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In this article, we examine the collective particle dynamics, as expressed by the time correlation function of the longitudinal particle current density, of several different fluids in the vicinity of their freezing points/lines. We consider and compare results obtained by dynamic light scattering for a suspension of hard spheres and by molecular dynamics for fluids with hard sphere and Lennard-Jones interactions. The latter are performed along both an isotherm and an isochore. In all cases, we find a qualitative change in the collective dynamics, within the resolution of the data, when their respective freezing lines are crossed. We associate this change with the onset of caging. The new results for the Lennard-Jones fluid reported here confirm that the occurrence of caging, found previously for systems of hard spheres, is a more general feature that distinguishes a metastable fluid from one in thermodynamic equilibrium.

The cage effect is generally invoked when discussing the delay in the decay of time correlation functions of dense fluids. In an attempt to examine the role of caging more closely we consider the spread of the displacement distributions of Brownian particles. These distributions are necessarily biased by the presence of neighbouring particles. Accommodation of this bias by those neighbours conserves the displacement distribution locally and presents a collective mechanism for exploring configuration space that is more efficient than the intrinsic Brownian motion. Caging of some particles incurs, through the impost of global conservation of the displacement distribution, a delayed, non-local collective process. This non-locality compromises the efficiency with which configuration space is explored. Both collective mechanisms incur delay or stretching of time correlation functions, in particular the particle number and flux densities. This paper identifies and distinguishes these mechanisms in existing data from experiments and computer simulations on systems of particles with hard sphere interactions., Comment: 18 pages, 5 figures

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

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...

When a suspension of hard spheres traverses the freezing volume fraction we find discontinuous changes in the character of the tagged particle density. In particular, the velocity auto-correlation function develops a negative algebraic decay and the fluctuations become subject to interruption. From these, and the exponent of the algebraic growth of the non-Gaussian parameter, the difference in mode of relaxation of the density fluctuations between the stable and metastable colloidal fluids can be quantified. A diagrammatic scheme is proposed that reconciles the dynamics of phase transitions observed in hard-sphere colloids.

The conventional view of freezing holds that nuclei of the crystal phase form in the metastable fluid through purely stochastic thermal density fluctuations. The possibility of a change in the character of the fluctuations as the freezing point is traversed is beyond the scope of this perspective. Here we show that this perspective may be incomplete by examination of the time autocorrelation function of the longitudinal current, computed by molecular dynamics for the hard-sphere fluid around its freezing point. In the spatial window where sound is overdamped, we identify a change in the long-time decay of the correlation function at the known freezing points of monodisperse and moderately polydisperse systems. The fact that these findings agree with previous experimental studies of colloidal systems in which particle are subject to diffusive dynamics, suggests that the dynamical signature we identify with the freezing transition is a consequence of packing effects alone.

We examine crystallization by settling for two different model hard-sphere suspensions. Sedimentation velocities, internal shock velocities, and crystal growth velocities are measured. Dynamic light scattering experiments measure volume fraction profiles in fluid phases, while Bragg scattering experiments determine volume fraction profiles in crystal phases. Centrifugation experiments determine the particle Péclet number above which samples will not crystallize. The sedimentation velocities, as a function of volume fraction, agree with other "hard-sphere" data. Remarkably, the value of the reduced crystal growth velocity (approximately 0.075) obtains for two orders of magnitude of the particle Péclet number. Kynch theory provides an adequate description of the data in the fluid phase, but is less adequate for volume fraction profiles in the crystal phase. The crystals in the dense sediment are compressed more along a vertical axis relative to the horizontal axis. Predicted Wilson-Frenkel crystal growth velocities, calculated using known hard-sphere equations of state and a short-time self-diffusion constant, rationalize the measured crystal growth velocities.

Dynamic light-scattering measurements are reported for suspensions at concentrations in the vicinity of the glass transition. In a mixture of identically sized but optically different particles having hard-sphere-like interactions, we project out the incoherent (or self-) intermediate scattering functions by adjusting the refractive index of the suspending liquid until scattering from the structure is suppressed. Due to polydispersity, crystallization is sufficiently slow so that good estimates of ensemble-averaged quantities can be measured for the metastable fluid states. Crystallization of the suspensions is still exploited, however, to set the volume fraction scale in terms of effective hard spheres and to eliminate (coherent) scattering from the structure. The glass-transition volume fraction is identified by the value where large-scale particle motion ceases. The nonequilibrium nature of the glass state is evidenced by the dependence on the waiting time of the long time decay of the relaxation functions. The self-intermediate scattering functions show negligible deviation from Gaussian behavior up to the onset of large-scale diffusion in the fluid or the onset of waiting time effects in the glass.

Structural relaxation of metastable fluids and the kinetics of subsequent crystallization are measured by dynamic light scattering and by the growth of the main interlayer reflection in binary mixtures of colloidal polymer particles with hard-sphere-like interactions. The size ratios of the particles lie in the range for which eutectic phase behavior is expected. We find that at a constant volume fraction, close to the melting value of the one-component hard-sphere solid, the addition of up to about 10% by volume of the second component has negligible effect on the particle dynamics in the metastable fluid but significantly retards crystal nucleation and growth.

We model interactions between charged colloidal particles by the screened Coulomb potential, and employ the analytically solved static structure factor in conjunction with the idealized mode-coupling theory to determine the liquid-glass transition phase boundary. We find that the presence of an ionic screening has the consequence of revealing a subtle competition between the hard-core geometric factor and the charge-induced screening effect, resulting, for the strong screening case, in an extended charge-stabilized suspension and, for the weak screening case, in a more restrictive ergodic domain. @S1063-651X~97!09807-3# The occurrence, in metastable liquid well above the calorimetric glass transition point, of a dynamical transition m c ~which is the temperature for quenching or density for supercompressing!, from ergodic to nonergodic behavior, has been the subject of considerable experimental and theoretical interest since it was first predicted by mode-coupling theory ~MCT !@ 1 #. The idealized version of MCT, which takes into account coupling of the density fluctuations only @2# ,i s a formal generalization of the equilibrium kinetic theory of liquids and of the Vlasov plasma theory. This theory has been applied to different monatomic systems such as neutral hard spheres @3#, charged hard spheres @4#, Lennard-Jones atoms @5#, and pure liquid metals @6#. Light @7# and neutron @8# scattering experiments on model colloids and a variety of molecular glass forming materials @9# suggest the existence of the predicted m c , and find compatibility with the scaling laws predicted by MCT despite the fact that, in many cases, the systems studied are far more complex than the onecomponent fluid for which the theory was developed. However, interpretation of both the experimental data and the theoretical predictions in terms of MCT is not without controversy @10#.

The crystallization of suspensions of sterically stabilized polymer particles, with hard-sphere-like interactions, is studied by laser light scattering. Over the range of volume fractions examined, from just below melting to the glass transition, crystallization occurs by homogeneous nucleation. After the suspensions are shear melted, the intensity, position, and width of the main interlayer Bragg reflection are measured as functions of time. From these the amount of crystal, the average linear crystal dimension, the number of crystals, and the volume fraction of the crystal phase are obtained. No assumptions are made concerning the functional time dependence of nucleation or growth processes. Below the melting concentration the observed crystallization process is compatible with the classical picture of sequential nucleation and growth of isolated crystals. However, when the melting concentration is exceeded, nucleation events are correlated, nucleation is accelerated, and high nucleation rate densities suppress crystal growth. Above the melting concentration we infer, with the aid of the equations of state for the hard-sphere fluid and solid, that the first identifiable crystals are in mechanical equilibrium with the embedding fluid and, consequently, strongly compressed by it. Ensuing nucleation lags expansion of the crystal lattice.

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.

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.

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.

Crystallization of a suspension of hard spherical polymer particles is measured by following the evolution of the main Bragg reflection from the close-packed planes. Crystal growth, evident below the melting concentration, is effectively suppressed at higher concentrations due to superlinear rates of crystal addition. We discuss these results in terms of classical nucleation theory. We conclude that superlinear nucleation rates may be a general characteristic of crystallization under constant volume conditions following a deep quench in systems where the crystal phase is of higher density than the fluid.

The design, alignment and operation of improved two-colour dynamic light scattering equipment which effectively suppresses multiple scattering and allows the study of the Brownian dynamics of optically turbid samples is described. The technique operates by cross-correlating scattered light of two different colours. With the appropriate scattering geometry, only singly scattered light contributes to the time-dependent part of the measured intensity cross-correlation function thus allowing straightforward interpretation of the data. To demonstrate the performance of the instrument we have studied various colloidal suspensions. Although dilute enough that free diffusion of the particles can be assumed, the samples ranged from quite transparent to distinctly turbid. We have confirmed that the measured cross-correlation functions accurately reflect only single scattering even in the presence of strong multiple scattering.

Recent experiments on suspensions of particles with steeply repulsive interactions are reviewed. These suspensions show the equilibrium freezing-melting transition and a glass transition similar to those obtained in computer simulations of the hard sphere system. The glass transition is identified, firstly, by the cessation of homogeneously nucleated crystallisation and, secondly, by failure of concentration fluctuations in the metastable colloidal fluid to decay on the experimental time scale. Intermediate scattering functions, measured by dynamic light scattering, clearly show the emergence of two slow relaxation processes as the glass transition concentration is approached from below. At the glass transition concentration the slower process is arrested. These slow relaxation processes, which span time windows of five decades, can be quantitatively described by the idealised version of mode coupling theory.

Coherent intermediate scattering functions are measured by dynamic light scattering for several wave vectors around the structure factor peak on metastable colloidal fluids and glasses of hard spherical particles. The results are quantitatively described by a combination of the \ensuremath{\alpha} and \ensuremath{\beta} processes. The scaling laws and factorization property predicted by mode-coupling theory are verified.

We report results of dynamic light scattering measurements of the coherent intermediate scattering function (ISF) of glasses of hard spheres for several volume fractions and a range of scattering vectors around the primary maximum of the static structure factor. The ISF shows a clear crossover from an initial fast decay to a slower non-stationary decay. Ageing is quantified in several different ways. However, regardless of the method chosen, the perfect "aged" glass is approached in a power-law fashion. In particular, the coupling between the fast and slow decays, as measured by the degree of stretching of the ISF at the crossover, also decreases algebraically with waiting time. The non-stationarity of this coupling implies that even the fastest detectable processes are themselves non-stationary., 28 pages, including 3 tables and 17 figures

A Comment on the Letter by G. Brambilla et al., Phys. Rev. Lett. 102, 085703 (2009). The authors of the Letter offer a Reply.

On sufficiently rapid cooling, many liquids show a glass transition (GT)1. Crystallization, which would occur on slower cooling, is bypassed and a long-lived amorphous solid or glass is formed. The transition to the glass appears to be largely dynamic rather than structural in nature. Thus, while the average structural arrangement of molecules varies only weakly through the GT, the dynamics of molecular diffusion change drastically. Given enough time, the molecules in the liquid state are able to diffuse throughout a sample. By contrast, in the glass long-distance diffusion is essentially suppressed and only local motions of the molecules about their fixed average positions in the amorphous structure are possible. It appears that the most important parameter which controls the glass transition is the density, rather than the temperature, of the system. Cooling is mainly a convenient way to increase the density at constant pressure.

The current correlation function is determined from dynamic light scattering measurements of a suspension of particles with hard spherelike interactions. For suspensions in thermodynamic equilibrium we find scaling of the space and time variables of the current correlation function. This finding supports the notion that the movement of suspended particles can be described in terms of uncorrelated Brownian encounters. However, in the metastable fluid, at volume fractions above freezing, this scaling fails.

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

Static structure factors S(q) were measured by laser light scattering for very concentrated systems of spherical, near-monosized, sterically stabilized particles dispersed in nonpolar liquids. A range of systems with particle concentrations beyond that corresponding to the disorder to order transition configurations were found to have amorphous structures. As the particles were stabilized by means of very short chain polymers, these systems were thought to closely approximate the fundamentally important amorphous, hard-sphere system. Subsequent analysis of S(q) carried out by means of data generated by the molecular-dynamics method for very concentrated, amorphous states of the hard-sphere system confirmed this interpretation. Thus we were able, by a combination of experiment and simulation, to give an extensive analysis and description of the structure of the amorphous state of a system of hard spheres. This study complements past work on the thermodynamic and transport properties of metastable, amorphous states of a system of hard spheres.

This paper describes a light-scattering study of the glass transition in nonaqueous suspensions of sterically stabilized colloidal spheres. The observed phase behavior, fluid, crystal, and glass, is consistent with an essentially hard-sphere interaction between the particles. Metastable fluid states were obtained upon shear melting the crystalline phases by tumbling the samples. Their intermediate scattering functions, measured by dynamic light scattering, showed the emergence of a nondecaying component, implying structural arrest, at essentially the same concentration as that at which homogeneously nucleated crystallization was no longer observed. The overall forms of the intermediate scattering functions are consistent with the predictions of mode-coupling theories for the glass transition. Supplementary studies of the static structure factors indicated only short-ranged spatial order for particle concentrations ranging from the equilibrium fluid through the metastable fluid to the glass.

The intermediate scattering function (ISF) is measured for a colloidal hard-sphere glass as functions of the scattering vector and waiting time. For scattering vectors near the structure factor peak, we show that the ISF and the stretching index, defined at the crossover time between the fast and slow processes, depend algebraically on the waiting time. By contrast, the Debye-Waller factor is independent of the waiting time.

By combining aspects of the coherent and self intermediate scattering functions, measured by dynamical light scattering on a suspension of hard sphere-like particles, we show that the arrest of particle number density fluctuations spreads from the position of the main structure factor peak. Taking the velocity auto-correlation function into account we propose that as density fluctuations are arrested the system's ability to respond to diffusing momentum currents is impaired and, accordingly, the viscosity increases. From the stretching of the coherent intermediate scattering function we read a quantitative manifestation of the undissipated thermal energy, the source of those, ergodicity restoring, processes that short-circuit the sharp transition to a perfect glass., Comment: 9 pages, 4 figures

A series of experiments on concentrated dispersions of hard colloidal spheres is discussed. The observed phase behavior is analogous to that of simple atomic systems: colloidal fluid, crystal and glass phases are found. The structure of the crystals, revealed by light diffraction, is a strongly faulted stacking of hexagonally-packed layers of particles. Dynamic light scattering confirms that the concentration of the metastable fluid phase for which long-ranged particle diffusion ceases coincides with the concentration where the glass transition is observed macroscopically. In studies of a binary mixture of colloidal spheres with a size ratio 0.61 eutectics, glass formation and the AB13 type alloy structure have been identified.

When suspended in a liquid, hard spherical colloidal particles can show fluid, crytalline and glassy phases. A light scattering study of the dynamics of the metastable fluid and glassy phases is reported. Comparison is made with the predictions of mode-coupling theories applied to the glass transition of simple atomic systems.

Colloidal particles are widely used both in fundamental research and in materials science. One important parameter influencing the physical properties of colloidal materials is the particle size distribution (polydispersity) of the colloidal particles. Recent work on colloidal crystallization has demonstrated that even subtle changes in polydispersity can have significant effects. In this study we present centrifugation techniques for subtly manipulating the width and the shape of the particle size distribution, for polydispersities less than 10%. We use scanning electron microscopy as well as dynamic and static light scattering to characterize the particle size distributions. We compare the results and highlight the difficulties associated with the determination of accurate particle size distributions.

Crystallization in nearly identical suspensions of hard spheres has been studied independently by small angle light scattering and by Bragg scattering. The results of those two measurement techniques are brought together here. Qualitatively different behavior is observed in different time domains. The reduced time and crystal size agrees for the different measurement techniques when compared at the boundary between the time domains. As a result the reduced nucleation rate density also correlates between the two techniques. Below the melting volume fraction a comparison of the characteristic length scale with either the small angle intensity or the Bragg crystal fraction indicates a constant nucleation rate density. Above the melting volume fraction an argument is given for accelerated nucleation. The theoretical classical nucleation rate varies more rapidy with volume fraction between melting and freezing than is observed experimentally.

The velocity autocorrelation function is derived from the mean-squared displacement measured on a colloidal suspension of particles with hard-sphere-like interactions. It decays to zero from below and follows a stretched exponential function of delay time for the thermodynamically stable suspension. For the metastable suspension a power-law decay emerges. The results are discussed in terms of the classical Lorentz gas and the model that describes diffusion confined to one dimension. With the aid of these models, the experimental results provide a characterization of the dynamical heterogeneities which are observed microscopically, and an explanation for the enhanced resistance to flow and diffusion usually found in undercooled fluids upon approaching the glass transition.

Molecular dynamics simulations are employed to obtain the velocity autocorrelation function (VAF) for hard spheres, spanning a wide range of volume fractions from dilute to high-density metastable fluids. For all volume fractions below freezing, Alder's classical positive 3/2 long-time tail is observed. For volume fractions from 0.45 to 0.48 the VAF becomes negative, before becoming positive and decaying with the positive 3/2 long-time tail. At the freezing volume fraction (0.494) the Alder 3/2 tail is not observed. At higher volume fractions a negative tail with an exponent of 5/2 emerges, which coincides with the long-time tail of a Lorentz gas.

The velocity autocorrelation function of concentrated colloidal fluids of hard-sphere particles, measured by dynamic light scattering, decays to the experimental noise floor from below. The decay follows a stretched exponential function of delay time for the colloidal fluid in thermodynamic equilibrium, and a power law for the nonequilibrium, undercooled colloidal fluid. Consideration of this difference between the two states points to a possible dynamical mechanism of freezing.

Mean-squared displacements (MSDs) of colloidal fluids of hard spheres are analyzed in terms of a random walk, an analysis which assumes that the process of structural relaxation among the particles can be described in terms of thermally driven memoryless encounters. For the colloidal fluid in thermodynamic equilibrium the magnitude of the stretching of the MSD is able to be reconciled by a bias in the walk. This description fails for the under-cooled colloidal fluid.

The creation of three-dimensional ordered colloidal crystals, for application in a range of nanotechnologies, has been a goal for many researchers in the past few years. The main difficulty in creating macroscopic sized crystals of densely packed colloidal particles is that colloidal particles always have a range of particle sizes - ie, they are polydisperse. This paper studied the crystallization kinetics of a hard-sphere colloid with a well defined Gaussian polydispersity. The authors find that crystallization occurs in two stages, and does not follow the simple classical nucleation picture. The paper discusses the implications of these results for research into colloidal crystals as possible nano-materials

Self-intermediate scattering functions (ISFs) are measured by dynamic light scattering for the colloidal fluid of hard spheres for both equilibrium and nonequilibrium (undercooled) conditions, i.e., for volume fractions below and above the known freezing transition of the hard-sphere system. The delay time tau(m) where the mean-squared displacement, or the low wave-vector limit of the ISF, exhibits its maximum stretching is identified as a characteristic of the non-Markovian process(es) and is used to separate the ISF into fast (tautau(m)) and slow (tautau(m)) contributions. Each of these contributions exposes qualitative differences in the dynamics of the particles between the equilibrium and nonequilibrium colloidal fluids. These changes in the relaxation scenario signal the colloidal fluid's awareness of its traversal of the freezing volume fraction.

In this paper we present measurements of the crystallization kinetics of binary mixtures of two different sized hard sphere particles. The growth of the Bragg reflections over time were analyzed to yield the crystallite scattering vector, the total amount of crystal, and the average linear crystal size. It was observed that a particle size distribution skewed to higher sized particles has a less detrimental effect on the crystal structure than a skew to smaller sized particles. In the latter case we observe that initial crystallite growth occurs at only a small number of sites, with further crystallization sites developing at later times. Based on these measurements we elaborate further on the previously proposed growth mechanism whereby crystallization occurs in conjunction with a local fractionation process in the fluid, which significantly affects the kinetic growth of crystallites in polydisperse systems.

We present the crystallization kinetics for two polydisperse hard-sphere particle stocks with differing particle size distributions. One of the latexes had a relatively symmetrical distribution, the other had a more polydisperse distribution, which was highly skewed to smaller sizes. The emerging Bragg reflections from the crystallizing samples were measured using a technique that provides improved statistical averaging over our previous methods. It was observed that, for the more polydisperse particles, the onset of nucleation was delayed by up to an order of magnitude in reduced time, and displayed qualitatively different growth behavior compared to the particles with the more symmetric size distribution. Based on these measurements and time lapse photographs, we propose a growth mechanism whereby crystallization occurs in conjunction with a local fractionation process near the crystal-fluid interface, which significantly alters the kinetics of crystallite nucleation and growth. This fractionation effect becomes more significant as polydispersity or skewness increases.

THE crystallization of 'hard' spheres, which interact only through repulsive forces on contact, is a purely entropic effect and provides a model system for testing theories of freezing transitions in liquids generally1. Micrometre-sized colloidal particles can be made to approximate hard spheres by grafting of polymers to their surfaces1. Their slow dynamics make them attractive systems for the study of crystallization kinetics2–4. As well as undergoing crystallization1,2, such particles also exhibit a glass transition at a well-defined volume fraction φg, which we have shown previously to be related to the cessation of large-scale diffusion on experimental timescales5,6.. Here we show that the glass transition coincides with a change in the mechanism of crystallization. At volume fractions less than φg isometric crystals are nucleated homogeneously throughout the sample, whereas above φg crystals forming slowly in the metastable glass phase are highly asymmetric. Regular rocking of the suspension above the glass transition induces more rapid formation of plate-like crystals, indicating that their nucleation is shear-induced.

Dynamic light-scattering experiments are performed on binary mixtures of hard-sphere-like colloidal suspensions with a size ratio of 0.6. The optical properties of the particles are such that the relative contrast of the two species is very sensitive to temperature, a feature that is exploited to obtain the three partial coherent intermediate scattering functions. The glass transition is identified by the onset of structural arrest, or arrest of the \ensuremath{\alpha} process, on the time scale of the experiment. This is observed in a one-component suspension at a packing fraction of 0.575. The intermediate scattering functions measured on the mixtures quantify how, on introduction of the smaller spheres, the \ensuremath{\alpha} process is released, i.e., how the glass melts. Increasing the fraction of smaller particles causes the \ensuremath{\alpha} process to speed up but, at a given wave vector, also incurs a change to its amplitude in proportion to the change in the (partial) structure factor.

Glassy states have been observed in hard-spherelike colloidal suspensions; however, some recent work suggests that a stable, one-component hard-sphere glass doesn't exist. A possible resolution of this dilemma is that colloidal glass formation results from a small degree of particle polydispersity. In order to investigate this further, we used the molecular-dynamics method to explore the phase behavior of both one- and two-component hard-sphere systems. It was found that the metastable fluid branch of the one-component system ceased to exist at a volume fraction marginally above melting, instead this system always crystallized within a relatively short period of time. Binary systems with a size ratio $\ensuremath{\gamma}=0.9$ were then used as the simplest approximation to model a polydisperse hard-sphere colloidal system. Here the crystallization process was slowed down dramatically for all volume fractions and the fluid state was maintained for many relaxation times. Indeed, at the lowest volume fraction $\ensuremath{\varphi}=0.55$ no sign of crystallization was seen on the simulation time scale. The systems at intermediate volume fractions did eventually crystallize but at the highest volume fraction of $\ensuremath{\varphi}=0.58,$ a dramatic slowing down in the crystallization process was observed. This is qualitatively in agreement with the experimental results on colloidal suspensions. Using the insight gained from this paper, the reasons behind a polydisperse system forming a stable glass, in contrast to the one-component system, are elucidated.

Particle dynamics have been measured by dynamic light scattering for mixtures of colloidal particles with hard sphere interactions. The diameter ratio (small:large) is 0.6. The optical properties of the suspended particles are such that the relative contrast of the two species is very sensitive to temperature, a feature we exploit to obtain the three partial intermediate scattering functions. The glass transition is identified by the onset of structural arrest, or the arrest of the alpha process, on the time scale of the experiment. This is observed in the one-component suspension at the packing fraction 0.57. Introduction of the smaller particles, at fixed packing fraction, releases the alpha process, ie, the glass melts. Increasing the fraction of smaller particles speeds up the alpha process but interestingly, increases its amplitude.

Dynamic light scattering measurements are reported for suspensions at concentrations around the glass transition. Self intermediate scattering functions are obtained by employing a mixture of identically interacting but optically contrasting polymer and silica particles and adjusting the refractive index of the suspending liquid until scattering from the structure is eliminated. A small spread in particle radii retards nucleation and crystal growth to an extent that allows characterization of (density) quenched metastable fluid states. The non-equilibrium nature of the glass state in evidenced by the dependence of the intermediate scattering functions on the waiting time.

Density fluctuations are monitored by small-angle light scattering during the crystallization of 0.22-\ensuremath{\mu}m-radius, hard colloidal spheres. Measured structure factors show an intensity maximum at finite-scattering vectors. The shape of the intensity distribution scales at early times during nucleation and growth and again at large times during ripening. At intermediate times there is a crossover region where scaling ceases to be valid. Both the amplitude and the position of the maximum intensity show quasi-power law behavior in time. The values of the observed exponents are within the range expected for classical growth models. The breadth of the intensity distribution increases with increasing volume fraction, suggesting greater crystal polydispersity with increasing volume fraction. The lower volume fraction intensity distributions suggest that crystals have a compound or internal structure, while the observed decrease in characteristic length in the crossover time regime may indicate breakup of crystals to this smaller internal structure. The results of measurements are compared with results calculated for nucleation and growth of crystals in suspensions of hard spheres. Results also are compared with earlier measurements made on samples containing 0.50-\ensuremath{\mu}m radius spheres. Differences in the two systems are discussed in terms of interparticle potential, polydispersity, and gravitational effects. \textcopyright{} 1996 The American Physical Society.