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A light scattering spectrometer is described that is used for the study of crystallization in hard sphere colloidal suspensions. Collimated laser light is Bragg scattered from the crystals as they form, and the diffracted light is focused by a liquid filled hemispherical lens onto low cost charge coupled device array detectors that are rotated about the optical axis to average the intensities around the whole Debye–Sherrer cone of scattered light. The temperature of the sample can be controlled, and this is utilized to control the amount of scattering from the sample. The spectrometer exploits the refractive index match of the colloidal particles, the solvent, the bath liquid, and the glass used for both the sample bottle and the hemispherical lens. Parameters measured are crystallization rate, the amount of crystal formed, and the average lattice spacing, all as functions of time. Results presented show that this spectrometer gives insight into the crystallization process more than 1 order of magnitude o...

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.

Using the Verlet–Weis hard sphere radial distribution functions and the Monte Carlo method we calculate the short time self‐ and mutual‐diffusion constants for a hard sphere dispersion at volume fractions up the disorder/order transition. The computations are based on two and three particle mobility tensors. The results show that the two particle mobility tensor alone is completely inadequate at high particle concentrations and considerable improvement is obtained when three particle interactions are included.

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.

We consider dilute suspensions of homogeneous polydisperse spherical particles for which the Rayleigh–Gans–Debye (RGD) approximation is valid. For two model particle size distributions we calculate the dependence on scattering vector Q of the average scattered intensity I(Q) and the effective diffusion coefficient De(Q) obtained from the first cumulant measured by photon‐correlation spectroscopy. If the mean particle radius R is large enough (≳170 nm) that the intensity form factor P(QR) shows at least one minimum in the accessible range of Q, we find that De(Q) exhibits a characteristic variation with Q which is very sensitive to the sample polydispersity. Under favorable conditions it should be possible to measure polydispersities (standard deviation/mean size) as small as 0.01. These theoretical considerations are supported, at least qualitatively, by experiment. We also discuss briefly the effect of relaxing the RGD approximation and the implications of this work for the more common PCS probes of polydispersity such as Laplace transformation of the light scattering correlation function.

Measurements are reported of the wave‐vector dependent diffusion coefficient D(q) in the region of the first two peaks in the static structure factor for concentrated nonaqueous dispersions of polymethylmethacrylate spheres (diam≂0.4 μm). The particle volume fractions are taken up to the point of spontaneous crystallization. In order to minimize the effects of multiple scattering in these dynamic light scattering experiments, the particles are dispersed in solvent mixtures having refractive indexes close to those of the particles. The measured data for D(q) are compared with the results of two recent theoretical predictions, viz. the approach of Beenakker and Mazur based on a partial resummation of many‐particle interactions, and the empirical effective two‐particle mobility method of Snook et al.

In view of the practical difficulties already encountered plus those anticipated in the implementation of many‐body hydrodynamic interactions, we examine a number of empirically screened two particle mobility tensors. The empirical screening constant(s), which depend on the dispersion concentration, are adjusted to give the best agreement with available experimental mutual and local self‐diffusion constants for concentrated hard sphere dispersions. These diffusion constants are obtained experimentally from the first cumulant of the coherent intermediate scattering function in the small and large wave vector limits. We find that the experimental data can only be satisfactorily reproduced by employing a pair mobility tensor containing two adjustable screening terms. The predictions of the final mobility tensor at intermediate wave vectors are also qualitatively consistent with experiments.

The grand canonical ensemble Monte Carlo method is used to calculate the density profile of a simple dense liquid, under conditions close to the vapor line, between two solid bodies and also the solvation force between the solids due to the simple fluid. The force is large compared with the van der Waals force at moderate surface separations, h, but is an oscillatory function of h. At small values of h the solvation force is strongly repulsive.

Brownian dynamics computer simulation has been employed in the diffusive limit to calculate the space‐time correlation functions and intermediate scattering functions of concentrated dispersions. At particle concentrations close to crystallization the results indicate that the magnitude of the departure of the single particle probability density from Gaussian behavior is comparable with that calculated and observed for atomic liquids. Also, the inclusion of tensorial hydrodynamic interactions between the particles has only a marginal effect on the magnitude of the non‐Gaussian terms.

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.

The grand canonical ensemble Monte Carlo method is used to calculate the distribution of liquid molecules and the solvation force between two parallel solid surfaces separated by a simple liquid. Neither this distribution nor the solvation force are very sensitive to changes in the chemical potential (i.e., bulk density) of the interstitial liquid.

Solvation forces in simple dense fluids, monolayer and submonolayer region, is explored. (AIP)