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PACS. 82.70 -Disperse systems. PACS. 64.75 -Solubility, segregation, and mixing.Abstract. -A new treatment of the phase behaviour of a colloid + nonadsorbing polymer mixture is described. The calculated phase diagrams show marked polymer partitioning between coexisting phases, an effect not considered in the usual effective-potential approaches to this problem. We also predict that under certain conditions an area of three-phase coexistence should appear in the phase diagram.Introduction. -Phase separation in colloidal suspensions, induced by the addition of nonadsorbing polymer, is a phenomenon of fundamental interest and considerable technological importance. A theoretical explanation was first advanced by Asakura and Oosawa [1], and also independently by Vrij [2], based on the exclusion of polymer from the region between two colloid particles when their surface-surface separation becomes smaller than the diameter of a free polymer coil. The resulting imbalance in osmotic pressure gives rise to an effective attractive «depletion» force between the colloid particles [3,4]. At high enough concentration of polymer this depletion force causes the suspension to separate into colloid-poor and colloid-rich phases. In the latter the particles can, depending on conditions (see below), be in either liquidlike or crystalline spatial arrangements.To predict the phase diagram of a colloid + polymer mixture, most workers to date have adopted an approach in which the depletion potential (an effective pair potential) is added to the parent interparticle potential; thermodynamic perturbation theory is then used to calculate phase stability boundaries [5,6]. Although experimental studies [6,7] show qualitative agreement with the predictions of these calculations, an important reservation

Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.

Experiments, theory, and simulation were used to study glass formation in a simple model system composed of hard spheres with short-range attraction ("sticky hard spheres"). The experiments, using well-characterized colloids, revealed a reentrant glass transition line. Mode-coupling theory calculations and molecular dynamics simulations suggest that the reentrance is due to the existence of two qualitatively different glassy states: one dominated by repulsion (with structural arrest due to caging) and the other by attraction (with structural arrest due to bonding). This picture is consistent with a study of the particle dynamics in the colloid using dynamic light scattering.Understanding the glass transition is an outstanding challenge for statistical and condensed-matter physics, with relevance throughout materials science as well as biology (1-3). In the multidisciplinary quest for understanding of glasses, the study of simple model systems occupies an important place. One of the simplest models amenable to theoretical study as well as experimentation is a collection of N hard spheres of radius R in volume V at density (volume fraction) ϭ (4/3)R 3 N/V. Although there have been speculations about a hardsphere glass at least since Bernal (4), substantial progress began in the 1980s with modecoupling theory (MCT) calculations (5) and experiments using colloids (6, 7). Further predictions from MCT have been substantially confirmed by colloid experiments and simulations (8), and novel features, such as spatially inhomogeneous particle dynamics, are still being revealed by new experimental probes (9). This close interplay between experiment, theory, and simulation has helped to give hard spheres the status of a reference system.In a system of hard spheres, particles are increasingly caged by their neighbors as increases. At a critical density, g , this caging becomes effectively permanent, stopping all long-range particle motion, and the system can be considered nonergodic, or glassy. MCT captures the essential nonlinear feedback in this mechanism. Each particle is both caged and forms part of the cage of its neighbors. We present a combined experimental, theoretical, and simulational study of how the hard-sphere glass transition is perturbed by a short-range interparticle attraction ("stickiness"). We find that such an attraction first "melts" the hardsphere glass, and then a second, qualitatively different, glassy state is formed (Fig. 1). Sticky hard spheres therefore represent perhaps the simplest system in which multiple glassy states occur.In our experiments, we used sterically stabilized polymethylmethacrylate (PMMA) particles (hard-sphere radius R ϭ 202 nm, polydispersity ϭ 7%) dispersed in cis-decalin. Computer simulations (10) predict that below ϭ 0.494, the lowest free energy state is an ergodic fluid consisting of amorphously arranged particles exploring all available space. For 0.494 Ͻ Ͻ 0.545, fluid and crystal coexist. Above ϭ 0.545, the system should fully crystallize. PMMA colloids follow this predi...

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Concentrated suspensions of submicron colloidal spheres were studied both by dynamic light scattering and by direct observation of their phase behavior. In agreement with recent theory and computer simulation, the measured dynamic structure factor developed an essentially nondecaying component, implying "structural arrest, " at almost the same concentration as that at which a long-lived amorphous or glass phase was first observed. PACS numbers: 64.70.Pf, 78.20.Dj, 82.70.Kj The idea that sufficiently rapid compression should transform a liquid composed of spherical particles into a long-lived metastable amorphous solid or glass dates back at least to the sphere-packing experiments of Bernal' and Scott and to the free-volume model of Cohen and Turnbull.More recently this simple "glass transition" has been studied in a number of computer experiments.Since 1984 the subject has gained impetus with the prediction of such a glass transition by kinetic and hydrodynamic theories of liquids which incorporate a nonlinear feedback mechanism.The transition from the ergodic liquid state to the nonergodic glass is signaled by divergence of the shear viscosity, by vanishing of the self-diffusion coefficient, and by "structural arrest, " the partial freezing-in of density fluctuations.To date these ideas have not been tested on real systems composed of spherical molecules because compression and/or temperature-quench rates high enough to bypass crystallization in a controllable fashion are not attainable. ' In this Letter we report the observation of a glass transition in concentrated suspensions in a liquid of solid submicron colloidal spheres having a narrow distribution of size. The static ' ' and dynamic ' properties of suspensions of identical spheres have many features in common with those of simple liquids. In particular, the full range of phase behavior, fluid crystal glass, is observed. ' However, the relaxation times of the diA'usive motions of particles in suspension are at least 10 times larger than those of atoms in a liquid. Thus the lifetimes of the metastable fluid phases of suspensions, observed before significant crystallization takes place, can be long enough, minutes to hours (see below), to allow detailed study of their properties. Here we compare measurements by dynamic light scattering (DLS) of F(Q, r) [Eq. (3)], the temporal correlation function of particle-density fluctuations, in the metastable fluid and glass phases with both theoretical predictions ' and a recent computer simulation. ' Good qualitative agreement is observed. Our main finding is that F(g, r) develops an essentially nondecaying component, associated with structural arrest, at almost exactly the same suspension concentration as that at which long-lived colloidal glasses are first observed. ' A useful, if oversimplified, picture of this archetypal glass transition can be given in terms of a neighbor cage in which the motion of individual particles is partially or completely constrained.At normal liquid densities, a particle is ab...

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