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In the present study, we elucidate the behaviour of a suspension composed of magnetic disk-like particles that flow in a Hagen-Poiseuille flow in a gradient magnetic field and the trapping characteristics of the magnetic particles. To this end, we utilise multiple pairs of magnetic poles located outside the cylindrical system. Brownian dynamics simulations were performed in order to clarify the dependence of these characteristics on a variety of factors. It is seen that an increase in the magnetic field strength improves the trapping characteristics of disk-like particles if the influence of an applied magnetic field is sufficiently more dominant than that of a flow field and that of the magnetic particle-particle interaction. In the case of a strong magnetic particle-particle interaction, thin chain-like clusters are formed from an anchored particle trapped around a magnetic pole. If the two poles are sufficiently close to each other, an arch-type cluster is formed between them. We expect that this arch-type cluster may give rise to good trapping performance even in the presence of a strong flow field if the formation arises at a sufficiently large separation distance between the pair of magnetic poles. Highlights We have performed Brownian dynamics simulations for a magnetic disk-like particle suspension in a Hagen-Poiseuille flow. If the magnetic particle-particle interaction is sufficiently more dominant than the flow field, thin chain-like clusters are formed from an anchored particle. As the pole separation distance decreases, an arch-type configuration tends to be formed along the surface of the wall. This arch-type cluster that is formed between the pair of magnetic poles may be expected to improve the trapping characteristics depending on the pole separation distance. [ABSTRACT FROM AUTHOR]

We have investigated a suspension composed of spherical magnetic Janus particles in thermodynamic equilibrium. The magnetic moment of Janus particles treated here is located at a position shifted from the particle centre, which is on the line perpendicular to the magnetic moment direction. We conducted quasi-2D Monte Carlo simulations to investigate the change in the regime of aggregate structures and the dependence of the regime on various factors, such as the strength of the magnetic particle-particle interaction, magnetic particle-field interaction, and the distance of the magnetic moment from the particle centre. In the absence of an applied magnetic field, the Janus particles aggregate to form stable clusters composed of a few particles with increased values of the magnetic particle-particle interaction strength. As the strength of the magnetic field is increased, these clusters collapse, and chain-like clusters are formed aligned in the direction of the magnetic field. The appearance of this regime change requires a stronger magnetic field as the position of the magnetic moment approaches the particle surface. This can be attributed to the fact that the magnetic interaction between neighbouring particles becomes more dominant as the magnetic moments of the constituent particles in the cluster units approach each other. Highlights We have addressed the aggregation phenomena in a suspension composed of spherical magnetic Janus particles. We have investigated the regime change in the aggregate structures of the Janus particles under various conditions. In a strong magnetic field, thick chain-like clusters are formed as the magnetic moment is shifted away from the particle centre. The present results are useful for elucidating the internal structure of complex aggregates in 3D systems. [ABSTRACT FROM AUTHOR]

We have first investigated a feasibility of a malt-particle collision dynamics (MPCD) method by addressing whether or not the translational and rotational Brownian motion is activated at a physically reasonable level. From the results with respect to the root mean square velocities, we conclude that the MPCD method is a quite feasible simulation technique for a suspension composed of magnetic cubic particles. Then we have applied the MPCD simulation method to a quasi-2D system of magnetic cubic particles in order to control a regime change in the internal structure of particle aggregates by means of the strength and direction of a uniform applied magnetic field. In the case of a magnetic field applied in a certain direction from the 2D plane, a regime change in the internal structure of large densely-packed clusters into a regular configuration is more effectively attained than a magnetic field applied along the 2D plane. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have addressed the problem of controlling a regime change in a quasi-2D system of magnetic disk-like particles by means of the strength and direction of an applied magnetic field. In the present study, two typical directions have been addressed, i.e. a y-direction field parallel to the 2D plane and a z-direction field normal to that plane of the particle disk. It is shown that the direction of an applied magnetic field is a significant factor in particle aggregation and can give rise to different regime changes. That is, an applied magnetic field parallel to the 2D system plane functions to induce a regime change in the aggregate structure from linear column-like particle clusters into a knitting-pattern formation, whereas in the case of an applied field normal to the 2D plane, a regime of long column-like clusters transforms into a particle distribution with no discernible internal structure. Key points of the present paper: The direction of an applied magnetic field may give rise to significantly different regime changes in particle aggregation. An applied magnetic field parallel to the 2D system plane induces a regime change from long linear column-like clusters into a knitting-pattern formation. A magnetic field in a direction normal to the 2D plane induces a regime change from long linear column-like clusters into a regime with no discernible internal structure. The strength and the direction of an applied magnetic field is a feasible tool for controlling a regime change in the aggregate structure. [ABSTRACT FROM AUTHOR]

We have addressed the aggregation phenomena of a cubic haematite particle suspension in an alternating magnetic field in order to investigate the regime change in the aggregate structure. Brownian dynamics simulations have been performed for a quasi-two-dimensional system with an alternating magnetic field applied in the plane of the system. In a weak alternating field, the cubic particles aggregate to form closely-packed structures tending to an aligned face-to-face configuration with increasing magnetic particle-particle interaction strength. In a closely-packed cluster, the orientation of the magnetic moment of a constituent particle tends to be more strongly influenced by the magnetic interaction between neighbouring particles and less influenced by the direction of the alternating magnetic field. As the strength of the applied field is increased, the particle moments become more restricted to the direction of the alternating field and the closely-packed structures transform into elongated aggregates with an offset face-to-face configuration. As the direction of the alternating magnetic field switches from the positive to the negative x-direction, the elongated aggregates tend to collapse temporarily and when the orientation of the moments is again strongly restricted to the switched field direction, elongated aggregate structures with offset face-to-face contact are re-formed in the system. Highlights of the present paper We have investigated a regime change in the internal structure of cubic particle aggregates in an alternating magnetic field. As the magnetic field strength is increased, closely-packed aggregates with an aligned face-to-face configuration transform into elongated aggregates with an offset face-to-face configuration. We have clarified that the elongated aggregates collapse temporarily and then re-form when the direction of the alternating magnetic field switches. We have suggested that the frequency of the magnetic field may be used as a means for controlling the local internal structure of particle aggregates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the present study, by means of quasi-two-dimensional Brownian dynamics simulations, we have addressed the physical phenomena of a suspension composed of cubic haematite particles in a rotating magnetic field in order to elucidate the relationship between aggregate structures and their response to a rotating magnetic field. In the case of a relatively weak magnetic particle-particle interaction, single particles remain without aggregating to form specific clusters, and the magnetic moment of single particles tends to follow a change in the direction of the rotating magnetic field. In the case of a strong magnetic particle-particle interaction, closely-packed clusters with a face-to-face configuration tend to form in the system, although the individual magnetic moments of the constituent particles do not follow the change in the magnetic field direction. As the magnetic field strength is increased, closely-packed structures are transformed into aggregate structures with an offset face-to-face configuration. In this situation, the magnetic moments of constituent particles are strongly restricted to the magnetic field direction, and the aggregates themselves may significantly rotate to follow the change in the orientation of the rotating magnetic field. [ABSTRACT FROM AUTHOR]

We have developed a Brownian dynamics simulation technique for a cubic magnetic particle suspension in a simple shear flow in order to elucidate the relationship between the particle aggregates and the magnetorheological characteristics. A magnetic field is applied in the direction normal to the shearing plane. In a weak applied magnetic field, if the magnetic particle–particle interaction strength is sufficiently large, the particles aggregate to form closely-packed clusters even when subject to the influence of the shear flow. As the magnetic field strength is increased, the closely-packed aggregate structures are transformed into chain-like structures. The net viscosity is increased because the chain-like clusters give rise to a larger resistance to the flow field. As the magnetic field strength is further increased, the chain-like clusters grow into wall-like clusters aligned in the direction of the magnetic field. If the wall-like clusters are the predominant clusters, a magnetic force arises due to a characteristic of the particle arrangement in the wall-like clusters that tends to accelerate the flow field and, as a consequence, decrease the net viscosity. From these results, it may be suggested that under certain conditions a magnetic cubic particle suspension may exhibit a negative contribution to the magnetorheological characteristic. Highlights of the present study We have developed a Brownian dynamics (BD) simulation technique for a magnetic cubic particle suspension. BD simulations have been performed in order to investigate the relationship between particle aggregates and magnetorheological properties. The net viscosity exhibits a complex dependence on the regime of particle aggregates. It is suggested that under certain conditions a magnetic cubic particle suspension may exhibit a negative contribution to the magnetorheological characteristics. An increase in the shear rate causes instability in the face-to-face configuration and leads to the collapse of closely-packed clusters. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The present study addresses the physical phenomena of a suspension composed of magnetic spherical particles in a rotating magnetic field in order to elucidate the influence of particle aggregation phenomena on the heat production from Brownian relaxation by means of Brownian dynamics simulations. In the case of a weak magnetic particle-particle interaction, particles do not aggregate to form specific cluster configurations. In this situation, the magnetic moment of each particle quickly inclines toward the field direction, and single particles do not give rise to a sufficiently large heating effect. With an increasing magnetic interaction between particles, chain-like clusters tend to form in the system and function to offer a larger resistance to the orientation of the magnetic moments, which leads to an increase in the heating effect. In the case of a significantly strong magnetic particle-particle interaction, the particles tend to aggregate to form stable ring-like clusters where the magnetic moments of the constituent particles are not able to be so responsive to the rotating magnetic field and this leads to a decrease in the heating effect. Highlights of the present paper The characteristics of particle aggregation have been clarified. The relationship between the strength of the magnetic particle-particle interaction and the frequency of the rotating magnetic field on the aggregation phenomena has been clarified. The response of the particle aggregates to an applied rotating magnetic field has been clarified. The relationship between the performance of the degree of heat production and the internal structure of magnetic particle configurations has been clarified. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have proposed a new repulsive layer model for describing the interaction between steric layers of coated cubic particles. This approach is an effective technique applicable to particle-based simulations such as a Brownian dynamics simulation of a suspension composed of cubic particles. 3D Brownian dynamics simulations employing this repulsive interaction model have been performed in order to investigate the equilibrium aggregate structures of a suspension composed of cubic haematite particles. It has been verified that Brownian dynamics employing the present steric interaction model are in good agreement with Monte Carlo results with respect to particle aggregate structures and particle orientational characteristics. From the viewpoint of developing a surface modification technology, we have also investigated a regime change in the aggregate structure of cubic particle in a quasi-2D system by means of Brownian dynamics simulations. If the magnetic particle–particle interaction strength is relatively strong, in zero applied magnetic field the particles aggregate in an offset face-to-face configuration. As the magnetic field strength is increased, the offset face-to-face structure is transformed into a more direct face-to-face contact configuration that extends throughout the whole simulation region. [ABSTRACT FROM AUTHOR]

The cubic particles magnetised along a diagonal direction are negatively charged and confined between two parallel electrode walls. These particles have a tendency to migrate toward and attach to the positive electrode together with the formation of chain-like clusters. We here have focused on three cases of external magnetic field direction. In the case of an external magnetic field applied in the oblique direction to the electrode wall, almost all the constituent particles of the chain-like clusters attach to the positive electrode wall in a face-to-face contact manner. In the case of an external magnetic field applied along the surface of the electrode wall, the chain-like clusters are formed in the space of the two parallel walls. In the case of an external magnetic field applied normal to the electrode wall, the tendency for the particles to form chain-like clusters along the electrode wall is the weaker among the three field directions. In this situation, the cubic particles attach to the wall surface with a vertex of the cube contacting with the surface, and from the anchored particle, a chain-like cluster is formed in the space between the two parallel walls. [ABSTRACT FROM AUTHOR]

By estimating the force and torque acting on the cube for the two cases of a uniform flow field and a rotational flow field, we have discussed whether or not there is a coupling between the translational and the rotational motion. From the characteristics of the friction coefficients, we may understand that there is no coupling between the translation motion and the rotational motion in the situation of the Reynolds number being sufficiently smaller than unity. In contrast, in the case of a non-slow flow field with the Reynolds number larger than unity, the coupling characteristics of the motion of a cube is certainly recognised and therefore the interaction with the ambient fluid is characterised by a variety of friction coefficients including friction coefficients that relate the forces acting on the cube to the angular velocities of the rotational motion. Hence, the employment of these translational and rotational diffusion coefficients for a cube enables the implementation of Brownian dynamics simulations for a suspension composed of cubic particles in order to analyse the dynamic characteristics of a cubic particle suspension. Highlights We have considered a flow problem around a cube in order to numerically clarify the characteristics of the translational and rotational friction or diffusion coefficients. In a slow flow field the motion of the cube need only to be characterised by two friction coefficients, i.e. the translational and rotational friction coefficients. In the case of a non-slow flow field, the coupling characteristics between the translational motion and the rotational motion are recognised. Employment of these diffusion coefficients enables the implementation of Brownian dynamics simulations for a suspension composed of cubic particles. [ABSTRACT FROM AUTHOR]

We have elucidated the feasibility of the multi-particle collision dynamics (MPCD) method as a technique for the simulation of a magnetic particle suspension, by addressing the dependence of the translational and rotational Brownian motion of the magnetic particles on the parameters that characterise a MPCD simulation. A reasonable level of activation of the Brownian motion is indispensable for simulating the aggregate structures of the magnetic particles because their internal structure is strongly dependent on particle Brownian motion. In the present study, we have employed a diffuse reflection model for treating the interaction between the fluid and the magnetic particles. The diffuse reflection model gives rise to the tendency that the translational Brownian motion of the magnetic particles is more significantly activated than the rotational Brownian motion. If a scaling coefficient is introduced for modifying the interaction between the fluid and the magnetic particles, a more accurate solution may be obtained in regard to the aggregate structures of the magnetic particles. We may conclude that the MPCD method with the diffuse reflection model is a promising simulation technique for analysing the behaviour of magnetic particles in a suspension. [ABSTRACT FROM AUTHOR]

We have developed a simple assessment method for the overlap between spheroidal particles, which neither requires the complex manipulation of vectors and matrices that is indispensable in the ordinary methods, nor is based on a model potential. Moreover, we have developed an evaluation method for the interaction energy arising from the overlap of the steric layer coating spheroidal particles. This is based on a sphere-connected particle model, but some modifications are introduced in order to express an appropriate repulsive interaction energy at the deepest overlapping position. We have investigated the phase change in a magnetic spheroidal particle suspension for a two-dimensional system by means of Monte Carlo simulations. In the case of no external magnetic field, if the magnetic particle-particle interaction is sufficiently strong to favour cluster formation, long raft-like clusters tend to be formed in a dilute situation. With decreasing values of area fraction, a chain-like structure in a dense situation transforms into a raft-like structure within a narrow range of the particle area fraction. Similarly, the raft-like clusters are preferred in a weak applied magnetic field, but an increase in the field strength induces a phase change from a raft-like into a chain-like structure. Highlights of the present paper: A simple assessment method has been proposed for the overlap between two spheroidal particles. The particle overlap assessment is free from a complex mathematical manipulation regarding vectors and matrices. A modified sphere-connected model has been proposed in order to more accurately evaluate a repulsive interaction due to the overlap of the steric layers coating spheroidal particles. 2D Monte Carlo simulations have been performed to elucidate the phenomenon of a phase change by magnetic spheroidal particles on a material plane surface. A phase change between a raft-like and a chain-like aggregate structure is able to be controlled by the area fraction of particles and an external magnetic field. [ABSTRACT FROM AUTHOR]

A strong microjet is generated in an electro-conjugate fluid between a positive and a ground electrode in a high external electric field. We here elucidate the mechanism and characteristics of this strong microjet by means of lattice Boltzmann simulations. In this analysis, we assume that charges are injected from the surface of a positive electrode at the position of a locally high concentrated electric field. When the effect of the Coulomb force is much more dominant than the viscous force, a strovnng microjet between the positive and the ground electrode is generated. This microjet attains a maximum velocity, collides with the ground electrode and disperses in oblique directions. The flow rate resulting from the appearance of a microjet increases with increasing electric field strength in a proportional manner. The present numerical results in respect to the visualisation of a microjet are in good agreement with a corresponding experiment. [ABSTRACT FROM AUTHOR]

We have investigated the aggregate structure of a suspension composed of magnetic particles with a cubic geometry by means of Monte Carlo simulations. From the viewpoint of application to the technology of surface modification, we have considered a quasi-two-dimensional suspension in thermodynamic equilibrium. As the magnetic interaction strength is increased, the effects of the thermal energy are reduced and the particles tend to aggregate together. These aggregates of cubic particles are not chain-like, but are designated as closely packed clusters. An applied magnetic field tends to enhance the formation of clusters along the field direction but does not significantly regularise the internal structure of the cluster. This is mainly due to the preference of a face-to-face contact configuration for the alignment of particles with cubic geometry. The regime of the internal structure of aggregates has a significant effect on the characteristics of the alignment of the magnetic moments with regard to the external magnetic field direction. Our simulations indicate that larger closely packed clusters are formed with increasing volumetric fraction, whereas the internal structure of the closely packed clusters is not found to be significantly influenced by the change in the volumetric fraction. [ABSTRACT FROM PUBLISHER]

We have investigated the behaviour of a suspension of magnetic rod-like hematite particles in a simple shear flow with the addition of an applied magnetic field. A significant feature of the present hematite particle suspension is the fact that the magnetic moment of the hematite particle lies normal to the particle-axis direction. From simulations, we have attempted to clarify the dependence of the negative magneto-rheological effect on the particle aggregation and orientational distribution of particles. The present Brownian dynamics method has a significant advantage in that it takes into account the spin rotational Brownian motion about the particle axis in addition to the ordinary translational and rotational Brownian motion. The net viscosity is decomposed into three components and discussed at a deeper level and in detail: these three viscosity components arise from (1) the torque due to the magnetic particle–field interaction, (2) the torque and (3) the force due to the interaction between particles. It is found that a slight change in the orientational distribution has a significant influence on the negative magneto-rheological effect. In a relatively dense suspension, the viscosity components arising from an applied magnetic field and the interaction between particles come to change rapidly for a certain strength of the magnetic particle–particle interaction, which is due to the onset of the formation of raft-like clusters. [ABSTRACT FROM PUBLISHER]

We have investigated aggregation phenomena in a suspension composed of rod-like haematite particles by means of Brownian dynamics simulations. The magnetic moment of the haematite particles lies normal to the particle axis direction and therefore the present Brownian dynamics method takes into account the spin rotational Brownian motion about the particle axis. We have investigated the influence of the magnetic particle–field and particle–particle interactions, the shear rate and the volumetric fraction of particles on the particle aggregation phenomena. Snapshots of aggregate structures are used for a qualitative discussion and the cluster size distribution, radial distribution function and the orientational correlation functions of the direction of particle axis and magnetic moment are the focus for a quantitative discussion. The significant formation of raft-like clusters is found to occur at a magnetic particle–particle interaction strength much larger than that required for a magnetic spherical particle suspension. This is because the rotational Brownian motion has a significant influence on the formation of clusters in a suspension of rod-like particles with a large aspect ratio. An applied magnetic field enhances the formation of raft-like clusters. A shear flow does not have a significant influence on the internal structure of the clusters, but influences the cluster size distribution of the raft-like clusters. [ABSTRACT FROM AUTHOR]

We have investigated various problems that arise in applying the Brownian dynamics method to a suspension composed of rod-like hematite particles, which have a magnetic moment normal to the particle axis direction. The accuracy and the deviation of simulation results from theoretical solutions have been discussed by comparison with the theoretical solutions that were obtained by solving the basic equations of the orientational distribution function. The characteristics of the negative viscosity are not observed to be dependent on a time interval unless a sufficiently short time interval is used. The present simulation results can satisfactorily reproduce the qualitative characteristics of the negative magneto-rheological effect that was predicted by the previous theoretical investigation. Good quantitative agreement is obtained in the range of small-applied magnetic fields, but agreement is not significantly good in the large magnetic field region. The deviation of the negative viscosity from the theoretical prediction cannot be improved by using a more accurate numerical algorithm such as moving from Euler to second-order or fourth-order Runge–Kutta. The results of the orientational distribution can well reproduce the characteristic features that the distribution has a gradual shape with low linear-like peak, which is in significant contrast to the sharp single-peak distribution of a ferromagnetic rod-like particle dispersion. The present orientational distributions are in significantly good agreement with those of the theoretical prediction in regard to the position and the height of a peak and the general shape of the overall profile. Good agreement of the present magnetisation curves with the theoretical prediction verifies that the spin rotational Brownian motion is activated at a physically reasonable level in the present simulations. [ABSTRACT FROM AUTHOR]

We have shown the basic equation of the orientational distribution function of prolate spheroidal haematite particles with rotational Brownian motion in a simple shear flow under an applied magnetic field. The equation has been solved numerically in order to investigate the influence of the spin Brownian motion on the orientational distribution, the negative magneto-rheological effect and the characteristics of magnetisation. With respect to the orientational distribution, the present results are in good agreement with those of the theory without that motion, which shows that the spin Brownian motion does not significantly influence the orientational distribution. In contrast, the influence of the spin Brownian motion appears more significantly in the negative viscosity: the negative viscosity effect decreases in comparison with that of the previous theory without the spin Brownian motion. Moreover, a more significant negative magneto-rheological effect is obtained for a larger particle aspect ratio. Since the magnetisation has a strong relationship with the orientation of the magnetic moment, the effect of the spin Brownian motion appears more significantly in this characteristic. [ABSTRACT FROM AUTHOR]

We have developed the basic equation of the orientational distribution function of oblate spheroidal hematite particles with rotational Brownian motion in a simple shear flow under an applied magnetic field. An oblate spheroidal hematite particle has an important characteristic in that it is magnetized in a direction normal to the particle axis. Since a dilute dispersion is addressed in the present study, we have taken into account only the friction force (torque) whilst neglecting the hydrodynamic interactions among the particles. This basic equation has been solved numerically in order that we may investigate the dependence of the orientational distribution on the magnetic field strength, shear rate and rotational Brownian motion and the relationship between the orientational distribution and the transport coefficients such as viscosity and diffusion coefficient. We found that if the effect of the magnetic field is more dominant, the particle inclines in such a way that the oblate surface aligns in the magnetic field direction. If the Peclet number increases and the effect of the shear flow becomes more dominant, the particle inclines such that the oblate surface tilts in the shear flow direction. The viscosity due to the magnetic torque is shown to increase as the magnetic field increases, since the magnetic torque due to the applied magnetic field becomes the more dominant effect. Moreover, the viscosity increase is shown to be more significant for a larger aspect ratio or for a more oblate hematite particle. We have applied the analysis to the problem of particle sedimentation under gravity in the presence of a magnetic field applied in the sedimentation direction. The particles are found to sediment with the oblate surface aligning more significantly in the sedimentation direction as the applied magnetic field strength increases. [ABSTRACT FROM AUTHOR]

We have developed a lattice Boltzmann method based on fluctuation hydrodynamics that is applicable to the flow problem of a particle suspension. In this method, we have introduced the viscosity-modifying method, rather than the velocity-scaling method, in which a modified viscosity is used for generating random forces in lattice Boltzmann simulations. The viscosity-modifying method is found to be applicable to the simulation of a magnetic particle suspension. We have applied this method to the two-dimensional Poiseuille flow of a magnetic suspension between two parallel walls in order to investigate the behavior of magnetic particles in a non-uniform applied magnetic field. From the results of the snapshots, the pair correlation function between the magnetic pole and the magnetic particles and the averaged local particle velocity and magnetization distributions, it was observed that the behavior of the magnetic particles changes significantly depending upon which factor dominates the phenomenon in the balance between the magnetic particle–particle interaction, the non-uniform applied magnetic field and the translational and rotational Brownian motion. [ABSTRACT FROM AUTHOR]

We have experimentally investigated the negative magneto-rheological effect that is theoretically predicted to appear in a dispersion composed of spindle-like hematite (α-Fe2O3) particles. The spindle-like hematite particles were synthesized by aging a solution of FeCl3 and KH2PO4 for 72 h at 373 K. The particle size distribution was determined by digital image analysis from electronic microscope observation of the particles. In the present study we considered a glycerol–water-based dispersion in order to clarify the influence of the shear rate and the magnetic field strength on the negative magneto-rheological effect. Measurement of the viscosity was carried out using a rotational-type rheometer in an external magnetic field generated by Helmholtz coils. The main results obtained can be summarized as follows. The viscosity of a hematite/glycerol–water dispersion relative to that with no applied magnetic field decreases with increasing magnetic field strength: that is, we observed the negative viscosity effect that has previously been predicted from theoretical considerations. Moreover, the negative magneto-rheological effect is observed to decrease with increasing shear rate, which also agrees qualitatively with the theoretical prediction. [ABSTRACT FROM PUBLISHER]

Objectives: The characteristics of serum catecholamine concentration at the hyper-acute phase of aneurysmal subarachnoid hemorrhage (SAH) and its relationship between patient outcome and delayed vasospasm were investigated. Methods: Patients with aneurysmal SAH (170) were prospectively studied between August 2008 and June 2011. Baseline demographic data and physiological parameters, including plasma concentrations of adrenaline (AD), noradrenaline (NA), and dopamine (DP) were evaluated for all patients. Results: On admission, plasma AD, NA, and DP levels were significantly higher in patients with a poor clinical grade on admission (Hunt and Kosnik grade: IV-V), compared to those with a good clinical grade on admission (Hunt and Kosnik grade: I-III). AD showed a markedly high concentration immediately after the onset of SAH and then rapidly decreased. NA levels peaked within 6 hours after onset, then significantly decreased. The increase of DP with time was not significant, but showed a similar trend to that of NA. The level of each catecholamine showed significant mutual correlation. Our multivariate model demonstrated that age, poor clinical grade at admission, plasma AD and NA levels were good predictors of poor patient outcome [receiver operating characteristic (ROC) area: 0.83]. And that poor clinical grade at admission, Fisher scale, blood sugar level and plasma AD level were good predictors of the development of delayed vasospasm (ROC area: 0.81) (1.3). Conclusions: The present findings suggest that sympathetic activation in patients in the acute phase of SAH reflects the severity of SAH, and is closely related to the development of delayed vasospasm, leading to the subsequent immune response and inflammatory reactions. Strategies for suppressing catecholamine at the hyper-acute phase may contribute to vasospasm prevention and improve patient outcome. [ABSTRACT FROM AUTHOR]

We discuss the feasibility of several methods for activating the Brownian motion of dispersed particles in the lattice Boltzmann method by considering aggregation phenomena in a dispersion composed of magnetic particles. The methods treated in the present study are a method based on fluctuation hydrodynamics (HF-type), in which the fluctuation term is combined with the basic equations of the usual lattice Boltzmann equations, and a method based on Brownian dynamics (hybrid-type), in which the random forces of the Brownian dynamics are simply added to the equations of motion of magnetic particles. The validity of the present results is discussed by a comparison with the results of Monte Carlo simulations. The HF-type method gives rise to a relatively emphasized activation of the Brownian motion in contrast to the hybrid-type method, which yields a weaker activation of such motion. In order to improve these Brownian-motion-activating techniques, the velocity-scaling method was introduced into both techniques. The modified HF-type method shows significantly good agreement with Monte Carlo results, implying that the translational and rotational Brownian motion of magnetic particles is activated with sufficient accuracy. In contrast, the implementation of the hybrid-type method is found to be unable to reproduce aggregate structures within the bounds that may be considered physically reasonable. [ABSTRACT FROM PUBLISHER]

The phase transitions and the internal aggregate structures of a highly dense suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis have been investigated by means of the Monte Carlo method. The present study considered a quasi-2D system in order to clarify the influences of the volumetric fraction of particles and the magnetic field strength on particle aggregations and phase transitions. The internal structures of particle aggregates have been discussed quantitatively in terms of pair correlation functions, orientational pair correlation functions, nematic and polar order parameters. The main results obtained here are summarized as follows. When the influence of the magnetic interaction between particles is of the same order of that of the perpendicular magnetic field strength, the particles form column-like clusters, and the internal structure of the suspension shows solid-like structures. For the case of a strong applied magnetic field, the internal structure is transformed from solid-like structures into isotropic ones. However, as the volumetric fraction increases, the particles form brick wall-like structures under the situation of a strong applied magnetic field, and the internal structure exhibits solid-like ones. The brick wall-like structures also appear for a relatively weak magnetic field applied along the in-plane direction despite a slightly smaller volumetric fraction compared with the case of the perpendicular applied magnetic field. [ABSTRACT FROM AUTHOR]

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modelled as disk-like particles with magnetic moment normal to the particle axis at the particle centre, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as the development of surface quality changing technology using such magnetic plate-like particles. A magnetic field is applied along the direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates are discussed quantitatively in terms of radial distribution and orientational pair correlation functions. For the case of strong magnetic interactions between particles, particles form long column-like clusters with their magnetic moments alternating in direction between the neighbouring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, particles incline towards the magnetic field direction, so that particles do not form such clusters. [ABSTRACT FROM AUTHOR]

We have developed the modified periodic-shell boundary condition (BC) for dissipative particle dynamics (DPD) simulations, which enables us to simulate an outer flow problem around an obstacle using a small simulation region. In order to clarify the validity of this BC, we have treated a uniform flow past a circular cylinder. The present BC has been compared with the ordinary BC such as the uniform flow condition. Also, the present results have been compared with those of the numerical results of the Navier-Stokes equation. The ordinary uniform BC is seen to give rise to significantly distorted flow fields and also to significant disappearance of dissipative particles from the simulation region. In contrast, for the present modified periodic-shell BC, the number density of dissipative particles is kept almost constant during a simulation run, and the flow field is in reasonable agreement with the result, which has been obtained by numerical simulations of the Navier-Stokes equation. [ABSTRACT FROM AUTHOR]

The negative viscosity of a colloidal dispersion composed of ferromagnetic rod-like particles, which have a magnetic moment normal to the particle axis, have been investigated. A simple shear flow problem has been treated to clarify the particle orientational distribution and rheological properties of such a semi-dense dispersion, under circumstances of an external magnetic field applied in the direction normal to the shear plane of a simple shear flow. The results obtained here are summarized as follows. For the cases of a very strong magnetic field and magnetic interactions between particles, the magnetic moment of the rod-like particles is significantly restricted in the magnetic field direction, so that the particle approximately aligns in the shear flow direction. Also, the particle can easily rotate around the axis of the cluster almost freely even in a simple shear flow. Characteristic orientational properties of the particle cause negative viscosity, as in the previous study for a dilute dispersion. However, magnetic particle-particle interactions have a function to make such negative viscosity decrease. [ABSTRACT FROM AUTHOR]

The validity of the application of the dissipative particle dynamics (DPD) method to ferromagnetic colloidal dispersions has been investigated by conducting DPD simulations for a two-dimensional system. First, the interaction between dissipative and magnetic particles has been idealized as some model potentials, and DPD simulations have been carried out using such model potentials for a two magnetic particle system. In these simulations, attention has been focused on the collision time for the two particles approaching each other and touching from an initially separated position, and such collision time has been evaluated for various cases of mass and diameter of dissipative particles and model parameters, which are included in defining the equation of motion of dissipative particles. Next, a multi-particle system of magnetic particles has been treated, and particle aggregates have been evaluated, together with the pair correlation function along an applied magnetic field direction. Such characteristics of aggregate structures have been compared with the results of Monte Carlo and Brownian dynamics simulations in order to clarify the validity of the application of the DPD method to particle dispersion systems. The present simulation results have clearly shown that DPD simulations with the model interaction potential presented here give rise to physically reasonable aggregate structures under circumstances of strong magnetic particle-particle interactions as well as a strong external magnetic field, since these aggregate structures are in good agreement with those of Monte Carlo and Brownian dynamics simulations. [ABSTRACT FROM AUTHOR]

Surfaces roughened by grit blasting influence the adhesion strength of plasma-sprayed ceramic coatings. The average surface roughness has been used to evaluate the surface topography of such surfaces. It is well known that the adhesion strength of ceramic coatings reaches a maximum value at a certain substrate surface roughness. However, this result cannot be understood based on only surface roughness. The blasted surface has fractal characteristics. There are two types of fractal surfaces, which are characterized by self-similarity and self-affinity. Using fractal analysis to evaluate the surface topography of substrates, the fractal dimension was measured for the roughened surfaces. The maximum fractal dimension was attained at a blasting angle of 75°, where the adhesion strength also reached approximately its maximum value. It is concluded that the fractal dimension is a more appropriate measure than the average surface roughness for evaluation of the adhesion strength of ceramic coatings. [ABSTRACT FROM AUTHOR]

The periodic-shell boundary condition, developed as an outer boundary condition for molecular dynamics simulations, is applied to soft molecular systems, using a molecular dynamics simulation of the flow of a Lennard-Jones liquid past a two-dimensional circular cylinder. The usual periodic boundary condition is found to give rise to a significantly distorted velocity field. For example, the fluid in the side areas of the cylinder near the boundary surfaces flows along the boundary surfaces, i.e., in the uniform flow direction. The velocities in the rear area near the boundary surface are much larger than the results of the periodic-shell boundary condition and the Navier-Stokes equations. Also the pair of vortices behind the cylinder is significantly compressed in the direction of the cylindrical centre. In contrast to these findings, the results of the periodic-shell boundary condition agree well with the Navier-Stokes result qualitatively; good agreement is obtained concerning the formation of a pair of vortices behind the cylinder and the flow fields in the side region of the cylinder. These results show clearly that the periodic-shell boundary condition is very useful for soft molecular systems as well as for rigid molecular systems. [ABSTRACT FROM AUTHOR]