P. N. Pusey

Dynamic light scattering by non-ergodic media

We consider dynamic light scattering (DLS) by non-ergodic media, such as glasses or gels, in which the scattering elements are able only to make limited Brownian excursions about fixed average positions. We point out that, for such media, the time-averaged correlation function of the intensity of scattered light, the quantity obtained from a single DLS measurement, is different from the ensemble-averaged function. An expression for this time-averaged intensity correlation function is derived and its properties and experimental analysis are discussed. Some of the literature on DLS by polymer gels is re-evaluated in the light of these new theoretical predictions.

Freezing of binary mixtures of colloidal hard spheres

The freezing phase transition in a binary suspension of colloidal hard spheres of diameter ratio α=0.61 was studied by light scattering and scanning electron microscopy. The suspensions consisted of sterically stabilized poly(methyl methacrylate) spheres of diameters about 670 and 407 nm suspended in a near refractive indexed matched suspension medium composed of carbon disulphide and cis‐decalin. With increasing volume fraction, binary suspensions of number fraction of larger component A xA>0.43 crystallized to give irregularly stacked close packed crystals containing almost entirely component A. As the number fraction xA decreased, the rate of crystallization decreased. Suspensions of xA≊0.28 remained amorphous and showed glassy behavior. Suspensions of xA≊0.057 showed a complex sequence of phase behavior with coexistence of crystals of component B, the ordered binary alloy phase AB13, and a binary fluid. In suspensions with xA<0.057, the only solid phase observed was irregularly stacked close packed cr...

Detection of small polydispersities by photon correlation spectroscopy

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 po...

Investigations of microemulsions by light scattering and neutron scattering

Microemulsions formed from water, xylene, sodium dodecyl benzene sulphonate and hexanol have been investigated by three different scattering techniques, namely, time-average light scattering (l.s.), small-angle neutron scattering (SANS) and photon correlation spectroscopy (p.c.s.). The water volume fraction in this water-in-oil system, which was investigated at a constant molar ratio (hexanol: sodium dodecyl benzene sulphonate) of 3.28, was varied from zero to 0.533. Over most of the volume-fraction range the droplets were strongly interacting. The problem of obtaining information about the variation of particle size with volume fraction from scattering techniques in strongly interacting systems is discussed in some detail and a possible theoretical approach suggested based on the use of a hard-sphere interaction model. The combination of l.s., SANS and p.c.s. is shown to be an excellent one for problems of this type, in that l.s. and SANS can provide time-average structural information and p.c.s. dynamic information about the motion of the particles in strongly interacting systems. With the model proposed the information from the three experimental techniques was found to be self-consistent. For the system investigated the radius of the water core of the droplet was found to increase with increase in the water volume fraction whilst apparently maintaining a constant water–oil interfacial area.

Measurement of the wave‐vector dependent diffusion coefficient in concentrated particle dispersions

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.

Diffusion in concentrated hard sphere dispersions: Effects of two and three particle mobilities

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.

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.

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.

Diffusion of polystyrene in solution studied by photon correlation spectroscopy

The diffusion behaviour in toluene, and in some cases toluene + cyclohexane mixtures, of three polystyrenes, two of narrow molecular weight distribution and one of broad molecular weight distribution, has been studied using photon correlation spectroscopy. The apparent diffusion coefficient text-decoration:overlineD increases with increasing concentration in all cases. For a narrow molecular weight sample the correlation function g(1)(K, τ) shows a marked departure from a single exponential decay as the concentration increases, and the possible causes of this behaviour are considered in terms of the theory of the light scattering process. The variation of text-decoration:overlineD with concentration and with molecular weight is examined.