Rod-like Magnetic Particles Research Articles

Directed Assembly of Magnetic Colloidal Rods in an External Field.

In fabricating new colloid-based materials via bottom-up design, particle-particle interactions are engineered to encourage the formation of the desired assemblies. One way to do this is to apply an external field, which orients magnetically polarized particles in the field direction. External fields have the advantage that they can be programmed to change in time (e.g., field rotation or toggling), tunably shifting the system away from equilibrium. Here, we apply a model for ferromagnetic colloidal rods that simulates their phase behavior in the presence of an external magnetic field with constant strength and direction. An annealing process slowly reduces the temperature during molecular dynamics simulations to estimate the system's equilibrium configuration in the ground state when the magnetic interactions between colloidal rods dominate the thermal forces. Numerous annealing simulations are performed at various particle densities and external field strengths. In the absence of an external field, the magnetic rods assemble into antiparallel configurations. When the strength of the external field is sufficiently strong, the magnetic rods are forced to orient in the direction of the field and therefore form head-to-tail structures. The formation of a head-to-tail state is associated with a net magnetic moment that results from the collective alignment of all magnetic particles in the field direction. Furthermore, when systems of magnetic rods assemble into a head-to-tail state, they occupy more space than they do in a phase in which most rods are assembled into antiparallel configurations. Phase diagrams predict that the magnetic properties of systems of rod-like magnetic particles can switch between magnetic and nonmagnetic states by tuning not only the external field strength but also the particle density.

The behavior and heat generation effect of a magnetic rod-like particle suspension in an alternating and a rotating magnetic field

We have investigated the behavior of magnetic rod-like particles and their relationship with the heat generation effect for both alternating and rotating magnetic fields, by means of Brownian dynamics simulations. As a common feature for both type of magnetic field variations, in the case of significantly strong particle-particle interaction strengths, densely-packed clusters are formed, which do not contribute to the heat generation. For the case of the alternating magnetic field, in the intermediate frequency range, linear thick chain-like clusters are formed and the constituent rod-like particles themselves tend to rotate to follow the change in the field. In contrast, for the case of the rotating field, linear clusters rotate as a whole body to respond to the magnetic field rotation. In both type of magnetic field variations, the magnetic interactions between the constituent particles in a cluster tend to function to suppress the relaxational motion of the rod-like particles, which, as a result, leads to increase in the heat generation effect in certain situations. In a relatively large frequency region, the rotating applied magnetic field gives rise to a larger heat generation effect, whereas in contrast, in the lower frequency region the alternating magnetic field yields the larger heating effect. Highlights The behavior of magnetic rod-like magnetic particles has been elucidated in the case of an alternating and a rotating magnetic field. The magnetic interactions between the constituent particles in a cluster tend to suppress the relaxation motion of the rod-like particles. The frequency and strength of a time-dependent magnetic field have complex influences on the heating effect. In a relatively large frequency region, the rotating applied magnetic field gives rise to a larger heat generation effect. In a relatively lower frequency region, the alternating magnetic field is superior to the rotating field.

Anticlinic order of long-range repulsive rodlike magnetic particles in two dimensions

In the field of liquid crystals, it is well known that rodlike molecules interacting via long-range attractive interactions or short-range repulsive potentials can exhibit orientational order. In this work, we are interested in what would happen to systems of rodlike particles interacting via a long-range repulsive potential. In our model, each particle consists of a number of point dipoles uniformly distributed along the particle length, with all dipoles pointing along the z axis so that the rodlike particles repel each other when they lie in the x-y plane. Dipoles from different particles interact via an r^{-3} potential, where r is the distance between the dipoles. We have considered two model systems, each with N particles in a unit cell with periodic boundary conditions. In the first, particle centers are fixed on a square or triangular lattice but they are free to rotate. In the second, particles are free to translate as well as rotate in cells with variable shapes. Here they self-assemble to form configurations where the stress tensors are isotropic. Our numerical results show that, at low temperatures, the particles tend to form stripes with alternating orientations, resembling herringbone patterns or the anticlinic Sm-C_{A} liquid crystal phase.

Improvement of trapping performance of magnetic particles by magnetic multi-poles via Brownian dynamics simulations of magnetic rod-like particles in a Hagen-Poiseuille flow

In the present study we have implemented a Brownian dynamics simulation of a Hagen-Poiseuille flow in a cylindrical vessel and have investigated the feasibility of multi-pairs of magnetic poles for exhibiting a good particle trapping performance. We have focused on two representative types of configuration consisting of two pairs of magnetic poles. The first is a vertical configuration of two pairs of poles arranged perpendicular to the direction of the flow field. The second is a parallel configuration, where each pair of magnetic poles is arranged along the surface of the vessel wall. In the case of the vertical pole arrangement, although densely-packed aggregates were formed between the poles, their location was unstable in the flow field and they tended to be carried away downstream. In contrast, a densely-packed cluster formed in the parallel arrangement tended locate in a more stable manner along the surface of the wall where the flow field is weaker. We conclude that under appropriate conditions of the magnetic particle-particle interaction strength, the applied magnetic field strength and the pole separation distance, the parallel arrangement of magnetic poles is the configuration of choice for giving rise to a superior trapping performance.

Brownian dynamics simulations on the trapping characteristics of magnetic rod-like particles via multi-magnetic poles

Brownian dynamics simulations on the trapping characteristics of magnetic rod-like particles via multi-magnetic poles

Feasibility of multi-particle collision dynamics for rod-like particles and its application to a change in the orientational regime of a hematite particle suspension

We have verified that the method of multi-particle collision dynamics (MPCD) is able to activate a physically reasonable level of random motion in the medium of virtual fluid particles and activate the translational, ordinary rotational and spin rotational Brownian motion of the magnetic rod-like particles. MPCD was then applied to a quasi-2D system of rod-like hematite particles. The particle Brownian motion was found slightly over-activated, but this tendency decreased with smaller simulation time steps. The deviation from the theoretical value in the present results may be regarded to be acceptable as a first approximation since the Brownian motion was activated without the addition of any special technique. The applied magnetic field strength is found to play a significant role to enhance the order of the alignment of the raft-like clusters along the direction of the applied magnetic field. A similar orientational regime change along the applied magnetic field may be accomplished by an increase in the volumetric fraction of magnetic particles. We have therefore concluded that both an applied magnetic field and the particle volumetric fraction may be feasible parameters for inducing a regime change in the ordering of the orientation of rod-like hematite particles.

Brownian Dynamics Simulations of the Trapping Characteristics of Magnetic Rod-like Particles in a Non-Uniform Magnetic Field

Brownian Dynamics Simulations of the Trapping Characteristics of Magnetic Rod-like Particles in a Non-Uniform Magnetic Field

Brownian Dynamics Simulations of Magnetic Rod-like Particles in an Alternating Magnetic Field

Brownian Dynamics Simulations of Magnetic Rod-like Particles in an Alternating Magnetic Field

Trapping characteristics of magnetic rod-like particles flowing in a cylindrical pipe by means of a non-uniform magnetic field (Brownian dynamics simulations)

We have elucidated the feasibility of trapping magnetic rod-like particles that flow in a cylindrical pipe in terms of a non-uniform magnetic field. We have performed Brownian dynamics (BD) simulations in order to clarify the influence that various factors have on the attachment or trapping characteristics of magnetic rod-like particles to the poles. The employment of a local linearisation approximation enables us to apply the BD method that was fully-developed for a simple shear flow to the present Hagen-Poiseuille flow. In the case of a sufficiently small magnetic interaction between the particles, numerous rod-like particles may attach to the magnetic poles if the influence of a non-uniform magnetic field is sufficiently more dominant than that of the flow field, otherwise, the particles flow away along the centreline of the cylindrical pipe. If the magnetic interactions are sufficiently more dominant than both the flow field and the non-uniform magnetic field, the particles aggregate to form densely-packed clusters that flow away along the centreline area of the pipe without being trapped at the poles. In the situation of insufficient magnetic interactions, thin chain-like clusters may be formed from several anchored particles trapped to the poles and incline in the downstream direction.

The Elucidation of the behavior of rod-like magnetic particles in the Hagen Poiseuille flow with a Non-uniform magnetic field

The Elucidation of the behavior of rod-like magnetic particles in the Hagen Poiseuille flow with a Non-uniform magnetic field

Magnetic field sensing using whispering-gallery modes in a cylindrical microresonator infiltrated with ferronematic liquid crystal.

An all-fiber magnetic field sensor based on whispering-gallery modes (WGM) in a fiber micro-resonator infiltrated with ferronematic liquid crystal is proposed and experimentally demonstrated. The cylindrical microresonator is formed by a 1 cm-long section of a photonic crystal fiber infiltrated with ferronematic materials. Both ferronematics suspensions are prepared based on the nematic liquid crystal 1-(trans-4-Hexylcyclohexyl)-4-isothiocyanatobenzene (6CHBT) doped with rod-like magnetic particles in the first case and with spherical magnetic particles in the second case. WGMs are excited in the fiber microresonator by evanescent light coupling using a tapered fiber with a micron-size diameter. The Q-factor of the microresonator determined from the experimentaly measured transmission spectrum of the tapered fiber was 1.975 × 103. Under the influence of an applied magnetic field the WGM resonances experience spectral shift towards shorter wavelengths. The experimentally demonstrated sensitivity of the proposed sensor was -39.6 pm/mT and -37.3 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for a magnetic field range (0-47) mT. Reducing the diameter of the cylindrical micro-resonator by tapering leads to enhancement of the magnetic field sensitivity up to -61.86 pm/mT and -49.88 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for the magnetic field range (0-44.7) mT.

Structural Changes in Liquid Crystals Doped with Rod-Like Magnetic Particles Studied by Surface Acoustic Waves

Structural Changes in Liquid Crystals Doped with Rod-Like Magnetic Particles Studied by Surface Acoustic Waves

Ferronematics based on the nematic 6CB in combined electric and magnetic fields

ABSTRACTThis work is devoted to the study of composite systems of the liquid crystal 4-n-hexyl-4-cyanobiphenyl (6CB) doped with differently shaped magnetite nanoparticles. The ferronematic samples were prepared with the volume concentration of =10 of spherical, as well as of rod-like magnetic particles. The structural transitions in ferronematic samples were observed by capacitance measurements in a capacitor made of indium-tin-oxide-coated glass electrodes in combined electric and magnetic fields.

The enhanced MR performance of dimorphic MR suspensions containing either magnetic rods or their non-magnetic analogs

A co-precipitation method was used to prepare non-magnetic rod-like ferrous oxalate dihydrate (Fe2CO4 · 2H2O) particles that were further turned into iron oxide (Fe3O4) magnetic rod-like particles. A simple precursor-assisted thermal decomposition technique enabled the preservation of the morphology and size of the precursor ferrous oxalate dihydrate particles, thus allowing their magnetic analogs to be obtained. Both types of rod-like particles were used as an additive together with spherical carbonyl iron (CI) particles in novel dimorphic magnetorheological (MR) suspensions. Controlled shear rate mode experiments were performed using a rotational rheometer with a source of an external magnetic field in order to investigate their MR behavior. Moreover, the properties of the novel prepared dimorphic MR (DMR) suspensions were compared with conventional MR suspensions based on spherical CI microparticles. It was found that the DMR suspensions exhibit enhanced MR performance as well as enhanced sedimentation stability in comparison with the MR suspension based on pure CI. The dimorphic suspensions containing magnetic rod-like additives further exhibited significant MR hardening at low shear rates. The properties of CI-based suspensions can be thus optimized by using various additive substances.

Monte Carlo simulations on phase change in aggregate structures of ferromagnetic spherocylinder particles

A suspension of ferromagnetic rod-like particles can be expected to exhibit strong magneto-rheological characteristics, which is a significantly important factor for application of magnetic particle suspensions to mechanical dampers and actuators. Hence, in the present study, we address a suspension composed of ferromagnetic rod-like particles in thermodynamic equilibrium. We here investigate the dependence of the phase change in aggregate structures on the various factors such as magnetic field strength, magnetic particle–particle interaction strength and volumetric fraction of magnetic particles. Monte Carlo simulations have been carried out to obtain results of snapshots, radial distribution function and order parameter of the system. In a weak applied magnetic field, the magnetic rod-like particles tend to aggregate to form raft-like clusters if the magnetic particle–particle interaction is much larger than thermal energy. If the magnetic field is increased, these raft-like clusters drastically dissociate into single-moving particles, that is, the phase change in aggregate structures arises. Moreover, the phase change in the aggregate structures is induced by the magnetic particle–particle interaction strength, from no cluster formation to long and thick chain-like clusters, in a strong applied magnetic field circumstance. As the length of the magnetic rod-like particles is increased, the orientational configuration comes to have a more significant effect on the cluster formation.

G0500303 強磁性棒状粒子からなるサスペンションの磁気粘性特性に関するブラウン動力学シミュレーション

Magneto-rheological properties of a magnetic particle suspension are strongly dependent on the formation of aggregates of magnetic particles. A suspension of ferromagnetic rod-like particles can be expected to exhibit strong magneto-rheological characteristics, which is a significantly important factor for application of magnetic particle suspensions to mechanical dampers and actuators. Hence, in the present study, we focus on a ferromagnetic rod-like particle suspension to discuss the magneto-rheological properties. The results were obtained by means of Brownian dynamics simulations in a simple shear flow situation. In a weak applied magnetic field, magnetic rod-like particles tend to aggregate to form raft-like clusters, and with increasing magnetic field strength, these raft-like clusters drastically dissociate into single-moving particles at a certain value of the magnetic field strength. The net viscosity and its viscosity components exhibit complex dependence on the magnetic field strength, which is mainly due to the raft-like cluster formation of magnetic particles.

Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition

The influence of the shape anisotropy of magnetic particles on the isotropic–nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The LC was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarising microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic–nematic phase transition in LC are demonstrated here. The results are in a good agreement with recent theoretical predictions.

Increasing the magnetic sensitivity of liquid crystals by rod-like magnetic nanoparticles

Magnetic Freedericksz transition was studied in ferronematics based on the nematic liquid crystal 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). 6CHBT was doped with rod-like magnetic particles of different size and volume concentration. The volume concentrations of magnetic particles in the prepared ferronematics were $\phi_1$ = 10$^{-4}$ and $\phi_2$ = 10$^{-3}$. The structural changes were observed by capacitance measurements that demonstrate a significant influence of the concentration, the shape anisotropy, and/or the size of the magnetic particles on the magnetic response of these ferronematics.

Structural changes in the 6CHBT liquid crystal doped with spherical, rodlike, and chainlike magnetic particles

In this work the 4-(trans- 4'-n -hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) liquid crystal was doped with differently shaped magnetite nanoparticles. The structural changes were observed by capacitance measurements and showed significant influence of the shape and size of the magnetic particles on the magnetic Fréedericksz transition. For the volume concentration phi= 2 x 10(-4) of the magnetic particles, the critical magnetic field was established for the pure liquid crystal, and for liquid crystals doped with spherical, chainlike, and rodlike magnetic particles. The influence of the magnetic field depends on the type of anchoring, which is characterized by the density of anchoring energy and by the initial orientation between the liquid crystal molecules and the magnetic moment of the magnetic particles. The experimental results indicated soft anchoring in the case of spherical magnetic particles and rigid anchoring in the case of rodlike and chainlike magnetic particles, with parallel initial orientation between the magnetic moments of the magnetic particles and director.

Effects of magnetic fields on fibrin polymerization and fibrinolysis

The authors' study first focuses on the role of ferromagnetic particles in the magnetic orientation of fibrin polymers. Rod-like magnetic particles were mixed into a solution of fibrinogen. The fibrin polymerization process was exposed to a magnetic field of 8 T. Magnetic particles agglomerated to form chains of clusters which were oriented parallel to magnetic fields. Magnetic orientation of fibrin polymers did not occur near the chains of magnetic particles. In a study of the possible effects of magnetic fields on the fibrinolytic process, dissolution of fibrin formed due to magnetic fields was observed. The fibrinolytic activities were evaluated by the absorbance changes at 500 nm and by a fibrin plate method. A distinguishable difference was observed in the dissolution of fibrin between fibrin gel formed with a magnetic field at 8 T and that formed without a field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>