Disk-like Particles Research Articles

Quasi-2D Monte Carlo simulations of a colloidal dispersion composed of magnetic plate-like particles with magnetic moment normal to the particle axis

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.

Complex dielectric response of ellipsoidal particles with surface conduction

Both particle shape and surface phenomena significantly affect the effective complex dielectric properties of colloidal systems. The treatment of particle shape has generally relied on the extrapolation from the solution of the spherical case proposed by O'Konski [J. Chem. Phys. 64, 605 (1960)] that treats ellipsoidal particles possessing surface conductivity as equivalent homogeneous anisotropic ellipsoids with bulk conduction. To test this approach, we have performed a rigorous analysis of the complex dielectric response of an ellipsoidal particle with surface conductivity using the generalization of the O'Konski boundary conditions to an ellipsoidal shape. The resulting closed-form solution obtained shows that surface conduction effects are represented by an equivalent inhomogeneous anisotropic ellipsoid. For the case of a spheroidal particle, the principle axes of the effective dielectric permittivity tensor of the equivalent particle are aligned with its geometrical principal axes; the effective permittivity varies in the direction of the unique spheroidal axis. In addition, numerical results indicate that the product of the surface area to volume ratio and the specific surface conductivity completely characterizes the effect of the surface phenomena on the response of spheroidal particles with a given shape. Numerical simulations show that spherical and prolate spheroidal particles exhibit a progressive dielectric enhancement while more disklike oblate spheroidal particles undergo an initial dielectric suppression followed by a subsequent enhancement with increasing surface conduction. A comparison of our model predictions with those obtained using the O'Konski approximation revealed significant differences in the magnitude of the low-frequency dielectric enhancement and relaxation frequency for ellipsoidal particle suspensions.

磁性扁平粒子からなるコロイド分散系の擬2次元モンテカルロ・シミュレーション(流体工学,流体機械)

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo method. Such plate-like particles have been modeled as disk-like particles with a magnetic moment normal to the particle axis at the particle center, with the side 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 development of surface changing technology using such magnetic plate-like particles. Internal structures of particle aggregates have been discussed quantitatively in terms of radial distribution and orientational pair correlation functions. The main results obtained here are summarised as follows. For the case of strong magnetic interactions between particles, particles form long cookie-like clusters and alternate between magnetic moments. This tendency appears under circumstances of a weak applied magnetic field. However, as the magnetic field strength becomes significant, particle inclines toward the magnetic field direction, so that the particles do not form the clusters.

Ordered Packing of Soft Discoidal System

A novel mesoscopic simulation method is adopted to study the ordered packing of the anisotropic disklike particles with a soft repulsive interaction, which possesses a modified anisotropic conservative force type used in dissipative particle dynamics. We examine the influence of the shape of the particles, the angular width of the repulsion, and the strength of the repulsion on the packing structures. Specifically, an ordered hexagonal columnar structure is obtained in our simulations. Our study demonstrates that an anisotropic repulsive potential between soft discoidal particles is sufficient to produce a relatively ordered hexagonal columnar structure.

Oriented Vaterite CaCO3Tablet-Like Arrays Mineralized at Air/Water Interface through Cooperative Regulation of Polypeptide and Double Hydrophilic Block Copolymer

Highly oriented vaterite CaCO 3 tablet-like arrays were formed at the air/water interface through the cooperative mineralization regulated by polypeptide and double hydrophilic block copolymer under ambient conditions. The nearly parallel arrangement of CaCO 3 vaterite tablets at the air/water interface shows the remarkable resemblance to the morphology of nacreous layers. The poly(aspartic acid) (PASP) with high molecular weight (M w = 11 000) and low solubility in water promoted the formation of vaterite tablets at the air/water interface, and stabilized the vaterite tablets by aggregation at the air/water interface and adsorption at the surface of vaterite tablets. Meanwhile, the highly hydrophilic poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) played an important role in regulating the arrangement and orientation of vaterite CaCO 3 tablets, leading to the oriented tablet-like arrays at the air/water interface. Detailed experiments revealed that hydrophilic PEG-b-PMAA alone did not produce any form of CaCO 3 crystals at the air/water interface, but could induce the formation of calcite CaCO 3 particles in the water phase. However, high molecular weight PASP alone led to the formation of disk-like vaterite particles composed of helically aggregated nanoplates at the air/water interface, suggesting the regulating role of PEG-b-PMAA in the growth of vaterite tablet arrays. These results reported here provide a better understanding of the growth mechanism of nacreous layers and shells in nature.

Dispersions of Internally Liquid Crystalline Systems Stabilized by Charged Disklike Particles as Pickering Emulsions: Basic Properties and Time-Resolved Behavior

The present paper reports on dispersions of internally liquid crystalline particles, formed from monoglyceride and oil mixtures, stabilized with discrete disklike particles of Laponite clay. Small-angle X-ray scattering (SAXS) was used to probe the presence of dispersed particles as well as their internal liquid crystalline structure. The data were compared to scattering results of reference systems, namely, from the bulk as well as from well-defined particles formed with a polymer as the emulsifier. The submicrometer sizes of the various particles could be derived using dynamic light scattering (DLS). The possible mechanisms involved in the stabilization of each of the different phases by the Laponite platelets, including the role of the residual salt, are discussed. Time-resolved experiments were performed over 60 days in order to follow the evolution of both the internal structure and size of the particles. In particular, we discuss the peculiar behavior of the sample without added oil, where the cubosomes transform into hexosomes over time. The effect of the high pH induced by the Laponite platelets in water, which could result in a hydrolysis of the monoglycerides, was shown to be responsible for the observed cubosome-to-hexasome transition, as well as for the decrease in the lattice parameters.

Distributed 3D Aspect Ratios of Toner

Two-dimensional shape analysis of toner may be extended to three dimensions by combining shape information from typical image analysis of oriented toner with information from instruments that measure toner volume such as the Coulter Multisizer. A mean aspect ratio for the hidden dimension perpendicular to the plane of the image can be estimated by assuming a model shape such as an ellipsoid. Furthermore, if the shape is independent of the size, a relationship between the aspect ratio of the hidden dimension and the aspect ratio of the visible dimensions may be developed. Shape is independent of size when brittle fracture dominates the grinding process in melt-pulverized toners (MPT) and is independent by design in the limited coalescence manufacturing method used to make chemically prepared toner (CPT). The relationship between the different aspect ratios is useful to distinguish toners that have flat disk-like particles from toners with spherical particles within a broader distribution of shapes that result in different performance in the electrophotographic process.

Aqueous foams stabilized by Laponite and CTAB

Abstract Aqueous foams prepared by cetyltrimethylammonium bromide (CTAB) and disk-like Laponite particles dispersions are studied. The particle/CTAB dispersions have a synergistic effect on foam stability at intermediate CTAB concentrations. The foam stability increases with increasing particle concentration. At a fixed particle concentration, the foam stability first increases with increasing CTAB concentration and reaches a maximum at a CTAB concentration around 1.7 cec. After that, the foam stability decreases and then remains unchanged. The effects of hydrophobicity and electrical property of modified Laponite particles on foam stability are discussed. The hydrophobicity of Laponite particles is predominant in stability of foams compared with the electrical property of particles. The most stable foams were obtained by particles with the maximum hydrophobicity (the contact angle is still smaller than 90°). Two mechanisms promote foam stability: (a) adsorption of modified particles on the bubble surface; (b) formation of an aggregate structure by the armored bubbles and particles in the bulk phase, which is confirmed by laser-induced fluorescent confocal microscopy.

Soft chemistry synthesis of the perovskite CaCu3Ti4O12

The perovskite CaCu3Ti4O12 (CCT) has been obtained after calcination of oxalate precursors at 900–1000 °C in air. Those precursors are prepared using a soft chemistry method, the coprecipitation. The oxalate powders consist of disk-like particles of 2–3 μm diameter and 300–400 nm thickness. By varying the ratio of the initial amounts of metal chlorides, additional phases (CaTiO3, TiO2 and CuO) could be obtained besides CCT. The corresponding multiphased ceramics present improved dielectric properties.

Helix Self-Assembly from Anisotropic Molecules

We explore the potential energy landscape for clusters composed of disklike ellipsoidal particles interacting via an anisotropic potential based on the elliptic contact function. Over a wide range of parameter space we find global potential energy minima consisting of helices composed of one or more strands. Characterizing the potential energy surface in the region of helical global minima reveals a topology associated with "structure-seeking" systems. This result indicates that the helices will self-assemble over a wide range of temperature.

Free-surface granular flows down heaps

The present paper extends the granular-flow constitutive model of Savage (1998 J Fluid Mech 377:1–26) to treat spherical particles. Savage accounted for both quasi-static and collisional stresses by considering: (i) strain-rate fluctuations embodied in a critical state plasticity model, as well as, (ii) individual particle velocity fluctuations modelled by granular-flow kinetic theory. In the present work, the governing equations of the kinetic theory of Jenkins (1998 In: Hermann HJ, Luding S (eds) Physics of Dry Granular Media. Kluwer Academic pp. 353–370) for identical spherical, smooth, inelastic particles are supplemented with additional quasi-static terms that have forms patterned after the corresponding terms in the equations of Savage for two-dimensional disk-like particles. The resulting equations along with side-wall and free-surface boundary conditions are applied to examine free-surface granular flow down a heap contained between two frictional vertical side walls. Width-averaged equations of motion are integrated to obtain depth profiles of mean velocity, granular temperature, solids fraction and the Savage–Jeffrey parameter. Detailed comparisons are made with particle-tracking experiments. When the gap between the vertical side walls is fairly narrow, good agreement is found between the predicted and the measured profiles of mean velocity and granular temperature.

Migration and orientation of disk-like particles in microcomposite processing

The objective of this study is to well understand the migration and orientation of thin micro-particles, such as talc and mica, in a suspension flow by means of both experiments and numerical simulations as well as to obtain the knowledge of the processing operations of thin micro-particle reinforced composites. A slit channel was mounted on the sidewall of a large reservoir in the experimental setup. For this reason, the thin disk-like particles were subjected by a planar extensional flow in a reservoir, then by a simple shear flow through a slit channel. The evolution of the orientation of thin disk-like particles was studied in both a planar extensional and simple shear flows by numerical calculation of the Jeffery equation: thin disk-like particles aligned in the parallel orientation to the upper- and lower-walls of the slit channel in a planar extensional flow through the reservoir, then entered into the inlet of the slit channel. On the other hand, in a simple shear flow through the slit channel, the disk-like particles kept this parallel orientation except the occurrence of flip-over. The period of the flip-over became longer with a decrease in aspect ratio (thickness/diameter) of the disk-like particles. Furthermore, the measurements of the orientation of the talc particles in a suspension flow through the slit channel clearly showed that almost the same period of the flip-over was found in spite of different particle size. The experimental result arises from complex geometries and no accurate data of the thickness of talc particles.

Rayleigh-Gans scattering approximation: surprisingly useful for understanding backscattering from disk-like particles

Recent computations of the backscattering cross section of randomly-oriented disk-like particles (refractive index, 1.20) with small-scale internal structure, using the discrete-dipole approximation (DDA), have been repeated using the Rayleigh-Gans approximation (RGA). As long as the thickness of the disks is approximately 20% of the wavelength (or less), the RGA agrees reasonably well quantitatively with the DDA. The comparisons show that the RGA is sufficiently accurate to be useful as a quantitative tool for exploring the backscattering features of disk-like particles with complex structure. It is used here to develop a zeroth-order correction for the neglect of birefringence on modeling the backscattering of detached coccoliths from E. huxleyi.

Backscattering of light from disk-like particles with aperiodic angular fine structure

Recent computations of the backscattering cross section (sigmab) of randomly-oriented disk-like particles (refractive index, 1.20) with small-scale periodic angular internal structure, have been repeated for similarly sized particles, but with the periodic structure replaced by an aperiodic structure. The latter is formed by randomly perturbing a periodic structure. Although sigmab for individual realizations of an aperiodic disk can differ significantly from that of its periodic counterpart, averaging over several realizations brings the two into confluence, unless the aperiodicity is too large. These computations suggest that using disks with perfectly periodic (as opposed to quasi-periodic) fine structure for modeling the backscattering of detached coccoliths from E. Huxleyi is justified.

Backscattering of light from disklike particles: is fine-scale structure or gross morphology more important?

The backscattering of light from disklike objects possessing periodic structures (e.g., resembling a wheel with spokes, hereafter called a pinwheel) or an object with a wavelength-sized deviation from a flat disk (e.g., a spherical cap) has been computed by using the discrete dipole approximation. The disks ranged in diameter from 1.5 to 2.7 microm with thicknesses from 0.04 to 0.15 microm. The goal of the study was to obtain some understanding of the differences between the backscattering of a collection of such objects in random orientation and a collection of randomly oriented homogeneous disks of the same size, i.e., the conditions under which the gross morphology (e.g., disklikeness) of these objects determines their backscattering. The computations for pinwheels showed that their backscattering cross sections were nearly identical to those of homogeneous disks of similar size (but with reduced effective refractive indices that are easily estimated) as long as the maximum separations between the spokes was less than one quarter of the wavelength. In this regime the backscattering is totally governed by the particle's gross morphology and effective index. For larger spoke separation, departures from a homogeneous disk are observed and manifest as significant increases (many times) in backscattering. In the case of spherical caps with the same projected area as the associated disk, the computations again show a complete similarity in their backscattering, and when the disks are sufficiently thin (with thickness divided by wavelength<0.15 to 0.25) there is very little difference between the backscattering of a cap and the associated disk.

Predicting the Phase Diagram of Two-Dimensional Colloidal Systems with Long-Range Interactions

The phase diagram of a two-dimensional model system for colloidal particles at the air-water interface was determined using Monte Carlo computer simulations in the isothermic-isobaric ensemble. The micrometer-range binary colloidal interaction has been modeled by hard disklike particles interacting via a secondary minimum followed by a weaker longer-range repulsive maximum, both of the order of kBT. The repulsive part of the potential drives the clustering of particles at low densities and low temperatures. Pinned voids are formed at higher densities and intermediate values of the surface pressure. The analysis of isotherms, translational and orientational correlation functions as well as structure factor gives clear evidence of the presence of a melting first-order transition. However, the melting process can be also followed by a metastable route through a hexatic phase at low surface pressures and low temperatures, before crystalization occurs at higher surface pressure.

Effect of electromagnetic interactions on plasmon excitations in silver particle ensembles

The optical properties of silver nano-particle ensembles are studied as a function of particle density by analyzing the light emission excited via electron injection from an STM tip. The particles are prepared with distinct dome and disk-like shapes on a Al 2O 3/NiAl(1 1 0) support. The particle density is varied over one order of magnitude by changing the Ag deposition temperature and the defect concentration in the oxide surface. With increasing density, a pronounced blue shift of the Ag plasmon mode is observed for ensembles of dome-like particles, whereas disk-like particles show relatively constant resonance energies. The observed evolution of plasmon energy as a function of particle density reflects the influence of electromagnetic interactions in the ensemble, as verified by model calculations.

Effect of pH and Lattice Distortion on the Luminescence of ( Y , Gd ) BO3 : Eu3 + Phosphor Prepared by the Coprecipitation Method

phosphor was produced by a coprecipitation method. The morphology of the particles could be controlled by adjustment of the pH value and the heat-treatment method used. The pH value of the coprecipitation solution was a critical parameter for the luminescent properties of the phosphor. The improved chromaticity in smaller or disk-like particles was attributed to higher lattice distortion, which gives rise to the loss of the inversion symmetry at the site of the ion, thus enhancing the transition.

State of the Art on Flow Kinematics and Particle Orientations during Composite Processing

The mechanism of orientation of fibers or thin micro-particles in various flows involving the processing of composite materials has not been fully understood although it is much significant to obtain the knowledge of the processing operations of particle reinforced composites as well as to improve the properties of the advanced composites. The objective of this article is to introduce and well understand the evolution of the particle orientation in a suspension flow and flow kinematics induced by suspended particles by means of both literature review and the following our two research work: Numerical simulation of flow kinematics and fiber orientation for fiber suspension: Orientation of disk-like particles in a micro-composite processing.

Effective interaction of charged platelets in aqueous solution: Investigations of colloid laponite suspensions by static light scattering and small-angle x-ray scattering

We study dilute aqueous solutions of charged disklike mineral particles (laponite) by a combination of static light scattering (SLS) and small-angle x-ray scattering (SAXS). Laponite solutions are known to form gels above a certain critical concentration that must be described as nonequilibrium states. Here we focus on the investigation by SLS and SAXS at concentrations below gelation (c<0.016 g/L) and at low concentrations of added salt (0.001M and 0.005M). Thus, we have obtained the scattering function of single Laponite platelets as well as the structure factor describing their interaction at finite concentration. A detailed analysis of the combined sets of data proves that the solutions are in a well-defined equilibrium state. Moreover, this analysis demonstrates the internal consistency and accuracy of the scattering functions obtained at finite concentrations. We find that laponite particles interact through an effective pair potential that is attractive on short range but repulsive on longer range. This finding demonstrates that Laponite solutions exhibit only a limited stability at the concentration of added salt used herein. Raising the ionic strength to 0.005M already leads to slow flocculation as is evidenced from the enhanced scattering intensity at smallest scattering angles. All data strongly suggest that the gelation occurring at higher concentration is related to aggregation.