Effects of metal oxide nanoparticles on the stability of dispersions of weakly charged colloids.

The stability behavior of dispersions of weakly charged silica colloids was studied in the presence of highly charged metal oxide nanoparticles. Experiments were performed using 5 nm zirconia as well as 10 nm alumina nanoparticles (both positively charged), which were added to 0.1 vol % suspensions of 1.

Adsorption and stabilizing effects of highly-charged latex nanoparticles in dispersions of weakly-charged silica colloids.

An experimental study was undertaken to determine the effectiveness of using highly-charged nanoparticles as stabilizers for colloidal dispersions. The specific systems used here involved cationic (amidine) and anionic (sulfate) polystyrene latex nanoparticles with an approximate diameter of 20 nm and silica microparticles of diameter 1.0 μm, and experiments were conducted at the isoelectric point of the silica.

Porous nanocomposites with integrated internal domains: application to separation membranes.

Asymmetric membranes with layered structure have made significant achievements due to their balanced properties and multi-functionalities that come from a combination of multiple layers. However, issues such as delamination and substructure resistance are generated by the intrinsic layered structure. Here, we present a strategy to integrate the traditional layered structure into an asymmetric but continuous porous network.

Depletion flocculation induced by synergistic effects of nanoparticles and polymers.

The depletion flocculation of stable suspensions of charged microparticles (1.2 μm diameter polystyrene) by mixtures of silica nanoparticles (7 nm) and poly(acrylic) acid (PAA, Mn = 24 500) was studied. The experiments revealed a very clear critical flocculation concentration of PAA that was lowered by the addition of the silica nanoparticles.

Synergistic effects of nanoparticles and polymers on depletion and structural interactions.

An experimental study was performed to investigate the synergistic effects of two different solution components on the depletion and structural forces between colloidal particles. Using silica nanoparticles and anionic poly(acrylic acid) polymer, it was found that the depletion and structural forces measured between a 30 μm diameter silica sphere and a flat silica plate (obtained using colloidal probe atomic force microscopy) were substantially greater than the sum of the forces obtained in systems containing only nanoparticles and only polymer. This result arises because the anionic polymer chains adsorb to the nanoparticles, creating a complex that is over twice as large as either component.

Stabilization of weakly charged microparticles using highly charged nanoparticles.

An experimental study was performed to understand the ability of highly charged nanoparticles to stabilize a dispersion of weakly charged microspheres. The experiments involved adding either anionic (sulfate) or cationic (amidine) latex nanoparticles to dispersions of micrometer-sized silica particles near the silica isoelectric point (IEP). Although both types of nanoparticles increased the zeta potential of the silica microspheres above the value at which dispersions containing only silica spheres remained stable, only with the amidine nanoparticles was stability obtained.

Interaction potentials between two colloidal particles surrounded by an extremely bidisperse particle suspension.

The results of a computational study predicting the equilibrium forces between two colloidal particles (microparticles) in a solution containing both submicroparticles and nanoparticles (e.g., one and two orders-of-magnitude smaller than the microparticles, respectively) are presented.

Fabrication of porous nanocomposites with controllable specific surface area and strength via suspension infiltration.

Porous ceramics are promising candidates for a variety of applications, including separation membranes, catalyst supports, tissue engineering scaffolds, energy storage devices, and microelectronics. We describe a novel method for creating porous ceramics with controllable specific surface area and high strength. The fabrication procedure involves infiltrating aqueous suspensions of silica nanoparticles into a porous ceramic scaffold.

Rheology of fluids measured by correlation force spectroscopy.

We describe a method, correlation force spectrometry (CFS), which characterizes fluids through measurement of the correlations between the thermally stimulated vibrations of two closely spaced micrometer-scale cantilevers in fluid. We discuss a major application: measurement of the rheological properties of fluids at high frequency and high spatial resolution. Use of CFS as a rheometer is validated by comparison between experimental data and finite element modeling of the deterministic ring-down of cantilevers using the known viscosity of fluids.

Hydrodynamic force on a microparticle approaching a wall in a nanoparticle dispersion: observation of a separation-dependent effective viscosity.

Colloid probe atomic force microscopy was used to measure the hydrodynamic force exerted on a 30-μm-diameter silica particle being moved toward or away from a silica plate in aqueous dispersions of 22-nm-diameter silica nanoparticles (6 or 8 vol %). Upon comparing the measured force to predictions made using the well-known expression of Cox and Brenner (Cox, R. G.

Interaction forces between colloidal particles in a solution of like-charged, adsorbing nanoparticles.

We have measured the force between a weakly charged micron-sized colloidal particle and flat substrate in the presence of highly charged nanoparticles of the same sign under solution conditions such that the nanoparticles physically adsorb to the colloidal particle and substrate. The objective was to investigate the net effect on the force profile between the microparticle and flat substrate arising from both nanoparticle adsorption and nanoparticles in solution. The experiments used colloidal probe atomic force microscopy (CP-AFM) to measure the force profile between a relatively large (5 μm) colloidal probe glass particle and a planar glass substrate in aqueous solutions at varying concentrations of spherical nanoparticles.

Influence of charged nanoparticles on colloidal forces: a molecular simulation study.

We employ the grand canonical Monte Carlo simulation technique to investigate the influence of charged nanoparticles (macro-ions) on the force between colloidal objects. Specifically, the structure and osmotic pressure of a system of screened Coulomb (Yukawa) particles confined between charged planar walls are simulated. We observe osmotic pressure to oscillate with wall separation and these oscillations to correspond to changes in the number of nanoparticle layers present in the slit pore.

Synergistic effects of polymers and surfactants on depletion forces.

This work investigates the synergistic effects of a neutral polymer and an anionic surfactant on depletion forces as a function of bulk polymer and bulk surfactant concentration. In this work, we measure the force between a silica particle and a silica plate in aqueous solutions of the polymer and the surfactant using atomic force microscopy. The polymer is the triblock copolymer poly(ethylene oxide-block-propylene oxide-block-ethylene oxide) (Pluronic F108), and the surfactant is sodium dodecyl sulfate (SDS).

Investigation of short-time particle dynamics near an interface in the presence of nonadsorbed macro-ions.

The optical technique of total internal reflection microscopy was used to study the normal Brownian motion of a single colloidal particle near an interface. The measurements were made using a recently developed technique in which the diffusion coefficient was determined by the variance of the short-time (Deltat --> 0) motion of the particle. Experiments were performed in solutions containing either silica nanospheres or clay platelets (Laponite RD) to investigate the effect of nonadsorbed material on the dynamics of near-contact particle motion.

Simultaneous investigation of sedimentation and diffusion of a single colloidal particle near an interface.

We describe here a new procedure for the simultaneous investigation of sedimentation and diffusion of a colloidal particle in close proximity to a solid, planar wall. The measurements were made using the optical technique of total internal reflection microscopy, coupled with optical radiation pressure, for dimensionless separation distances (gap width/radius of particle) ranging from 0.01 to 0.

Structuring of macroions confined between like-charged surfaces.

We investigate the structuring of charged spherical nanoparticles and micelles (i.e., "macroions") between two surfaces as a function of bulk macroion concentration.

Direct measurement of depletion and structural forces in polydisperse, charged systems.

The effect of polydispersity in macromolecule size or surface potential on the depletion interaction between a spherical silica particle and a silica flat in solutions containing two different types of nonadsorbing charged spherical macromolecules was studied with an atomic force microscope (AFM). The macromolecules used here were negatively charged nanospheres of either polystyrene or silica. To investigate the effect of size polydispersity, experiments were performed under the condition of either constant macromolecule number density or constant volume fraction as the relative proportions of smaller and larger polystyrene nanospheres in the suspension were varied.

Prediction and measurement of the interparticle depletion interaction next to a flat wall.

A theoretical and experimental study was performed to investigate the depletion interaction between two colloidal particles next to a solid wall in a solution of nonadsorbing macromolecules. By calculating the change in free volume available to the macromolecules upon approach of the two particles, a relatively simple expression was developed for the interparticle depletion attraction in hard sphere systems as a function of the particle-particle and particle-plate spacing. Perhaps the most useful result obtained from this analysis was that the wall has no effect whenever the ratio of the particle radius to the macromolecule radius is greater than four.

Effect of depletion interactions on transport of colloidal particles in porous media.

The influence of depletion interactions on the transport of micrometer-sized, negatively charged polystyrene latex particles through porous media was studied by analysis of particle breakthrough curves as a response to short-pulse particle injections to the inlet of a packed column of glass beads. The column outlet latex particle concentration profiles and the total amount of particles exiting the column were determined as a function of the concentration of small, silica nanoparticles in the solution and the bulk flow rate. Because of similar charges, the silica particles do not adsorb to either the latex particles or glass beads and thus induce an attractive depletion force between the latex particles and glass bead collectors.

Relationship between scattered intensity and separation for particles in an evanescent field.

We describe measurements of the scattering of visible light from an evanescent field by both spherical particles (R = 1-10 mum) that are glued to atomic force microscopy (AFM) cantilevers, and by sharp tips (R < 60 nm) that were incorporated onto the cantilevers during manufacture. The evanescent wave was generated at the interface between a flat plate and an aqueous solution, and an atomic force microscope was used to accurately control the separation, h, between the particle and the flat plate. We find that, for sharp tips, the intensity of scattered light decays exponentially with separation between the tip and the plate all the way down to h approximately 0.

A new approach for analyzing particle motion near an interface using total internal reflection microscopy.

A new method was developed for analyzing the normal motion of a single colloidal particle near an interface. The optical technique of total internal reflection microscopy (TIRM) was used to determine the distribution of vertical displacements of a particle from a specific starting position as a function of time. At very small displacement times, the displacements are normally distributed with a variance that is proportional to the diffusion coefficient times the displacement time.

Atomic force microscopy colloid-probe measurements with explicit measurement of particle-solid separation.

We describe the use of evanescent wave scattering to measure the separation between the surface of a solid and a particle that is attached to an atomic force microscope (AFM) cantilever. Termed evanescent wave atomic force microscopy, our approach involves measuring the intensity of the light scattered from an evanescent field formed by the total internal reflection of a laser beam at a solid/fluid interface. In a conventional AFM "colloid probe" measurement, this separation must be inferred from an examination of the surface forces.

An approximate method for calculating depletion and structural interactions between colloidal particles.

An approximate method is evaluated for calculating the second virial coefficient in a dilute macromolecular solution bounded by two interfaces. The approximation is essentially the superposition of the coefficients calculated independently for each surface. To test this approach, the depletion interaction between two particles in a solution of nonadsorbing, spherical macromolecules is calculated in systems with either hard-wall or long-range electrostatic interactions.

A Model for Calculating Electrostatic Interactions between Colloidal Particles of Arbitrary Surface Topology.

A numerical model for calculating the electrostatic interaction between two particles of arbitrary shape and topology is described. A key feature of the model is a generalized discretization program, capable of simulating any desired analytical shape as a set of flat, triangular elements. The relative sizes of the elements are adjusted using a density function to better match the desired shape and the spatial variation of the electrical surface properties on each particle.