Crystallization mechanism of hard sphere glasses.

In supercooled liquids, vitrification generally suppresses crystallization. Yet some glasses can still crystallize despite the arrest of diffusive motion. This ill-understood process may limit the stability of glasses, but its microscopic mechanism is not yet known. Here we present extensive computer simulations addressing the crystallization of monodisperse hard-sphere glasses at constant volume (as in a colloid experiment). Multiple crystalline patches appear without particles having to diffuse more than one diameter. As these patches grow, the mobility in neighboring areas is enhanced, creating dynamic heterogeneity with positive feedback. The future crystallization pattern cannot be predicted from the coordinates alone: Crystallization proceeds by a sequence of stochastic micronucleation events, correlated in space by emergent dynamic heterogeneity.

Crystallization and aging in hard-sphere glasses.

We report new results from our programme of molecular dynamics simulation of hard-sphere systems, focusing on crystallization and glass formation at high concentrations. First we consider a much larger system than hitherto, N = 86 400 equal-sized particles. The results are similar to those obtained with a smaller system, studied previously, showing conventional nucleation and growth of crystals at concentrations near melting and crossing over to a spinodal-like regime at higher concentrations where the free energy barrier to nucleation appears to be negligible. Second, we investigate the dependence on the initial state of the system. We have devised a Monte Carlo 'constrained aging' method to move the particles in such a way that crystallization is discouraged. After a period of such aging, the standard molecular dynamics programme is run. For a system of N = 3200, we find that constrained aging encourages caging of the particles and slows crystallization somewhat. Nevertheless, both aged and unaged systems crystallize at volume fraction φ = 0.61 whereas neither system shows full crystallization in the duration of the simulation at φ = 0.62, a concentration still significantly below that of random close packing.

Differential dynamic microscopy of bacterial motility.

We demonstrate a method for the fast, high-throughput characterization of the dynamics of active particles. Specifically, we measure the swimming speed distribution and motile cell fraction in Escherichia coli suspensions. By averaging over ∼10(4) cells, our method is highly accurate compared to conventional tracking, yielding a routine tool for motility characterization. We find that the diffusivity of nonmotile cells is enhanced in proportion to the concentration of motile cells.

Hard spheres: crystallization and glass formation.

Motivated by old experiments on colloidal suspensions, we report molecular dynamics simulations of assemblies of hard spheres, addressing crystallization and glass formation. The simulations cover wide ranges of polydispersity s (standard deviation of the particle size distribution divided by its mean) and particle concentration. No crystallization is observed for s>0.07. For 0.02

Crystallization of hard-sphere glasses.

We study by molecular dynamics the interplay between arrest and crystallization in hard spheres. For state points in the plane of volume fraction (0.54 varphi_{g}. This happens on time scales for which the system is aging, and a diffusive regime in the mean square displacement is not reached; by those criteria, the system is a glass. Hence, contrary to a widespread assumption in the colloid literature, the occurrence of spontaneous crystallization within a bulk amorphous state does not prove that this state was an ergodic fluid rather than a glass.

Dynamics of a colloid-stabilized cream.

We use x-ray photon correlation spectroscopy to investigate the dynamics of a high-volume-fraction emulsion creaming under gravity. The dodecane-in-water emulsion has interfaces stabilized solely by colloidal particles (silica). The samples were observed soon after mixing: as the emulsion becomes compact we discern two regimes of aging with a crossover between them. The young emulsion has faster dynamics associated with creaming in a crowded environment accompanied by local rearrangements. The dynamics slow down for the older emulsion, although our studies show that motion is associated with large intermittent events. The relaxation rate, as seen from the intensity autocorrelation function, depends linearly on the wave vector at all times; however, the exponent associated with the line shape changes from 1.5 for young samples to less than 1 as the emulsion ages. The combination of ballisticlike dynamics, an exponent that drops below 1, and large intermittent fluctuations has not been reported before to our knowledge.

Spinodal decomposition in a model colloid-polymer mixture in microgravity.

We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.

Stability of the binary colloidal crystals AB2 and AB13.

Suspensions of binary mixtures of hard-sphere poly-methylmethacrylate colloidal particles were studied at six different size ratios alpha. The main aim was to determine the range of size ratios over which the binary colloidal crystals AB2 and AB13 are stable. Combining these results with those of earlier work, we found stability of AB2 for 0.60 approximately > alpha approximately > 0.425, in good agreement with theoretical predictions by computer simulation and cell model methods. AB13 was observed for 0.62 approximately > alpha approximately > 0.485, the lower limit being significantly smaller than the theoretical prediction of about 0.525. Rough measurements of crystallization rates showed that AB2 tended to crystallize fastest at small size ratios, whereas the opposite was true for AB13. These findings should provide a guide to the optimum conditions for materials applications of these binary colloidal crystals.

Glasses in hard spheres with short-range attraction.

We report a detailed experimental study of the structure and dynamics of glassy states in hard spheres with short-range attraction. The system is a suspension of nearly hard-sphere colloidal particles and nonadsorbing linear polymer which induces a depletion attraction between the particles. Observation of crystallization reveals a reentrant glass transition. Static light scattering shows a continuous change in the static structure factors upon increasing attraction. Dynamic light scattering results, which cover 11 orders of magnitude in time, are consistent with the existence of two distinct kinds of glasses, those dominated by interparticle repulsion and caging, and those dominated by attraction. Samples close to the "A3 point" predicted by mode coupling theory for such systems show very slow, logarithmic dynamics.

Structural aging of crystals of hard-sphere colloids.

We report a detailed experimental study of the aging of the (initial) random hexagonal close-packed (rhcp) crystals formed in suspensions of hard-sphere colloids near the melting point. By suspending the same colloidal particles in two different mixtures of solvents we are able to tune the strength of the gravitational forces acting on the particles. The crystal structure is deduced from diffraction patterns measured by the light scattering equivalent of powder x-ray crystallography. A spontaneous aging of the structure is observed over long periods of time, consisting of a fraction of pure face-centered cubic (fcc) crystals growing at the expense of the randomly stacked crystallites. The rate of growth of the new crystals is small and consistent with the predictions by Pronk and Frenkel [13]. Gravity is also revealed to affect the crystals and favor fcc order but through a slow gradual rearrangement of the random stacking. An important new observation is that small mechanical perturbations can strongly affect the structure of the colloidal crystals, promoting fcc growth and interfering with the spontaneous aging process. Previous experimental results are also discussed in the light of these new findings.

Multiple glassy states in a simple model system.

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.

Rearrangements in hard-sphere glasses under oscillatory shear strain.

We investigate particle rearrangements in colloidal glasses subjected to oscillatory shear strain by the technique of light scattering (LS) echo. LS echo directly follows the motion of the particles through peaks (echoes) in the intensity autocorrelation function; the height of the peak measures the reversible motion in the sample. Polydisperse hard-sphere poly-methylmethacrylate particles were used to avoid crystallization under shear. The yielding behavior is monitored through irreversible particle rearrangements at several volume fractions in the glass phase region. At high volume fractions the glasses are found to yield at strains as high as 15% while the irreversible rearrangements have a more gradual onset with strain for low volume fraction glasses. The behavior of high order echoes at long times is related to the effects of shear on the frozen-in fluctuations of the glass.

Phase separation in star-polymer-colloid mixtures.

We examine the demixing transition in star-polymer-colloid mixtures for star arm numbers f=2,6,16,32 and different star-polymer-colloid size ratios 0.18< or =q< or =0.50. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecular computer simulations. The numerical results show a spinodal instability. The demixing binodals are approximately calculated and found to be consistent with experimental observations.

Real-space imaging of nucleation and growth in colloidal crystallization.

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.

Multiple scattering suppression in static light scattering by cross-correlation spectroscopy.

Cross-correlation techniques have been used successfully to suppress multiple scattering in dynamic light-scattering experiments on turbid samples. This allows dynamic information to be obtained straightforwardly by processing the remaining single scattering. Here we show that cross-correlation techniques can also be used to suppress multiple scattering in static light-scattering measurements. We use the two-color dynamic light-scattering method and exploit the fact that the amplitude of the time-dependent part of the measured intensity cross-correlation function depends on the ratio of the single-scattered intensity to the total (single + multiple) scattered intensity. The method is illustrated by measurements of the static structure factors of concentrated suspensions of "hard-sphere" colloids. Good agreement is found with those calculated in the Percus-Yevick approximation.