Disk-like Particles Research Articles

Liquid Crystal Phases in Suspensions of Charged Plate-Like Particles.

Anisotropic interactions in colloidal suspensions have recently emerged as a route for the design of new soft materials. Nonisotropic particles can form nematic, smectic, hexatic, and columnar liquid crystals. Although the formation of these phases is well rationalized when excluded volume is solely at play, the role of electrostatic interactions still remains unclear and even less so when particles present a charge heterogeneity, for example, clays. Here, we use Monte Carlo simulations of concentrated suspensions of charged disk-like particles to reveal the role of Coulomb interactions and charge anisotropy underlying liquid crystal formation and structures. We observe a vast zoo of exotic structures, going from hexatic to columnar phases, which are shown to be controlled by the charge anisotropy. The particle volume fraction at which these phases start to form is found to decrease with increasing Coulomb interactions and charge anisotropy, which suggests a route to tune the structure of aqueous liquid crystals.

Biochemical characterization and structural modeling of human cathepsin E variant 2 in comparison to the wild-type protein.

Cathepsin E splice variant 2 appears in a number of gastric carcinomas. Here we report detecting this variant in HeLa cells using polyclonal antibodies and biotinylated inhibitor pepstatin A. An overexpression of GFP fusion proteins of cathepsin E and its splice variant within HEK-293T cells was performed to show their localization. Their distribution under a fluorescence microscope showed that they are colocalized. We also expressed variants 1 and 2 of cathepsins E, with propeptide and without it, in Escherichia coli. After refolding from the inclusion bodies, the enzymatic activity and circular dichroism spectra of the splice variant 2 were compared to those of the wild-type mature active cathepsins E. While full-length cathepsin E variant 1 is activated at acid pH, the splice variant remains inactive. In contrast to the active cathepsin E, the splice variant 2 predominantly assumes β-sheet structure, prone to oligomerization, at least under in vitro conditions, as shown by atomic force microscopy as shallow disk-like particles. A comparative structure model of splice variant 2 was computed based on its alignment to the known structure of cathepsin E intermediate (Protein Data Bank code 1TZS) and used to rationalize its conformational properties and loss of activity.

How clay colloids surround internally self-assembled phytantriol drops.

The stabilisation of emulsified microemulsions using different concentrations of colloidal disk-like particles (Laponite) is investigated. The resulting structures are characterised by cryo-TEM, small-angle and very small-angle neutron scattering methods. We show that the Laponite colloids are effectively attached onto the droplet interfaces, creating a protective layer around them, dense enough to be statistically observed by means of neutron scattering, although the mean coverage remains still rather low. The mean size of the internally organised droplets does not change with the colloidal concentration (up to 1 wt%), and a few free colloids are found in the continuous water solvent. However, the colloids are shown to be able to deform the soft interfaces of the microemulsion phase to create droplets that are not always spherical, but can have angled interfaces.

Hierarchical Self-Assembly of Soft Disklike Particles under Shear Flow

We develop a mesoscale nonequilibrium simulation model to study the effect of steady shear on the hierarchical self-assembly of soft disklike particles in dilute solutions. By properly tuning shear rates and solvent conditions, soft disklike particles can self-assemble into flexible threads and bundle-like structures along the flow direction. Shear flow facilitates the self-assembly of soft disklike particles into one-dimensional long threads along the flow direction; however, it suppresses the formation of flexible bundles from the threads while decreasing the solvent quality. The relatively well-defined bundle structures along the flow direction can only be obtained when the solvent condition becomes even worse. Our study elucidates how the solvent condition and shear rate can be utilized to control the shear-induced self-assembled structures, which would enable designed nanofabrication.

Estimation of Interdomain Flexibility of N-Terminus of Factor H Using Residual Dipolar Couplings

Characterization of segmental flexibility is needed to understand the biological mechanisms of the very large category of functionally diverse proteins, exemplified by the regulators of complement activation, that consist of numerous compact modules or domains linked by short, potentially flexible, sequences of amino acid residues. The use of NMR-derived residual dipolar couplings (RDCs), in magnetically aligned media, to evaluate interdomain motion is established but only for two-domain proteins. We focused on the three N-terminal domains (called CCPs or SCRs) of the important complement regulator, human factor H (i.e., FH1-3). These domains cooperate to facilitate cleavage of the key complement activation-specific protein fragment, C3b, forming iC3b that no longer participates in the complement cascade. We refined a three-dimensional solution structure of recombinant FH1-3 based on nuclear Overhauser effects and RDCs. We then employed a rudimentary series of RDC data sets, collected in media containing magnetically aligned bicelles (disklike particles formed from phospholipids) under three different conditions, to estimate interdomain motions. This circumvents a requirement of previous approaches for technically difficult collection of five independent RDC data sets. More than 80% of conformers of this predominantly extended three-domain molecule exhibit flexions of <40°. Such segmental flexibility (together with the local dynamics of the hypervariable loop within domain 3) could facilitate recognition of C3b via initial anchoring and eventual reorganization of modules to the conformation captured in the previously solved crystal structure of a C3b:FH1-4 complex.

Synthesis, characterization, UV and dielectric properties of hexagonal disklike ZnO particles embedded in polyimides

A series of novel ZnO/polyimide composite films with different ZnO contents was prepared through incorporation hexagonal disklike ZnO particles into poly(amic acid) of the pre polymer of the polyimide. The hexagonal disklike ZnO particles with a diameter of 300–500 nm were synthesized from zinc acetate and NaOH in water with citric acid. The prepared zinc oxide–polyimide composites were characterized for their structure, morphology, and thermal behavior employing Fourier transform infrared spectroscopy, scanning electron micrograph, X-ray diffraction and thermal analysis techniques. Thermal analyses show that the ZnO particles were successfully incorporated into the polymer matrix and these ZnO/polymer composites have a good thermal stability. Scanning electron microscopy studies indicate the ZnO particles were uniformly dispersed in the polymer and they remained at the original size (300–500 nm) before immobilization. All composite films with ZnO particle contents from 1 to 5 wt% show good transparency in the visible region and luminescent properties.

Physicochemical and mechanical properties of plasma cermet coatings containing the strengthening phase formed from carbon nanotubes

Plasma cermet Ni-Mo coatings containing 30 or 68 vol % carbon nanotubes as a strengthening phase are produced by the mechanical alloying method in a planetary mill. Dense coatings consisting of disklike particles and having a microhardness of 9.83 GPa are obtained by spraying powder containing 30 vol % carbon nanotubes. The coatings containing 68 vol % carbon nanotubes are less dense, and their microhardness is 3.03 GPa.

Connectivity percolation of polydisperse anisotropic nanofillers

We present a generalized connectedness percolation theory reduced to a compact form for a large class of anisotropic particle mixtures with variable degrees of connectivity. Even though allowing for an infinite number of components, we derive a compact yet exact expression for the mean cluster size of connected particles. We apply our theory to rodlike particles taken as a model for carbon nanotubes and find that the percolation threshold is sensitive to polydispersity in length, diameter, and the level of connectivity, which may explain large variations in the experimental values for the electrical percolation threshold in carbon-nanotube composites. The calculated connectedness percolation threshold depends only on a few moments of the full distribution function. If the distribution function factorizes, then the percolation threshold is raised by the presence of thicker rods, whereas it is lowered by any length polydispersity relative to the one with the same average length and diameter. We show that for a given average length, a length distribution that is strongly skewed to shorter lengths produces the lowest threshold relative to the equivalent monodisperse one. However, if the lengths and diameters of the particles are linearly correlated, polydispersity raises the percolation threshold and more so for a more skewed distribution toward smaller lengths. The effect of connectivity polydispersity is studied by considering nonadditive mixtures of conductive and insulating particles, and we present tentative predictions for the percolation threshold of graphene sheets modeled as perfectly rigid, disklike particles.

Chromatin Ordering in the SV40 Virus

The Simian Virus 40 (SV40) is composed of an outer shell formed from capsid proteins, enveloping a minichromosome that is made of double stranded DNA wound around ca. 20 nucleosomes. In contrast to many bacteriophages and RNA viruses, the structure and order of the packaged viral chromatin has remained elusive. Using small angle x-ray diffraction as well as computer modeling, we show that a unique ordering of the nucleic acid emerges, indicating at least two concentric shells of higher minichromosomal electron density. Analysis shows that packaging can be explained by considering the competition of interactions between disk-like nucleosomal particles that favor columnar ordering versus wall-nucleosome interactions that tend to align particles with the capsid interior.

Relaxation dynamics in the columnar liquid crystal phase of hard platelets

We perform Monte Carlo simulations to analyze the equilibrium dynamics and the long-time structural relaxation decay of columnar liquid crystals of disk-like colloidal particles. In the wake of recent studies on the columnar mesophase of hard calamitic (rod-like) colloids, we now focus on the diffusion of their discotic counterparts, here modeled as oblate hard spherocylinders. These systems exhibit a non-Gaussian column-to-column diffusion due to the combined action of transient cages and periodic free-energy barriers. We find that at fixed packing fraction the barrier height increases with decreasing particle thickness, resulting into a more heterogeneous and non-Gaussian dynamics for thinner platelets, and reducing the inter-column diffusion coefficient. Moreover, we observe the characteristic two-step relaxation decay of the structure in the plane perpendicular to the column axis. By contrast, the in-column dynamics is similar to the typical single-file diffusion of one-dimensional dense fluids, with a relatively fast decay of the correlation functions.

3D Monte Carlo simulations of a magnetic disk-like particle dispersion

We have investigated the aggregate structures of a colloidal dispersion composed of ferromagnetic disk-like particles with a magnetic moment normal to the particle axis at the particle center, by means of 3D Monte Carlo simulations. Such disk-like particles have been modeled as a circular disk-like particle with the side section shape of spherocylinder. We have attempted to clarify the influences of the magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the aggregate structures of clusters, we have focused on the radial distribution and orientational pair correlation functions, etc. For no applied magnetic field cases, long column-like clusters come to be formed with increasing magnetic particle–particle interactions. The internal structures of these clusters clearly show that the particles incline in a certain direction and their magnetic moments alternate in direction between the neighboring particles in the clusters. For applied magnetic field cases, the magnetic moment of each particle inclines in the magnetic field direction and therefore the column-like clusters are not formed straightforwardly. If the magnetic field is much stronger than magnetic particle–particle interactions, the particles do not have a tendency to form the clusters. As the influence of magnetic particle–particle interactions is significantly strong, thick chain-like clusters or column-like clusters or brick-wall-like clusters come to be formed along the magnetic field direction.

Shear-induced orientation in polymer/clay dispersions via in situ X-ray scattering

We report in situ X-ray scattering measurements of shear-induced orientation in polymer–clay dispersions. Two different organically modified clays, montmorillonite and fluorohectorite, are dispersed in a low molecular weight, viscous polymer melt, facilitating studies at room temperature. Orientation measurements are performed in the flow-gradient plane, allowing characterization of both the average degree and direction of particle orientation during shear. In all cases, the orientation angle is finite, indicating systematic misalignment of the particle long axes relative to the flow direction. In concentrated fluorohectorite and montmorillonite dispersions, anisotropy and orientation angle are roughly independent of shear rate, and negligible relaxation is observed upon flow cessation. Conversely, a lower concentration montmorillonite sample exhibits orientation that is more responsive to shear flow, and partially relaxes upon flow cessation. In this sample, the orientation behavior is interpreted in light of rotational diffusion of the clay particles. This same sample exhibits oscillatory structural dynamics upon shear flow reversal, attributed to tumbling rotations of the disk-like clay particles in shear. Large-amplitude oscillatory shear is similarly demonstrated to be capable of inducing significant particle orientation; the degree of orientation is principally determined by the applied strain amplitude. Complementary measurements of rheological properties exhibit many characteristics commonly reported in polymer–clay nanocomposites. Based on the structural measurements reported here, the rheological phenomena are interpreted to arise from a combination of flow-induced particle orientation and rate- and time-dependent destruction or reformation of particle networks.

Dispersions of plate-like colloidal particles – Cubatic order?

Experimental evidence for the existence of ‘cubatic’ order in a colloidal dispersion of plate-like particles is presented. In a ‘cubatic’ phase, disk-like particles self-assemble with domains of a few parallel plates and the director tends to be orthogonal in adjacent domains. This phase has been predicted previously by computer simulation. The domains are approximately equiaxial and are predicted to exist only within a limited range of aspect ratios and volume fractions. This locally ordered structure cannot be identified readily using scattering techniques, since the patterns are expected to be similar to those of isotropic liquid phases. For this reason, we have used a real-space technique of cryo-transmission electron microscopy that directly probes such locally ordered structures to study dispersions of nickel hydroxide particles. Polydispersity of particle size is expected to require some local tilting in order to include larger particles in a dense structure and this is discussed with respect to the concentration range for which cubatic order is observed. This new structure offers the possibility of novel materials that could be prepared by self-assembly and have applications in a wide range of fields.

Simulation Model for Hierarchical Self-Assembly of Soft Disklike Particles

We develop a novel mesoscale simulation model in order to study the hierarchical self-assembly of soft disklike particles in dilute solutions. In suitable solvent conditions, the soft anisotropic disklike particles first self-assemble into one-dimensional flexible threads, in accord with experiments. Then, intriguingly, the threads reversibly pack into flexible hexagonal bundles by decreasing the solvent quality. Hierarchical self-assembly of this type may be important to provide a strategy to create bundle structures by bottom-up self-assembly with a single type of soft and flexible building block and mimic the bundles commonly found in biological structures.

Effective viscosity of semidilute suspensions of rigid ellipsoids

Hard and refractory particles with various shapes have been added to ceramic materials to enhance creep resistance. Also, many of the particles in food emulsions, magnetic particle suspensions, and other industrial systems may have nonspherical shapes. Hence, it is important to understand the effects of particle shape on the viscosity of suspensions. While the shape of many particles can be approximated as prolate or oblate spheroids, the purpose of the present study is to model the effective viscosity of semidilute suspensions of rigid ellipsoids. Closed-form expressions are obtained; however, their formulations are formidable. For the special cases of rodlike or disklike particles, simple closed-form expressions can be obtained. The present solutions are compared with existing solutions.

扁平粒子からなる強磁性コロイド分散系の凝集構造に関する3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 aggregate structures of a colloidal dispersion composed of ferromagnetic plate-like particles with a magnetic moment normal to the particle axis at the particle center, by means of three-dimensional Monte Carlo simulations. Such plate-like particles have been modeled as disk-like particles with the side section shape of a spherocylinder. In concrete, we have attempted to clarify the influences of the magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the aggregate structures of clusters, we have focused on the radial distribution and orientational pair correlation function, etc. For no applied magnetic field cases, long column-like clusters are formed as magnetic particle-particle interactions increase. Characteristics of these clusters are that particles incline in a certain direction and their magnetic moments alternate in direction between neighboring particles. For applied magnetic field cases, the particles incline in the magnetic field direction so that the column-like clusters are not formed easily. When the magnetic field has the same order of influence as the magnetic particle-particle interactions, the particles do not form the clusters. However, as the influence of the magnetic field and magnetic particle-particle interactions become significantly strong, thick chain-like clusters are formed along the magnetic field direction.

Clay/Epoxy Nanocomposites

Nanocomposites are a novel class of composite materials where one of the constituents has dimensions in the range between 1 and 100 nm. Recent and ongoing research on polymer/inorganic nanocomposites has shown dramatic enhancements in stiffness, strength and thermal properties over those of polymers, without compromising on density, toughness or processibility. Such improvement in properties has been estimated to correspond to approxi

Liquid-Crystalline Nematic Phase in Aqueous Suspensions of a Disk-Shaped Natural Beidellite Clay

After size-selection and osmotic pressure measurements at fixed ionic strength, the behavior of aqueous colloidal suspensions of anisotropic disklike beidellite clay particles has been investigated by combining optical observations under polarized light, rheological, and small angle X-ray scattering (SAXS) experiments. The obtained phase diagrams (volume fraction/ionic strength) reveal, for ionic strength below 10(-3) M/L, a first-order isotropic/nematic (I/N) phase transition before gel formation at low volume fractions, typically around 0.5%. This I/N transition line displays a positive slope for increasing ionic strength and shifts toward lower volume fraction with increasing particle size, confirming that the system is controlled by repulsive interactions. The swelling laws, derived from the interparticle distances obtained by SAXS, display a transition from isotropic swelling at low volume fractions to lamellar swelling at higher volume fractions. The liquid-crystal properties have then been investigated in detail. Highly aligned nematic samples can be obtained in three different ways, by applying a magnetic field, an ac electric field, and by spontaneous homeotropic anchoring on surfaces. The birefringence of the fluid nematic phase is negative with typical values around 5 x 10(-4) at a volume fraction of about 0.6%. High nematic order parameters have been obtained as expected for well-aligned samples. The nematic director is aligned parallel to the magnetic field and perpendicular to the electric field.

Structure and Interdomain Dynamics of Apoptosis-associated Speck-like Protein Containing a CARD (ASC)

The human protein ASC is a key mediator in apoptosis and inflammation. Through its two death domains (pyrin and CARD) ASC interacts with cell death executioners, acts as an essential adapter for inflammasome integrity, and oligomerizes into functional supramolecular assemblies. However, these functions are not understood at the structural-dynamic level. This study reports the solution structure and interdomain dynamics of full-length ASC. The pyrin and CARD domains are structurally independent six-helix bundle motifs connected by a 23-residue linker. The CARD structure reveals two distinctive characteristics; helix 1 is not fragmented as in all other known CARDs, and its electrostatic surface shows a uniform distribution of positive and negative charges, whereas these are commonly separated into two areas in other death domains. The linker adopts residual structure resulting in a back-to-back orientation of the domains, which avoids steric interference of each domain with the binding site of the other. NMR relaxation experiments show that the linker is flexible despite the residual structure. This flexibility could help expand the relative volume occupied by each domain, thus increasing the capture radius for effectors. Based on the ASC structure, a tentative model is proposed to illustrate how ASC oligomerizes via CARD and pyrin homophilic interactions. Moreover, ASC oligomers have been analyzed by atomic force microscopy, showing a predominant species of disk-like particles of approximately 12-nm diameter and approximately 1-nm height. Taken together, these results provide structural insight into the behavior of ASC as an adapter molecule.

Nanoscale Organization of GaSe Quantum Dots on a Gold Surface

Precise spatial organization and electronic coupling between quantum dots are pivotal for many potential applications. Typical spherical quantum dots in assemblies are separated by organic ligands and hence weakly coupled. GaSe nanoparticles are disk-like particles that are four atoms thick with tunable lateral dimensions. Previous spectroscopic investigations indicate the formation of nanoscale aggregates in which the quantum dots are strongly coupled. In this report, we show that the anisotropic properties of these particles may be exploited to assemble surface-stabilized superstructures with well-defined distances between the quantum dots. By changing the ligands adsorbed on the nanoparticle edges, three distinct aggregate morphologies can be produced. The surface chemistry of GaSe orients the nanoparticles on a gold surface and induces stacking in the surface normal direction. The discrete heights of such stacked aggregates suggest that the layers are held together by van der Waals interactions with a...