S. M. Underwood

Tracer diffusion in concentrated colloidal dispersions. III. Mean squared displacements and self‐diffusion coefficients

The incoherent dynamic structure factor Fs has been measured on concentrated dispersions of particles with interactions approximating hard spheres. The particle mean squared displacements and self‐diffusion coefficients are obtained from Fs at times and scattering vectors where the Gaussian approximation (for Fs ) is expected to be reasonable.

EFFECT OF PARTICLE SIZE DISTRIBUTION ON CRYSTALLISATION AND THE GLASS TRANSITION OF HARD SPHERE COLLOIDS

Phase behaviour, crystallisation kinetics and particle dynamics are compared for two colloidal suspensions of hard spherical particles with different particle size distributions; one is narrow and roughly symmetrical and the other is broader and skewed towards smaller particles. Both suspensions exhibit the equilibrium phase behaviour expected for a system of identical hard spheres and they show a glass transition, indicated by the cessation of homogeneous nucleation and the partial arrest of concentration fluctuations, at approximately the same volume fraction, Φg ≈ 0.58. Interestingly, compared to the suspension with the narrower size distribution, crystallisation rates are significantly slower in the more polydisperse suspension. In its colloidal glass state no crystal growth occurs on secondary nuclei, such as the shear-aligned crystals that can be induced in both suspensions by regular rocking.

Tracer diffusion in concentrated colloidal dispersions

A colloidal tracer system suitable for light scattering experiments has been prepared. The nonaqueous dispersion comprises host and tracer particles of 0.66 μm diameter in which the refractive index of the dispersion medium is adjusted to that of the host particles. Both the host polymer particles and the inorganic tracer particles are sterically stabilized by the same polymeric adsorbed layer and, therefore, differ only in their scattering properties. Dynamic light scattering measurements were carried out on these systems at volume fractions in the range 0.36 to 0.48; this includes the coexisting disordered and crystalline phases of the dispersion.

Tracer diffusion in concentrated colloidal dispersions. II. Non‐Gaussian effects

The incoherent dynamic structure factor for concentrated dispersions of interacting, monodisperse, hard, spherical, colloidal particles has been measured by dynamic light scattering and displays weak but systematic departures from Gaussian behavior. At particle concentrations just below the crystallization transition, these non‐Gaussian effects are comparable with those measured and calculated for simple atomic liquids.

Particle diffusion in concentrated dispersions

Three dynamic light scattering experiments on concentrated non-aqueous dispersions of spherical particles are discussed. The first two consist of measurements of the diffusion of tracer particles in different systems. In each case, host dispersions were rendered transparent by adjusting the refractive index of the dispersion medium to be the same as that of the particles. Trace amounts of particles of different refractive index, but of similar size and sterically stabilized by the same polymeric layer as the host particles, were added to the host dispersion. Thus the tracer particles provided the dominant incoherent light scattering. The measured correlation functions were analysed to provide particle mean-square displacements from which short- and long-time self-diffusion coefficients were obtained. In the third experiment the coherent scattering from concentrated dispersions of a single particle species was studied up to very high concentrations. Clear evidence of the glass transition, recently predicted, was found.

Dynamics of hard spherical colloids from the fluid to the glass

Particle dynamics, measured by dynamic light scattering, in hard-sphere colloidal dispersions over a range of volume fractions covering the equilibrium and metastable phases are discussed. The dynamics are considered in the framework of the phase behaviour of the dispersion which mimics that of a simple atomic system.

Refractive index variation in nonaqueous sterically stabilized copolymer particles

We present procedures for preparing sterically stabilized polymer particles whose refractive index can be controlled over a range of a few percent. Particle sphericity and size distribution are such that suspensions crystallize at high concentrations. This at least ensures that Brownian motion dominates over particle settling and that the polydispersity is no more than about 10%. Of particular interest are new particles comprising poly(methylmethacrylate-co-trifluoroethylacrylate) that can be optically matched in single solvent, namely cis-decalin.

Colloidal fluids, crystals and glasses

Various aspects of the behaviour of essentially hard spherical colloidal particles, suspended in a liquid, are outlined. The authors consider the phase behaviour and crystal structure of one- and two-component suspensions and the glass transition of a one-component system.

OBSERVATION OF COLLOIDAL CRYSTALS WITH THE CESIUM CHLORIDE STRUCTURE

Binary mixtures of hard sphere-like colloidal particles, of the same size but different polymer composition, when suspended in some liquids phase separate and form crystals with the Cesium Chloride structure. We attribute this behavior to a cross-attraction between particles of the different species. In other suspending liquids this apparent cross-attraction is absent; hard sphere-like behavior is observed with the expected randomly stacked close packed crystal structure with no positional correlation of the two particle types.

Particle Diffusion and Crystallisation in Suspensions of Hard Spheres

Hard sphere colloidal suspensions composed of polymer particles in a non-aqueous solvent are studied using light scattering techniques. Single particle motions in the metastable fluid are obtained from measurement of the incoherent intermediate scattering function. The emergence of crystalline order is observed by monitoring the development of the main Bragg reflection