Aging in a Laponite colloidal suspension: a Brownian dynamics simulation study

The authors report Brownian dynamics simulation of the out-of-equilibrium dynamics (aging) in a colloidal suspension composed of rigid charged disks, one possible model for Laponite, a synthetic clay deeply investigated in the last few years by means of various experimental techniques. At variance with previous numerical investigations, mainly focusing on static structure and equilibrium dynamics, the authors explore the out-of-equilibrium aging dynamics. They analyze the wave vector and waiting time dependence of the dynamics, focusing on the single-particle and collective density fluctuations (intermediate scattering functions), the mean-squared displacement, and the rotational dynamics. Their findings confirm the complexity of the out-of-equilibrium dynamical behavior of this class of colloidal suspensions and suggest that an arrested disordered state driven by a repulsive Yukawa potential, i.e., a Wigner glass, can be observed in this model.     

Long-time self-diffusion of charged colloidal particles: electrokinetic and hydrodynamic interaction effects

The authors analyze the long-time self-diffusion of charge-stabilized colloidal macroions in nondilute suspensions using a mode-coupling scheme developed for multicomponent suspensions of interacting Brownian spheres. In this scheme, all ionic species, including counterions and electrolyte ions, are treated on an equal footing as charged hard spheres undergoing overdamped Brownian motion. Hydrodynamic interactions between all ions are accounted for on the far-field level. We show that the influence on the colloidal long-time self-diffusion coefficient arising from the relaxation of the microionic atmosphere surrounding the colloids, the so-called electrolyte friction effect, is usually insignificant in comparison with the friction contributions arising from direct and hydrodynamic interactions between the colloidal particles. This finding is true even for small colloid concentrations unless the mobility difference between colloidal particles and microions is not large. Furthermore, we observe an interesting nonmonotonic density dependence of the colloidal long-time self-diffusion coefficient in suspensions with low amount of added salt. We show that this unusual density dependence is due to colloid-colloid hydrodynamic interactions.     

Simulations of the PDF functions for dilute colloidal suspensions of molecular particles flowing in mesopores with rough surface boundaries

Simulations have been carried out to analyze the dynamics of dilute colloidal suspensions of macromolecular particles in solutions flowing in pores, subject to hydrodynamic forces, Brownian motion and stochastic collisions at rough pore boundaries in a two-dimensional spatial frame. A theoretical model is developed and intensively analyzed for the treatment of the mechanical restitution of the particles due to dynamic collisions at these boundaries. In particular we are able to calculate the Probability distribution functions for the spatial positions and the orientations of rod-like particles inside the pores. The results are presented for different widths of pore channels referenced to the size of a rod-like particle. These simulations are general in the sense that they are developed for confining and open pore channels, rough at the nano scale. The simulations also permit calculating the nematic order parameters for colloidal suspensions; the model calculation is applied for dilute colloidal suspensions of carbon nano-tubes in an aqueous single-stranded DNA solution flowing inside pores. Our calculated nematic order results for dilute suspensions of particles of known lengths flowing inside porous systems should indicate, when coupled to birefringence and dichroism experimental results, the possibility to estimate the pore widths for these systems.     

Sedimentation of concentrated monodisperse colloidal suspensions: role of collective particle interaction forces

The sedimentation velocities and concentration profiles of low-charge, monodisperse hydroxylate latex particle suspensions were investigated experimentally as a function of the particle concentration to study the effects of the collective particle interactions on suspension stability. We used the Kossel diffraction technique to measure the particle concentration profile and sedimentation rate. We conducted the sedimentation experiments using three different particle sizes. Collective hydrodynamic interactions dominate the particle-particle interactions at particle concentrations up to 6.5 vol%. However, at higher particle concentrations, additional collective particle-particle interactions resulting from the self-depletion attraction cause particle aggregation inside the suspension. The collective particle-particle interaction forces play a much more important role when relatively small particles (500 nm in diameter or less) are used. We developed a theoretical model based on the statistical particle dynamics simulation method to examine the role of the collective particle interactions in concentrated suspensions in the colloidal microstructure formation and sedimentation rates. The theoretical results agree with the experimentally-measured values of the settling velocities and concentration profiles.     

无机溶致液晶研究进展

目前发现的液晶多数为有机液晶,无机液晶非常少见。非球形无机胶体(棒状或盘状)体系在排斥体积熵的作用下可形成液晶相,即无机溶致液晶。由于其具有的理论意义和潜在的应用价值,无机液晶近年来引起了人们的关注。本文综述了无机溶致液晶的研究历史和最新进展。     

Dynamic light scattering,a probe of brownian particle dynamics

The principles and techniques of dynamic light scattering (DLS) are outlined and its application to the study of suspensions of interacting colloidal particles is discussed. We show how, under appropriate conditions, DLS can measure long-time collective and self-diffusion coefficients as well as study short-time motions (characterized by the cumulants). These theoretical considerations are illustrated by experimental data. Finally, we discuss the relevance of certain characteristic timescales to theories of the diffusion of interacting particles.     

AC electrokinetic templating of colloidal particle assemblies: effect of electrohydrodynamic flows

The use of spatially nonuniform electric fields for the contact-free colloidal particle assembly into ordered structures of various length scales is a research area of great interest. In the present work, numerical simulations are undertaken in order to advance our understanding of the physical mechanisms that govern this colloidal assembly process and their relation to the electric field characteristics and colloidal system properties. More specifically, the electric-field driven assembly of colloidal silica (d(p) = 0.32 and 2 μm) in DMSO, a near index matching fluid, is studied numerically over a range of voltages and concentration by means of a continuum thermodynamic approach. The equilibrium (u(f) = 0) and nonequilibrium (u(f) ≠ 0) cases were compared to determine whether fluid motion had an effect on the shape and size of assemblies. It was found that the nonequilibrium case was substantially different versus the equilibrium case, in both size and shape of the assembled structure. This dependence was related to the relative magnitudes of the electric-field driven convective motion of particles versus the fluid velocity. Fluid velocity magnitudes on the order of mm/s were predicted for 0.32 μm particles at 1% initial solids content, and the induced fluid velocity was found to be larger at the same voltage/initial volume fraction as the particle size decreased, owing to a larger contribution from entropic forces.     

Sedimentation of aggregating colloids

We investigate the combined effects of gravity, attractive interactions, and brownian motion in suspensions of colloidal particles and nonadsorbing polymer. Depending on the effective strength of gravitational forces, resulting from a density mismatch between the colloids and the solvent, and the magnitude and range of the depletion interactions induced by the polymer, sedimentation in these suspensions can result in an equilibrium structure or a kinetically arrested state. We employ large-scale molecular dynamics simulations to systematically classify the different regimes that arise as a function of attraction strength and gravitational stress. Whereas strong attractions lead to cluster aggregation and low-density arrested states, moderate attractions can enhance crystallization of the colloidal particles in the sediment. We make direct comparisons to experimental results to infer general conclusions about the mechanisms leading to mechanically stable sediments.     

Colloidal suspensions of N-modified graphene nano-platelets in water and organic solvent/water mixed systems

The production of colloidal suspensions of graphene-based nano-platelets in large scale is highly important for their use in practical applications. In this work, we developed a new route for generation of colloidal suspensions of N-modified graphene-based nano-platelets (A-rG-O) with high concentration in water or organic solvent/water mixed systems by the reaction between graphene oxide and ammonium hydroxide. Graphene oxide was reduced by the ammonium hydroxide and N atoms (up to 6 at%) were introduced into A-rG-O materials using solution process without further thermal treatment. Such A-rG-O nano-platelets were well dispersed as individual layers in both systems. Macroscopic free-standing A-rG-O paper materials with good electrical conductivity were produced by filtration of such colloidal suspensions.     

DLVO AND NON-DLVO SURFACE FORCES AND INTERACTIONS IN COLLOIDAL DISPERSIONS

Some aspects of DLVO and non-DLVO forces in colloidal systems are over-viewed. The influence of long range interactions on some kinetic properties of dispersions, as Brownian diffusion, is discussed. It is shown, both theoretically and experimentally, that the electrostatic repulsion increases the collective diffusivity. The film stratification and oscillatory structure forces in colloidal suspensions are considered within the framework of an uniform approach The presence of small colloidal species (e. g. micelles or polymer molecules) may lead to several maxima and minima in the disjoining pressure isotherm. The particular case of interacting emulsion droplets is examined accounting for the interfacial deformability. The droplet deformation acts as a soft repulsion but affects also the remaining contributions to the interaction energy due to changes of the droplet shape. A general procedure for calculating the inter-droplet interaction energy, as well as the equilibrium film radius and thickness in a doublet of droplets, is suggested. The energy of interaction between charged colloidal particles, due to correlations of the density fluctuations in the electric double layer is also studied. It is found that this effect may lead to attraction greater than the van der Waals contribution, especially when multivale counter ions are present.     

Microviscoelasticity of soft repulsive sphere dispersions: tracer particle microrheology of triblock copolymer micellar liquids and soft crystals

Tracer particle microrheology using diffusing wave spectroscopy-based microrheology is demonstrated to be a useful method to study the dynamics of aqueous Pluronic? F108 solutions, which are viewed as solutions of repulsive soft spheres. The measured zero-shear microviscosity of noncrystallizing micellar dispersions indicates micelle corona dehydration upon increasing temperature. Colloidal sphere thermal motion is shown to be exquisitely sensitive to the onset of crystallization in these micellar dispersions. High temperature dynamics are dominated by an apparent soft repulsive micelle-micelle interaction potential indicating the important role played by lubrication forces and ultimately micelle corona interpenetration and compression at sufficiently high concentrations. The measured microscopic viscoelastic storage and loss moduli are qualitatively similar to those experimentally observed in mechanical measurements on colloidal dispersions and crystals, and calculated from mode coupling theory of colloidal suspensions. The observation of subdiffusive colloidal sphere thermal motion at short time-scales is strong evidence that the observed microscopic viscoelastic properties reflect the dynamics of individual micelles rather than a dispersion of micellar crystallites.     

Video microscopy of colloidal suspensions and colloidal crystals

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.     

Synthetic electrically driven colloids: A platform for understanding collective behavior in soft matter

The collective motion of synthetic active colloids is an emerging area of research in soft matter physics and is important both as a platform for fundamental studies ranging from non-equilibrium statistical mechanics to the basic principles of self-organization, emergent phenomena, and assembly underlying life, as well as applications in biomedicine and metamaterials. The potentially transformative nature of the field over the next decade and beyond is a topic of critical research importance. Electrokinetic active colloids represent an extremely flexible platform for the investigation and modulation of collective behavior in active matter. Here, we review progress in the past five years in electrokinetic active systems and related topics in active matter with important fundamental research and applicative potential to be investigated using electrokinetic systems.     

Qualitative and quantitative analysis of crystallographic defects present in 2D colloidal sphere arrays

In this work, we present a study of the typical spontaneous defects present in self-assembled colloidal monolayers grown from polystyrene and silica microspheres. The quality of two-dimensional crystals from different colloidal suspensions of beads around 1 μm in diameter has been studied qualitatively and quantitatively, evaluated in 2D hexagonal arrays at different scales through Fourier analysis of SEM images and optical characterization. The crystallographic defects are identified to better understand their origin and their effects on the crystal quality, as well as to find the best conditions colloidal suspensions must fulfill to achieve optimal quality samples.     

Distorted colloidal crystal of similar-sized aggregates (1.5 μm in diameter) of nano-sized diamond particles (4 nm in diameter)

Distorted colloidal crystal suspension of similar-sized aggregates of diamonds (1.5 μm in diameter) was obtained by the deionization of aqueous suspension of the pre-particles of diamond, 4 nm in diameter. The stability, characteristics, and the rigidity of the crystal-like suspensions were studied. The main cause for the formation of the similar-sized aggregates is deduced to be the cooperation between the van de Waals inter-particle attraction and the repulsion induced by the vigorous thermal motion of the pre-particles. The rigidity was evaluated from the microscopic observation in the sedimentation equilibrium. Fluctuation parameters of the distorted colloidal crystals estimated from the rigidities were between 0.03 and 0.06, which are quite similar to those of typical colloidal crystals and solids of hard spheres.     

On the influence of size, zeta potential, and state of motion of dispersed particles on the conductivity of a colloidal suspension

The dependence of the DC conductivity of diluted colloidal suspensions on the size, zeta potential, and state of motion of the dispersed particles is analyzed both theoretically and numerically. It is shown that the simple formula that represents the conductivity as a sum of products: charge times mobility, taken over all the carriers present in the suspension, is only valid for exceedingly low values of the product kappaa. In contrast, the formulation based on the value of the dipolar coefficient of the suspended particles seems to be valid for all the range of particle sizes. This assertion is only true if the dipolar coefficient is calculated taking into account the electrophoretic motion of the particles. For very low values of the product kappaa, the dipolar coefficient of particles free to move can be several orders of magnitude larger than that of immobile particles.     

Highly charging of nanoparticles through electrospray of nanoparticle suspension

Patterned deposition of nanoparticles is a prerequisite for the application of unique properties of nanoparticles in future nanodevices. Recent development of nanoxerography requires highly charged aerosol nanoparticles to avoid noise deposition due to random Brownian motion. However, it has been known that it is difficult to charge aerosol nanoparticles with more than two elementary charges. The goal of this work is to develop a simple technique for obtaining highly charged monodisperse aerosol nanoparticles by means of electrospray of colloidal suspension. Highly charged aerosol nanoparticles were produced by electrospraying (ES) and drying colloidal suspensions of monodisperse gold nanoparticles. Size and charge distributions of the resultant particles were measured. We demonstrate that this method successfully charges monodisperse nanoparticles very highly, e.g., 122 elementary charges for 25.0 nm, 23.5 for 10.5 nm, and 4.6 for 4.2 nm. The method described here constitutes a convenient, reliable, and continuous tool for preparing highly charged aerosol nanoparticles from suspensions of nanoparticles produced by either wet chemistry or gas-phase methods.     

Pattern formation in draining thin film suspensions

We demonstrate the emergence of complexity from remarkably simple and ubiquitous systems: draining thin-film suspensions exhibiting a striking transition between two classes of self-organizing patterns. Vertical channels form when attractive forces lead to transient gelation, while horizontal bands result from granular mixtures. We propose an explanation whereby the generic physical mechanisms require only the existence of viscous and excluded-volume couplings among the particles, solvent, and substrate. System-specific, small inhomogeneities trigger large-scale pattern formation, through collective dynamics, where jamming plays a crucial role. Our results shed light on emergent complexity in bio- and geophysical processes and have implications for coatings and food industries.     

A new model system for diffusion NMR studies of concentrated monodisperse and bidisperse colloids

A method to prepare monodisperse and simultaneously NMR-visible and fluorescent colloidal particles is described, and a systematic approach to obtain spectrally resolved diffusion coefficient for every component in a monodisperse colloidal suspension is presented. We also prepared bidisperse colloidal suspensions, where each colloid component has a distinct NMR spectral signature, and obtained the diffusion coefficients of both colloid species simultaneously in concentrated colloidal suspensions, with volume fractions between 20 and 50%. The colloidal model system developed in this work enables the study of colloidal phase behavior in binary mixtures for different number and size ratios.