Chain-like Clusters Research Articles

Three-dimensional Monte Carlo simulations of internal aggregate structures in a colloidal dispersion composed of rod-like particles with magnetic moment normal to the particle axis

We have treated a suspension composed of ferromagnetic rod-like particles with a magnetic moment normal to the particle axis in order to investigate aggregation phenomena of such a suspension by means of cluster-moving Monte Carlo simulations. In the present study, we have considered a three-dimensional mono-dispersed model system composed of such rod-like particles. Internal structures of self-assembled clusters have been discussed quantitatively in terms of radial distribution, pair correlation, orientational pair correlation functions, number distributions of clusters, and order functions. The main results obtained here are summarized as follows. Rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction more significantly with magnetic particle–particle interactions. In such raft-like clusters, the direction of each particle axis has a tendency to incline in parallel formation, but is not so parallel as in a two-dimensional dispersion. As the volumetric fraction increases, longer raft-like clusters are formed, but such raft-like clusters do not aggregate further to form thicker clusters, which is in significantly contrast with a dispersion of spherical particles, where thicker chain-like clusters are observed under certain conditions. For the case of strong magnetic particle–particle interactions, sufficiently long raft-like clusters are formed along the magnetic field direction, even if the influence of an external magnetic field is of the same order of that of the thermal energy. However, rod-like particles in such clusters do not necessarily incline in significantly parallel formation along a certain direction. Self-assembled tube-like clusters are formed when magnetic particle–particle interactions are much more dominant than the rotational Brownian motion under circumstances of rod-like particles inclining in a certain direction.

Effects of oscillatory shear flow on chain-like cluster dynamics in ferrofluids without magnetic fields

Brownian dynamics simulations of interacting magnetic particles in a quasi-two-dimensional ferrofluid system are performed at zero temperature, under the influence of oscillatory shear flow in the absence of external magnetic fields. Starting from chain-like clusters of the particles, we study the time-dependent behavior of both magnetization and microstructures of the ferrofluid by changing values of two parameters, the shear rate strength and frequency of oscillatory shear flow. Simulation results show that there are three different dynamical regimes for the chain clusters dynamics, depending on these two parameters. Scaling behavior of the asymptotic magnetization is also observed for a certain range of parameters.

Dynamic Simulation of Shear-induced Particle Migration in a Two-dimensional Circular Couette Device

The shear-induced migration of neutrally-buoyant non-colloidal circular particles in a two-dimensional circular Couette flow is investigated numerically with a distributed Lagrange multiplier based fictitious domain method. The effects of inertia and volume fraction on the particle migration are examined. The results indicate that inertia has a negative effect on the particle migration. In consistence with the experimental observations, the rapid migration of particles near the inner cylinder at the early stage is observed in the simulation, which is believed to be related to the chain-like clustering of particles. The migration of circular particles in a plane Poiseuille flow is also examined in order to further confirm the effect of such clustering on the particle migration at early stage. There is tendency for the particles in the vicinity of outer cylinder in the Couette device to pack into concentric rings at late stage in case of high particle concentration.

Effects of Magnetic Interactions between Ferromagnetic Spherocylinder Particles on Orientational Distributions of a Nondilute Colloidal Dispersion

We have analyzed the time-dependent behaviors of the ferromagnetic spherocylinder particles which make a nondilute colloidal dispersion influenced by the external magnetic field. In order to understand the effects of the magnetic interactions between the particles, we have derived a basic equation for the orientational distributions of the particles in steady two dimensional flow by applying the mean-field theory and the perturbation expansion method. An exact solution of the basic equation is obtained theoretically. By using the solution, the mean-field, which is derived from the self-consistent condition, is analytically calculated. The magnetic interactions between the particles restrict the particle directions and make the particle incline to the direction of the chainlike cluster. The strength of the restriction is, by an approximate estimation, inversely proportional to the aspect ratio of the particle. Compared with a dilute system, in the nondilute system, the existence of the particle-particle interactions provides the increased stability of the particle orientation and the swift response to the external magnetic field.

Agglomeration Simulation of Chain-like Inclusions in Molten Steel Based on Fractal Cluster-Cluster Agglomeration Model

Based on the fractal cluster-cluster agglomeration (CCA) model, two-dimensional simulations of cluster aggregation process have beencarried out by varying initial particle number, particle size and sticking probability between clusters. Particle size and amount of seed particleshave small effects on the shape of clusters. Actual alumina clusters observed in tundish show the chain-like appearance and are very similar tothe simulatedagglomerated clusters with highstickingprobability.The agglomeration process of alumina chain-like clusters aroundimmersionnozzle in the tundish and RH process are plausibly controlled by the random collision process with high sticking probabilityvalues. [doi:10.2320/matertrans.MRA2007037](Received February 15, 2007; Accepted May 18, 2007; Published July 19, 2007)Keywords: fractal, fractal dimension, inclusion, chain-like inclusion, deoxidation, cluster-cluster aggregation

Formation of Magnetic Particle Chains in Ultra High Magnetic Field

Magnetic particles form chain-like clusters in a magnetic field. These chains are used to supply toner particles on a photoreceptor drum in the two-component magnetic brush development system used for high-speed and/or color laser printers. Characteristics of chains influence image quality of the printer. Although experiments had been performed in low magnetic field, characteristics in the high magnetic field are necessary to investigate actual performance of the system and thus an experiment on the chain formation has been performed in the ultra high magnetic field created by a superconducting magnet. It has been clarified that the chain length first increased and then decreased in accordance with the increase of the magnetic field. The result has been confirmed by a numerical calculation with the Distinct Element Method that included the contact force between particles, the gravitational force, and the magnetic force due to the field created by the coil and adjacent magnetized particles. Lastly, it has been discussed that the static stability of chain was determined by a minimization principle of the potential energy that consists of the gravitational and magnetic potential energy.

Three-dimensional Monte Carlo Simulations of Internal Aggregate Structures in a Colloidal Dispersion Composed of Rod-like Particles with Magnetic Moment Normal to the Particle Axis

We have investigated aggregation phenomena of a colloidal dispersion composed of ferromagnetic rod-like particles with a magnetic moment normal to the particle axis, by means of three-dimensional cluster-moving Monte Carlo simulations. Concretely, we have treated a monodispersed model system in order to clarify the influences of the volumetric fraction of particles, magnetic interactions between particles, etc., on particle aggregation phenomena. Aggregate structures have been discussed qualitatively in terms of radial distribution, orientational pair correlation functions, etc. The main results obtained here are summarized as follows. Rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction as magnetic particle-particle interactions increase. In such raft-like clusters, the direction of each particle axis has a tendency to align in parallel formation, but is not significantly parallel as expected. As the volumetric fraction increases, longer raft-like clusters are formed. However, such raft-like clusters do not aggregate further to form thicker clusters, which is in significantly contrast with dispersions of spherical or rod-like particles with a magnetic moment along the particle axis, where thicker chain-like clusters are observed under certain conditions.

Formation of Magnetic Particle Chains in Ultra High Magnetic Field

Magnetic particles form chain-like clusters in the magnetic field. This phenomenon is of interest in two separate fields, one is a development system of the laser printer and another is an electromagnetic manipulation of biological cells. Experiments on the chain formation in air and oil have been performed in the ultra high magnetic field created by a superconducting magnet. It has been clairfied that the chain length first increased and then decreased in accordance with the increase of the magnetic field. The result has been confirmed by a numerical calculation with the Distinct Element Method that included the mechanical contact force between particles and the magnetic force due to the field created by the coil and adjacent magnetized particles. A periodic boundary condition was employed to perform realistic calculation in a reasonable calculation time. The effect of the density and the diameter of particles was investigated experimentally. The effect of the dry friction between particles was also examined by comparing experimental results performed in air and oil. Lastly, it has been discussed that the static stability of the chain was determined by a minimization principle of the potential energy that consists of the gravitational and magnetic potential energy.

Aggregation and Sedimentation of Aqueous Nanoscale Zerovalent Iron Dispersions

Nanoscale zerovalent iron (NZVI) rapidly transforms many environmental contaminants to benign products and is a promising in-situ remediation agent. To be effective, NZVI should form stable dispersions in water such that it can be delivered in water-saturated porous media to the contaminated area. Limited mobility of NZVI has been reported, however, attributed to its rapid aggregation. This study uses dynamic light scattering to investigate the rapid aggregation of NZVI from single nanoparticles to micrometer size aggregates, and optical microscopy and sedimentation measurements to estimate the size of interconnected fractal aggregates formed. The rate of aggregation increased with increasing particle concentration and increasing saturation magnetization (i.e., the maximum intrinsic magnet moment) of the particles. During diffusion limited aggregation the primary particles (average radius = 20 nm) aggregate to micrometer-size aggregates in only 10 min, with average hydrodynamic radii ranging from 125 nm to 1.2 microm at a particle concentration of 2 mg/L (volume fraction(phi= 3.2 x 10(-7)) and 60 mg/L (phi = 9.5 x 10(-6)), respectively. Subsequently, these aggregates assemble themselves into fractal, chain-like clusters. At an initial concentration of just 60 mg/L, cluster sizes reach 20-70 microm in 30 min and rapidly sedimented from solution. Parallel experiments conducted with magnetite and hematite, coupled with extended DLVO theory and multiple regression analysis confirm that magnetic attractive forces between particles increase the rate of NZVI aggregation as compared to nonmagnetic particles.

Assembly of Linear Clusters of Iodobenzene Dimers on Cu(110)

Low-temperature STM observations of the low-coverage chemisorption behavior of iodobenzene on Cu(110) are presented at two annealing temperatures. When a random distribution of isolated iodobenzene molecules deposited at T < 100 K is annealed to approximately 170 K, the undissociated molecules assemble into chainlike clusters composed of identical subunits. An STM tip-induced dissociation reaction is used to determine that the individual units in the chains are composed of pairs of iodobenzene molecules. A model is proposed in which iodine atoms on each member of the pair are directed toward one another. The driving force for the formation of such clusters is suggested to be the dispersion interactions between the polarizable iodobenzene molecules.

Forming chainlike filaments of magnetic colloids: The role of the relative strength of isotropic and anisotropic particle interactions

The influence of the interplay between anisotropic magnetic and isotropic electrostatic interactions on the aggregation behavior of aqueous suspensions of electric double layered magnetic particles was studied. Therefore, the particles were aggregated under the action of an external magnetic field and in the presence of different amounts of an indifferent 1:1 electrolyte. After removing the field, linear aggregates remained in the sample. Static light scattering and electron micrographs confirmed the chainlike cluster morphology. Dynamic light scattering was used for monitoring the average diffusion coefficient of these magnetic filaments. A theoretical model that allows the experimental mean diffusion coefficient to be related to the average chain length was successfully employed. The results show that, at fixed exposure time and field strength, the average filament size is proportional to the amount of electrolyte added. The light scattering data and transmission electron microscopy micrographs prove that permanent chains coexist with a relatively large fraction of individual particles when no or little electrolyte was added to the samples. A plausible explanation for this "selective aggregation" phenomenon could be given in terms of surface charge heterogeneities. The chain growth was found to follow a power law with a similar exponent for all the electrolyte concentrations studied. Scaling theories were employed for estimating the ratio of particles taking part in the aggregation process.

Rheological properties and particle behaviors of a nondilute colloidal dispersion composed of ferromagnetic spherocylinder particles subjected to a simple shear flow: Analysis by means of mean-field approximation for the two typical external magnetic field directions

We have analyzed the orientational distributions and rheological properties of a nondilute colloidal dispersion composed of ferromagnetic spherocylinder particles subjected to a simple shear flow. In order to understand the effects of the magnetic interactions between the particles, we have applied the mean-field theory to a nondilute colloidal dispersion for the two typical external magnetic field directions, that is, the direction parallel to the shear flow and the direction parallel to the angular velocity vector of the shear flow. The main results are summarized as follows. The particle–particle interactions suppress the Brownian motion of the particles and, therefore, make the particles incline toward the same direction. For the magnetic direction parallel to the shear flow, the influence of the particle–particle interactions makes the peak of the orientational distribution sharper and higher. The viscosity generally increases as the interactions between particles become stronger in the case where the effects of the shear flow and magnetic field are relatively small. For the magnetic direction parallel to the angular velocity vector of the shear flow, the influence of the particle–particle interactions on the orientational distribution appears significantly, when the influences of the shear flow and magnetic field are not so strong that the particles can be aligned sufficiently to form stable chainlike clusters in a certain direction.

2302 印加磁場による磁性流体,MR流体中の超音波伝播速度への影響(S42-1 複雑流体の流動現象(1),S42 複雑流体の流動現象)

The properties of ultrasonic propagation velocity in water-based and kerosene-based magnetic fluids and an MR fluid are examined experimentally. The ultrasonic measurement scheme is based on the pulse method. The external magnetic field intensity is varied from 0mT to 550mT and the angle between the magnetic field direction and the direction of ultrasonic wave propagation is adjusted from 0°to 90°. The ultrasonic propagation velocity changes with the elapsed time of external magnetic field application and with magnetic field intensity. Moreover, the ultrasonic propagation velocity in MR fluid is larger than that in magnetic fluids. These properties of ultrasonic propagation seem to be highly influenced by the formation of chain-like clusters (in the magnetic fluids) and robust clusters (in the MR fluid).

Lattice-Boltzmann Simulation of Particle Suspensions in Shear Flow

Inclusion of short-range particle–particle interactions for increased numerical stability in a lattice-Boltzmann code for particle-fluid suspensions, and handling of the particle phase for an effective implementation of the code for parallel computing, are discussed and formulated. In order to better understand the origin of the shear-thickening behavior observed in real suspensions, two simplified cases are considered with the code thus developed. A chain-like cluster of suspended particles is shown to increase the momentum transfer in a shear flow between channel walls, and thereby the effective viscosity of the suspension in comparison with random configurations of particles. A single suspended particle is also shown to increase the effective viscosity under shear flow of this simple suspension for particle Reynolds numbers above unity, due to inertial effects that change the flow configuration around the particle. These mechanisms are expected to carry over to large-scale particle-fluid suspensions.

Molecular Dynamics Simulation Study of the Role of Evenly Spaced Poly(ethylene oxide) Tethers on the Aggregation of C60 Fullerenes in Water

The aggregation behavior of C60 fullerenes and C60 fullerenes with six symmetrically tethered poly(ethylene oxide) oligomers [(PEO)-6-C60] in aqueous solutions has been studied using implicit solvent molecular dynamics simulations. Our simulations reveal that while the attraction between two (PEO)-6-C60 fullerenes in aqueous solution is stronger and longer range than that between two bare C60 fullerenes, the (PEO)-6-C60 fullerenes do not phase-separate in water but rather aggregate in chain-like clusters at concentrations where unmodified fullerenes completely phase-separate.

Two-dimensional Monte Carlo simulations of a colloidal dispersion composed of polydisperse ferromagnetic particles in an applied magnetic field

We have investigated aggregation phenomena in a polydisperse colloidal dispersion of ferromagnetic particles simulated by employing the cluster-moving Monte Carlo method in an applied magnetic field. The influence of both particle–particle and particle–field interactions on the aggregate structures is analyzed in terms of a pair correlation function. The results obtained in this study are summarized as follows: Under a strong magnetic field, chainlike clusters are formed along the magnetic field direction, and they become thickly clustered with an increase in the strength of the external magnetic field. Moreover, the thickly clustered chains are formed for a polydisperse system that has a large standard deviation of particle diameters. In contrast, for a very weak magnetic field, the strong interaction between the larger particles gives rise to the formation of various shapes in the chainlike clusters, including bending, looping, and branching. With an increase in the external magnetic field, these structures reorganize to form straight chainlike clusters. Furthermore, the thickness of the chainlike clusters for the polydisperse system is found to depend on the standard deviation of the particle-size distribution but is found to be independent of the magnetic field strength.

Systematic UHV‐AFM experiments on Na nano‐particles and nano‐structures in NaCl

Results of systematic AFM (atomic force microscopy) experiments on heavily and moderatly irradiated NaCl samples are presented. The sodium nanoparticles and structures of nanoparticles are poduced in sodium chloride during irradiation. The AFM images of the nanoparticles have been obtained in ultra high vacuum (UHV) in the non-contact mode with an Omicron UHV AFM/STM system. The sizes and arrangements of the observed particles depend on the irradiation conditions. The melting behaviour of the sodium nanoclusters has been studied for these samples by differential scanning calorimetry (DSC). A phenomenological model has been proposed which explains the observed behaviour by the existence of two different structural states in the population of sodium colloids. AFM measurements of heavily irradiated natural rock salt samples have revealed ring- and chain-like clusters (or agglomerates) of Na colloid particles. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of External Magnetic Field on Ultrasonic Propagation Velocity in Magnetic Fluids

Experimental results for the properties of ultrasonic propagation velocity in kerosene-based and water-based magnetic fluids are reported. Ultrasonic wave frequencies of 1 MHz, 2 MHz and 4 MHz are used and the measurement scheme is based on the pulse method. The external magnetic field intensity is varied from 0 mT to 550 mT and the angle between the magnetic field direction and the direction of ultrasonic wave propagation is varied from 0° to 90°. The ultrasonic propagation velocity in magnetic fluids is dependent on temperature, elapsed time of applying the magnetic field, and magnetic field intensity. Hysteresis and anisotropy of ultrasonic propagation velocity are observed. These interesting results seem to be related to chain-like cluster formation in the magnetic fluids and the characteristic period of Brownian motion of the magnetic particles.

Vacancy-type defects in strained-Si layers deposited on SiGe∕Si structures probed by using monoenergetic positron beams

Vacancy-type defects in strained-Si layers deposited on Si0.75Ge0.25∕graded-SiGe∕Si structures were probed by using monoenergetic positron beams. The Doppler broadening spectra of the annihilation radiation and the lifetime spectra of the positrons were measured for samples before and after annealing (800–1050 °C). For an as-received sample, the defects in the strained-Si layer were identified as vacancy-type defects coupled with Ge. The mean open size of these defects was estimated to be close to that of a divacancy. The line-shape parameter, S, corresponding to the positron annihilation in the strained-Si layers decreased with increasing annealing temperature, but no large change in the positron lifetime was observed. From a comparison between the Doppler broadening profiles for the strained-Si films and those calculated using the projector augmented-wave method, it was found that the number of Ge atoms forming a complex by coupling with a defect increased with increasing annealing temperature. The number was estimated to be three or four after annealing at 1050 °C. Since the defect complexes were stable even after annealing at such a high temperature, the defects detected by the positrons could be part of chainlike vacancy clusters.

Influence of magnetic field on ultrasonic propagation velocity in magnetic fluids

Experimental results for anisotropy and hysteresis of ultrasonic propagation velocity in a kerosene-based magnetic fluid are reported. The ultrasonic frequencies used are 1, 2 and 4 MHz and the measurement scheme is based on the pulse method. The external magnetic field intensity is varied from 0 to 550 mT and the angle between the magnetic field direction and the direction of ultrasonic wave propagation is varied from 0 ∘ to 90 ∘ . The ultrasonic propagation velocity increases with magnetic field intensity and ultrasonic frequency. Anisotropy due to the configuration of chain-like clusters is also observed. These results seem to be caused by cluster formation and the characteristic time of Brownian motion of the magnetic particles.