Suspensions Of Colloidal Particles Research Articles

Fabrication of Large‐Area Two‐Dimensional Colloidal Crystals

Nanoparticle coating: A suspension of colloidal particles in a water/propanol solution was layered onto a water surface, where the particles self-assembled into ordered two-dimensional hexagonal crystal arrays (>280 cm2) within two minutes. These arrays were transferred from the water surface to other substrates (see picture) and embedded in a chitosan hydrogel for visual detection of the pH value.

Optimizing the solar water disinfection (SODIS) method by decreasing turbidity with NaCl

Solar water disinfection (SODIS) has proven to be effective at reducing diarrheal incidence in epidemiological intervention studies. However, the SODIS method is limited to waters of low turbidity (<30 NTU). This study investigates the use of common table salt (NaCl) to reduce the turbidity of water containing suspended colloidal clay particles for use in the SODIS method. Three representative clays found in tropical soils (kaolinite, illite and bentonite) were tested at three levels of turbidity (50, 100 and 200 NTU) for their flocculating behavior with multiple NaCl concentrations to find the optimum. Supernatants were tested for sodium concentration for comparison against health and taste thresholds. Results show that unlike kaolinite and illite, pure bentonite solutions were shown to be very responsive to NaCl and produced supernatants with as low as 4 NTU (98% particle removal efficiency). This study has shown that NaCl, in combination with high-activity clay particles in solution, may effectively reduce turbidity to levels suitable for SODIS treatment, thereby expanding the number of people who can utilize the technology effectively.

Coulomb self-energy of a uniformly charged three-dimensional cylinder

We calculate exactly the Coulomb self-energy of a uniformly charged three-dimensional cylinder. We derive a general analytical formula which, in the limit of zero length, gives also the correct result for the Coulomb self-energy of a uniformly charged two-dimensional disk. The exact analytical expression that we derive can be used in models that incorporate finite thickness effects in studies of two-dimensional electronic systems in the fractional quantum Hall regime as well as models that describe cylindrical beams of charged particles, certain colloidal suspensions of charged rigid rod-like particles and biological systems consisting of macromolecules with cylindrical symmetry.

Diffusion, sedimentation, and rheology of concentrated suspensions of core-shell particles

Short-time dynamic properties of concentrated suspensions of colloidal core-shell particles are studied using a precise force multipole method which accounts for many-particle hydrodynamic interactions. A core-shell particle is composed of a rigid, spherical dry core of radius a surrounded by a uniformly permeable shell of outer radius b and hydrodynamic penetration depth κ(-1). The solvent flow inside the permeable shell is described by the Brinkman-Debye-Bueche equation, and outside the particles by the Stokes equation. The particles are assumed to interact non-hydrodynamically by a hard-sphere no-overlap potential of radius b. Numerical results are presented for the high-frequency shear viscosity, η(∞), sedimentation coefficient, K, and the short-time translational and rotational self-diffusion coefficients, D(t) and D(r). The simulation results cover the full three-parametric fluid-phase space of the composite particle model, with the volume fraction extending up to 0.45, and the whole range of values for κb, and a/b. Many-particle hydrodynamic interaction effects on the transport properties are explored, and the hydrodynamic influence of the core in concentrated systems is discussed. Our simulation results show that for thin or hardly permeable shells, the core-shell systems can be approximated neither by no-shell nor by no-core models. However, one of our findings is that for κ(b - a) ≳ 5, the core is practically not sensed any more by the weakly penetrating fluid. This result is explained using an asymptotic analysis of the scattering coefficients entering into the multipole method of solving the Stokes equations. We show that in most cases, the influence of the core grows only weakly with increasing concentration.

Graphene oxide added carbonyl iron microsphere system and its magnetorheology under applied magnetic fields

Novel graphene oxide (GO) particle was introduced as an additive to soft magnetic carbonyl iron (CI) micron-sized particle based magnetorheological (MR) fluid to improve its sedimentation and agglomeration problems in which the MR fluid is a colloidal suspension of magnetizable particles dispersed in a nonmagnetic carrier. The added plate-like GO gap-fillers filled the interspaces of CI particles and improved the resistance to sedimentation. The MR properties of two systems with and without additives were evaluated and compared under different magnetic field strengths using a rotational rheometer. Morphology of pure CI and GO/CI mixture were examined via scanning electron microscopy (SEM) images, and X-ray diffraction (XRD) patterns confirm the differences between pure CI and GO/CI mixture further, the sedimentation problem was observed to be improved.

Liquid Crystal Order in Colloidal Suspensions of Spheroidal Particles by Direct Current Electric Field Assembly

DC electric fields are used to produce colloidal assemblies with orientational and layered positional order from a dilute suspension of spheroidal particles. These 3D assemblies, which can be visualized in situ by confocal microscopy, are achieved in short time spans (t < 1 h) by the application of a constant voltage across the capacitor-like device. This method yields denser and more ordered assemblies than had been previously reported with other assembly methods. Structures with a high degree of orientational order as well as layered positional order normal to the electrode surface are observed. These colloidal structures are explained as a consequence of electrophoretic deposition and field-assisted assembly. The interplay between the deposition rate and the rotational Brownian motion is found to be critical for the optimal ordering, which occurs when these rates, as quantified by the Peclet number, are of order one. The results suggest that the mechanism leading to ordering is equilibrium self-assembly but with kinetics dramatically accelerated by the application of the DC electric field. Finally, the crystalline symmetry of the densest structure formed is determined and compared with previously studied spheroidal assemblies.

Deterministic solution of the kinetic theory model of colloidal suspensions of structureless particles

A direct modeling of colloidal suspensions consists of calculating trajectories of all suspended objects. Due to the large time computing and the large cost involved in such calculations, we consider in this paper another route. Colloidal suspensions are described on a mesoscopic level by a distribution function whose time evolution is governed by a Fokker–Planck-like equation. The difficulty encountered on this route is the high dimensionality of the space in which the distribution function is defined. A novel strategy is used to solve numerically the Fokker–Planck equation circumventing the curse of dimensionality issue. Rheological and morphological predictions of the model that includes both direct and hydrodynamic interactions are presented in different flows.

Unique Origin of Colors of Armchair Carbon Nanotubes

The colors of suspended metallic colloidal particles are determined by their size-dependent plasma resonance, while those of semiconducting colloidal particles are determined by their size-dependent band gap. Here, we present a novel case for armchair carbon nanotubes, suspended in aqueous medium, for which the color depends on their size-dependent excitonic resonance, even though the individual particles are metallic. We observe distinct colors of a series of armchair-enriched nanotube suspensions, highlighting the unique coloration mechanism of these one-dimensional metals.

Microscopy imaging and quantitative phase contrast mapping in turbid microfluidic channels by digital holography

We show that sharp imaging and quantitative phase-contrast microcopy is possible in microfluidics in flowing turbid media by digital holography. In fact, in flowing liquids with suspended colloidal particles, clear vision is hindered and cannot be recovered by any other microscopic imaging technique. On the contrary, using digital holography, clear imaging is possible thanks to the Doppler frequency shift experienced by the photons scattered by the flowing colloidal particles, which do not contribute to the interference process, i.e. the recorded hologram. The method is illustrated and imaging results are demonstrated for pure phase objects, i.e. biological cells in microfluidic channels.

Electrical phenomena in a suspension of soft particles

The stability of a suspension of colloidal particles is determined by the interparticle electrostatic interaction, which in turn depends on the particle surface potential. The surface potential or charge of a charged colloidal particle can be evaluated by measuring its electrophoretic mobility. In the present article we focus on soft particles, that is, hard particles covered with an ion-penetrable surface layer of polyelectrolytes and provide various approximate analytic expressions for the electrostatic interaction energy between soft particles and for the electrophoretic mobility of soft particles. These expressions, which depend on the Donnan potential in the surface layer, are quite different from those for hard particles without surface structures.

A Novel Aqueous Route To Fabricate Ultrasmall Monodisperse Lipophilic Cerium Oxide Nanoparticles

Ultrasmall monodisperse 2.3 nm CeO2 nanoparticles have been synthesized by simple ammonia precipitation of cerium nitrate in a mixed glycol–water solvent and phase transferred into apolar solvents. Cerium oxide crystal surfaces were passivated with oleic acid (OA) by reflux under ambient pressure conditions. OA molecules were chemisorbed on the ceria nanoparticle surfaces. Preferential growth of the {100} planes has been observed, ascribed to restricted growth of {111} faces due to adsorbed OA. The surfacted CeO2 nanoparticles were dispersed into a stable, colloidal suspension of fine particles in hydrocarbon solvents. The CeO2 nanocrystals were characterized by X-ray diffraction, Fourier transform infrared spectra, and Raman spectral methods. Transmission electron microscopic images and photon scattering studies proved that the colloidal particles in hydrocarbon solvent were indeed monocrystalline with one crystallite per particle, monodispersed with a narrow size distribution.

Periodicity-Controlled Two-Dimensional Crystalline Colloidal Arrays

We developed a convenient and fast approach to preparing close-packed two-dimensional (2-D) particle arrays on mercury surfaces. Addition of cosolvents, such as alcohols, to aqueous colloidal particle suspensions induces spreading and self-assembly of the particles into 2-D arrays on top of the mercury surface. We can fabricate large-area close-packed 2-D arrays (>70 cm(2)) within 30 s. We attached these 2-D arrays to functional hydrogel films such that the 2-D array spacings were altered by the hydrogel volume response to the environment. We directly observed the hydrogel volume induced 2-D array spacing changes by using confocal laser scanning microscopy to monitor the spacings of fluorescent polystyrene particle 2-D arrays in response to changes in pH, solvent composition, temperature, etc.

High throughput screening of hydrolytic enzymes from termites using a natural substrate derived from sugarcane bagasse.

BackgroundThe description of new hydrolytic enzymes is an important step in the development of techniques which use lignocellulosic materials as a starting point for fuel production. Sugarcane bagasse, which is subjected to pre-treatment, hydrolysis and fermentation for the production of ethanol in several test refineries, is the most promising source of raw material for the production of second generation renewable fuels in Brazil. One problem when screening hydrolytic activities is that the activity against commercial substrates, such as carboxymethylcellulose, does not always correspond to the activity against the natural lignocellulosic material. Besides that, the macroscopic characteristics of the raw material, such as insolubility and heterogeneity, hinder its use for high throughput screenings.ResultsIn this paper, we present the preparation of a colloidal suspension of particles obtained from sugarcane bagasse, with minimal chemical change in the lignocellulosic material, and demonstrate its use for high throughput assays of hydrolases using Brazilian termites as the screened organisms.ConclusionsImportant differences between the use of the natural substrate and commercial cellulase substrates, such as carboxymethylcellulose or crystalline cellulose, were observed. This suggests that wood feeding termites, in contrast to litter feeding termites, might not be the best source for enzymes that degrade sugarcane biomass.

Dynamics of suspended colloidal particles near a wall: Implications for interfacial particle velocimetry

Interfacial transport due to surface forces is significant in many microfluidic devices with their relatively large surface areas and small volumes. Recently, velocimetry methods where evanescent waves illuminate fluorescent particles less than 1 μm in radius have been used to obtain fluid velocities within 500 nm or less of the wall. This review considers some of the phenomena affecting the dynamics of suspended colloidal particles near a wall. The measurement of interfacial flow velocities using such particles as tracers, illustrated by studies of Poiseuille and electrokinetically driven flows, is then discussed.

Processing of thin film ceramic membranes for oxygen separation

Abstract Processing of dense and thin ceramic membrane layers for high temperature selective oxygen separation is addressed in this study. Mixed oxygen-ionic and electronic conducting perovskite oxide system based on La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3− δ composition is employed for processing of structural and functional layers. Special focus is aimed at obtaining thin layer and final microstructure with particle size in the sub-micron range. Thin layer deposition is performed by dip coating technique using stable colloidal suspension of perovskite particles dispersed within ethanol media. Two polymer based surfactants were screened for their effect on particle agglomeration and rheological response. By using optimum quantity of 2.5 wt.% addition of selected surfactant it is possible to obtain dense 15–60 μm thick functional layers. The thermal cycle applied resulted in final particle sizes within sub-micron range. By employing suspension with pore former it was possible to significantly increase the surface area of the functional layer.

Short-time rheology and diffusion in suspensions of Yukawa-type colloidal particles

A comprehensive study is presented on the short-time dynamics in suspensions of charged colloidal spheres. The explored parameter space covers the major part of the fluid-state regime, with colloid concentrations extending up to the freezing transition. The particles are assumed to interact directly by a hard-core plus screened Coulomb potential, and indirectly by solvent-mediated hydrodynamic interactions. By comparison with accurate accelerated Stokesian Dynamics (ASD) simulations of the hydrodynamic function H(q), and the high-frequency viscosity η(∞), we investigate the accuracy of two fast and easy-to-implement analytical schemes. The first scheme, referred to as the pairwise additive (PA) scheme, uses exact two-body hydrodynamic mobility tensors. It is in good agreement with the ASD simulations of H(q) and η(∞), for smaller volume fractions up to about 10% and 20%, respectively. The second scheme is a hybrid method combining the virtues of the δγ scheme by Beenakker and Mazur with those of the PA scheme. It leads to predictions in good agreement with the simulation data, for all considered concentrations, combining thus precision with computational efficiency. The hybrid method is used to test the accuracy of a generalized Stokes-Einstein (GSE) relation proposed by Kholodenko and Douglas, showing its severe violation in low salinity systems. For hard spheres, however, this GSE relation applies decently well.

Phase field crystal model of drying induced ordering of colloidal droplet

Controlled evaporation of solvent from a suspension of colloidal particles can induce interesting patterns of the residue on a substrate. Under certain circumstances, the colloidal particles can self-assemble in large crystalline arrays. The conditions needed for obtaining such a homogeneous pattern are poorly understood because of the complex interactions between many forces. Here we report on a simple phase-field crystal modeling of the phenomenon which shows that the ability to crystallize depends both on the initial density of the suspension as well as the drying rate.

Issues in Assessing Environmental Exposures to Manufactured Nanomaterials

Manufactured nanomaterials (MNs) are commonly considered to be commercial products possessing at least one dimension in the size range of 10−9 m to 10−7 m. As particles in this size range represent the smaller fraction of colloidal particles characterized by dimensions of 10−9 m to 10−6 m, they differ from both molecular species and bulk particulate matter in the sense that they are unlikely to exhibit significant settling under normal gravitational conditions and they are also likely to exhibit significantly diminished diffusivities (when compared to truly dissolved species) in environmental media. As air/water, air/soil, and water/soil intermedium transport is governed by diffusive processes in the absence of significant gravitational and inertial impaction processes in environmental systems, models of MN environmental intermedium transport behavior will likely require an emphasis on kinetic approaches. This review focuses on the likely environmental fate and transport of MNs in atmospheric and aquatic systems. Should significant atmospheric MNs emission occur, previous observations suggest that MNs may likely exhibit an atmospheric residence time of ten to twenty days. Moreover, while atmospheric MN aggregates in a size range of 10−7 m to 10−6 m will likely be most mobile, they are least likely to deposit in the human respiratory system. An examination of various procedures including the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal particle suspension stability in water indicates that more sophisticated approaches may be necessary in order to develop aquatic exposure models of acceptable uncertainty. In addition, concepts such as Critical Coagulation Concentrations and Critical Zeta Potentials may prove to be quite useful in environmental aquatic exposure assessments.

Pore size distribution from challenge coreflood testing by colloidal flow

The transport of colloidal and suspension particles and the resultant particle retention occur in a wide range of porous media. The micro scale pore throat size distribution is an important characteristic of porous media, allowing for evaluation of important transport properties. An effective method based on micro scale modelling for the determination of overall pore throat size distribution (PSD) by injection of colloidal particle suspensions into engineered porous media with monitored inlet and breakthrough particle concentrations is developed. The treatment of inlet and outlet colloidal particle concentrations obtained in coreflooding results in a good agreement between the modelling and experimental data. Yet, some deviation was observed between the obtained PSD and that calculated by the Monte Carlo simulation based on the Descartes’ theorem.

When shape matters

The well-known boundaries of coffee stains are caused by the outward flow of particles suspended in the liquid. Experiments show that ellipsoidal particles can prevent the formation of such boundaries. See Letter p.308 When a drop of coffee dries, a halo of particles accumulates at the drop's edge. This 'coffee-ring effect', first described formally in a Nature paper in 1997, is a common occurrence when a solution of suspended colloidal particles evaporates. Far from being just a household curiosity, it has turned out to have relevance for many applications in which a uniform particle deposition is required, such as inkjet printing, assembly of photonics components and manufacture of DNA chips. In this issue, Peter Yunker and colleagues show that ellipsoidal particles suppress the coffee-ring effect. Attractive interparticle interactions between ellipsoids are sufficiently strong to counteract the forces that drive spherical particles towards the drop's edge as the drop evaporates. The coffee-ring effect can be restored for ellipsoids in solution containing surfactant, and 'designed' mixtures of spheres and ellipsoids can lead to uniform deposition.