluid and between the molecules of fluid and an external magnetic field is calculated. For the uniform magnetic field, the contact angle increases with increasing magnetic field, approaching an asymptotic value that depends on the strength of the fluid-fluid magnetic interactions. In the nonuniform field generated by a permanent magnet, the contact angle first increases with increasing magnetic field B(M) and then decreases, with the decrease being almost linear for large values of B(M). The obtained results are in qualitative agreement with the experimental data on the contact angle of magnetic drops on a solid surface available in the literature.

Optical negative refraction in ferrofluids with magnetocontrollability

We numerically demonstrate optical negative refraction in ferrofluids containing isotropic Fe3O4 nanoparticles, each having an isotropic Ag shell, in the presence of an external dc magnetic field H. The all-angle broadband optical negative refraction with magnetocontrollability arises from H-induced chains or columns. They result in hyperbolic equifrequency contour for transverse magnetic waves propagating in the system. The finite element simulations verify the analyses using the effective medium approximation. Experimental demonstration and potential applications are suggested and discussed.

Phase transitions and ordering of confined dipolar fluids

We apply a modified mean-field density functional theory to determine the phase behavior of Stockmayer fluids in slit-like pores formed by two walls with identical substrate potentials. Based on the Carnahan-Starling equation of state, a fundamental-measure theory is employed to incorporate the effects of short-ranged hard-sphere-like correlations while the long-ranged contributions to the fluid interaction potential are treated perturbatively. The liquid-vapor, ferromagnetic-liquid-vapor, and ferromagnetic-liquid-isotropic-liquid first-order phase separations are investigated. The local orientational structure of the anisotropic and inhomogeneous ferromagnetic liquid phase is also studied. We discuss how the phase diagrams are shifted and distorted upon varying the pore width.

Structure of ferrofluid nanofilms in homogeneous magnetic fields

We report molecular dynamics simulations results for model ferrofluid films subject to an external, homogeneous magnetic field directed parallel or perpendicular to the film surfaces. The interactions between the magnetic nanoparticles are modeled via the Stockmayer potential. In a previous study [J. Jordanovic and S. H. L. Klapp, Phys. Rev. Lett. 101, 038302 (2008)] we have shown that an external field can control the number and internal structure of the layers characterizing the fluid films, in qualitative agreement with experiments. Here we explore the dependence of the layering effects on thermodynamic conditions, and we analyze the results from an energetic (microscopic and macroscopic) perspective. As a special case we investigate a monolayer to bilayer transition induced via a perpendicular field.