The Correlation Functions of a Suspension of Large Particles in Amorphous Polybutadiene: A Molecular Dynamics Simulation Study.

The correlation functions of a suspension of Lennard-Jones large particles in a model 1,4-cis-polybutadiene solvent have been investigated by molecular dynamics simulations. We present the effects of temperature, the degree of polymerization, and the solvent/large particle density on the effective interactions between the large particles and the solvent. It is found that as the temperature increases, the structure between solvent-solvent, solvent-large particle, and large-large particle decreases. Additionally, as the bulk density or the chain length is increased, the attractive part of the large-large interaction becomes weaker and small. We believe that in part, this is due to the polymer having collapsed onto itself and entangling the large particles and lessening their interaction until they are actually in contact. Increasing the length of the polymer also entangles the large particles. However, we believe that this may be a general feature that is a characteristic of a polymer solvent containing macroscopically large colloidal particles even though entanglement should be of less significance. Copyright 2000 Academic Press.

Structure of charged colloids under a wedge confinement

Monte Carlo simulations have been performed to study the influence of a wedge confinement by hard walls on the ordering of charged colloidal particles interacting through screened Coulomb repulsive potential in an aqueous medium. The density distribution of particles for a fixed wedge angle straight theta(0) is studied for different suspension parameters, viz., bulk volume fraction straight phi, salt concentration C(s), and charge Ze on the particles. The density distribution rho(straight theta) along the angular direction and that along the radial direction, rho(r), have been analyzed in different regions of the wedge. Simulations show the formation of layered structure along the angular direction and a large gathering of particles along the wall. The number of layers as well as the density of particles within the layer are found to change as the strength and range of the interaction are varied, whereas the density profiles calculated close to the vertex region showed no significant variation in the density. The radial density profiles rho(r) corresponding to the vertex region show one-dimensional (1D) ordering of particles parallel to the vertex at a distance that is close to a wedge height, h equal to diameter of the particle. This 1D ordering is found to be destroyed upon the addition of salt or lowering the straight phi. The reported experimental observations on the "vacuum phase" are discussed in the light of the present results.

Gas-liquid transition in a two-dimensional system of millimeter-sized like-charged metal balls

Metal balls with a diameter of 1.59 mm, gently rubbed against a dielectric surface using a shaker, are seen to spontaneously exhibit a two-dimensional liquidlike order with macroscopic dimensions, viz., interball distances of several millimeters. This liquidlike order transforms to a gaslike order through coexistence upon decreasing the area fraction of the balls. The measured pair interaction of like-charged balls surprisingly exhibits a long-range attractive term analogous to that in charged colloids.

Role of Silicone Surfactant in Flexible Polyurethane Foam.

Grafted copolymers which consist of a polydimethylsiloxane backbone and polyethylene oxide-co-propylene oxide pendant groups are used as surfactants to stabilize the foam cells in the flexible polyurethane foaming process. The mechanical properties of the cured polyurethane foam such as air permeability and foam cell size are affected significantly by the structure of the silicone surfactant used in the formulation. It is shown that silicone surfactant has an important impact on both the bubble generation and the cell window stabilization stage. A series of silicone surfactants with different structures was tested. Surfactants with higher silicone content will provide lower surface tension and thus help increase the number of air bubbles introduced during mixing. These air bubbles serve as the starting point for foam cell growth. As a result, the cured polyurethane foam made with higher silicone content surfactant has a smaller bubble size. It is also shown that silicone surfactant can reduce the cell window drainage rate due to the surface tension gradient along the cell window. The Gibbs film elasticity, the dynamic film elasticity, and the film drainage rate were measured for the first time versus surfactant composition. Surfactants with longer siloxane backbones are shown to give higher film elasticity. Using the vertical film drainage and foam column tests, it is shown that surfactants with higher film elasticity will yield slower drainage rate and better foam cell stability. Copyright 1999 Academic Press.

A Molecular Theory of the Hydration Force in an Electrolyte Solution.

A model, consisting of a pair of large macroions in a dipolar hard sphere-point ion electrolyte, is considered in order to evaluate the hydration force (solvent-mediated) contribution to the force between colloidal particles, which is missing in the DLVO theory. Using the mean spherical approximation (MSA), an explicit expression for this force is obtained. It is shown that the force consists of the hard-core exclusion term that was proposed recently by Henderson and Lozada-Cassou (HLC) [J. Colloid Interface Sci. 121, 486 (1988)], and a dipole alignment contribution that originates from the orientational ordering of the solvent molecules near the colloidal particles. The long-range asymptotic form of the total force is given by a Coulomb contribution and is described by the Poisson-Boltzmann or Derjaguin-Landau-Verwey-Overbeek (DLVO) result. The hydration force is short-ranged and extends about ten solvent layers and is responsible for the oscillations of the total force. The total force that we obtain is similar to the semiempirical result of HLC. The comparison with the experimental results for a 10(-3) M KCl electrolyte solution is discussed. Copyright 1999 Academic Press.

Biconical Bob Oscillatory Interfacial Rheometer.

This paper describes a biconical bob oscillatory interfacial rheometer designed to measure the dynamic viscoelastic response of a liquid-liquid interface subjected to a small amplitude oscillatory shear stress. This instrument is used to examine the rheological behavior of interfaces in the presence of surfactants, especially macromolecular types. Rheological parameters are calculated from a hydrodynamic analysis incorporating a linear viscoelastic interfacial rheological model. The general response of this instrument is compared with the oscillatory deep channel interfacial rheometer which is also capable of similar measurements. Measurements of interfacial viscoelasticity for the same liquid-liquid system with the two rheometers, the biconical bob and the deep channel rheometers, are shown to be comparable. This study demonstrates the intrinsic nature and, therefore, the instrument independence of these dynamic interfacial rheological properties. Accurate measurements of interfacial shear viscoelasticity can be carried out over a wide range of systems by combining measurements with the oscillatory interfacial rheometers. The limitations and regime of usefulness of these instruments are discussed. Copyright 1998 Academic Press.

The Effect of Many-Body Interactions on the Sedimentation of Monodisperse Particle Dispersions

An experimental investigation was made of the sedimentation rate of low-charged monodisperse silica and polystyrene latex particle dispersions as a function of the particle volume fraction. It was found that the normalized sedimentation velocity U/U0, corrected for the effect of the two-body hydrodynamic interaction, increases with the particle volume fraction, which indicates that the degree of particle aggregation inside the dispersions increases with the particle volume fraction. This phenomenon results from attractive many-body hydrodynamic interactions between colloidal particles. It is reported for the first time that the many-body hydrodynamic interaction becomes important at the particle concentration of 6.5 vol% in monodisperse dispersions, and the many-body thermodynamic interaction is negligible at a low particle concentration, i.e., less than 15 vol%. The effect of many-body hydrodynamic interaction on the particle microstructure was also experimentally examined by using a nondestructive Kossel diffraction technique based on the principle of back-light scattering. It was found that the particle packing structure inside the dispersion initially becomes more ordered with the increase of the particle volume fraction. However, there is less increase in the particle ordering structure after 6 vol%. Furthermore, after the particle concentration reaches 10 vol%, the particle packing structure decreases to a value lower than that of 6 vol% due to the increased particle aggregation, as found in the sedimentation experiments. Predictions of a statistical thermodynamic model were compared with the experimental data on structure factors. It is found that particle dimerization occurs around 10 vol%, which agrees with the sedimentation results. Copyright 1998 Academic Press. Copyright 1998Academic Press

Particle Structure and Stability of Colloidal Dispersions as Probed by the Kossel Diffraction Technique

A nondestructive Kossel diffraction technique based on the principle of back-light scattering was used to characterize the structure formation which results from the interparticle interactions in colloidal dispersions. Static structure factors, radial distribution functions, and average pair potentials, which characterize the particle packing structures in model systems of hydroxylate latex dispersions, were obtained. The effects of particle concentration and polydispersity on particle structuring and dispersion stability were also studied. Computer simulations based on the Ornstein-Zernike method were compared with the experimental results. Good quantitative agreement was observed.

Dynamic Film and Interfacial Tensions in Emulsion and Foam Systems

In concentrated fluid dispersions the liquid films are under dynamic conditions during film rupture or drainage. Aqueous foam films stabilized with sodium decylsulfonate and aqueous emulsion films stabilized with the nonionic Brij 58 surfactant were formed at the tip of a capillary and the film tension was measured under static and dynamic conditions. In the stress relaxation experiments the response of the film tension to a sudden film area expansion was studied. These experiments also allowed the direct measurement of the Gibbs film elasticity. In the dynamic film tension experiments, the film area was continuously increased by a constant rate and the dynamic film tension was monitored. The measured film tensions were compared with the interfacial tensions of the respective single air/water and oil/water interfaces, which were measured using the same radius of curvature, relative expansion, and expansion rate as in the film studies. It was found that under dynamic conditions the film tension is higher than twice the single interfacial tension (IFT) and a mechanism was suggested to explain the difference. When the film, initially at equilibrium, is expanded and the interfacial area increases, a substantial surfactant depletion occurs inside the film. As a result, the surfactant can be supplied only from the adjoining meniscus (Plateau border) by surface diffusion, and the film tension is controlled by the diffusion and adsorption of surfactant in the meniscus. The results have important implications for the stability and rheology of foams and emulsions with high dispersed phase ratios (polyhedral structure).

Attractive Interaction between Similarly Charged Colloidal Particles

The pair interactions between the charged colloidal particles dispersed in a solvent are studied theoretically by the integral equation method. The pair potential of the mean forces, accounting for the effective pair interaction between colloidal particles, is calculated from the solution of the Ornstein-Zernike equation with the mean spherical approximation (MSA). An attractive interaction was found between two similarly charged colloidal particles in contrast with the purely repulsive force predicted by the Debye-Huckel theory. Such an attractive interaction provides physical insight for the "condensed" phenomena in charged colloidal dispersions, that is, the coexistence of a "condensed" phase and an "expanded" phase (voids). At the higher concentration and charge on colloidal particles, the effective pair interaction becomes oscillatory.