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...

The relevance of anisotropic interactions in colloidal systems has recently emerged in the context of the rational design of new soft materials. Patchy colloids of different shapes, patterns and functionalities are considered the new building blocks of a bottom-up approach toward the realization of self-assembled bulk materials with predefined properties. The ability to tune the interaction anisotropy will make it possible to recreate molecular structures at the nano- and micro-scales (a case with tremendous technological applications), as well as to generate new unconventional phases, both ordered and disordered. Recent theoretical studies suggest that the phase diagram of patchy colloids can be significantly altered by limiting the particle coordination number (that is, valence). New concepts such as empty liquids—liquid states with vanishing density—and equilibrium gels—arrested networks of bonded particles, which do not require an underlying phase separation to form—have been formulated. Yet no experimental evidence of these predictions has been provided. Here we report the first observation of empty liquids and equilibrium gels in a complex colloidal clay, and support the experimental findings with numerical simulations.

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.

The addition of enough non-adsorbing polymer to a hard-sphere suspension causes the particles to aggregate to form a space-Ðlling gel. The integrity of the gel persists for a Ðnite period of time, and then the space-Ðlling structure collapses suddenly to form a denser sediment. This phenomenon of " delayed sedimentation Ï is ubiquitous in many weakly-Ñocculated suspensions. In this work, we observe the processes occurring in the bulk of a colloidÈpolymer gel using dark-Ðeld imaging, and probe the arrangement and dynamics of the particles in the system using two-colour dynamic light scattering. The e †ect of shear is also studied. A number of physical mechanisms relevant to a comprehensive explanation of delayed sedimentation are proposed and discussed.

Using experiments, theory and simulations, we show that the arrested state observed in a colloidal clay at intermediate concentrations is stabilized by the screened Coulomb repulsion (Wigner glass). Dilution experiments allow us to distinguish this high-concentration disconnected state, which melts upon addition of water, from a low-concentration gel state, which does not melt. Theoretical modelling and simulations reproduce the measured Small Angle X-Ray Scattering static structure factors and confirm the long-range electrostatic nature of the arrested structure. These findings are attributed to the different timescales controlling the competing attractive and repulsive interactions.PACS numbers: 82.70. Dd, 64.70.kj, 64.70.pv Dynamical arrest in soft colloidal systems has recently become the subject of an intense research activity. The fine tuning of control parameters opens the possibility to tailor the macroscopic properties of the resulting nonergodic states. Several mechanisms of dynamical arrest have been identified. Building on the knowledge on the hard spheres glass [1], it has become recently clear that when both attractive and repulsive terms are present in the interaction potential, a re-entrant liquid-glass line, surrounded by two distinct glasses, has been predicted and experimentally observed in short-ranged attractive colloids at high concentrations [2]. A rich phenomenology also takes place at low concentrations: here gelation occurs, which may result from different routes [3]. Interesting scenarios arise when, in addition to a short-ranged attraction, particles have a residual electrostatic charge which builds up a long-range repulsion in the effective potential. In this case, particles can form equilibrium clusters [4], which provide the building blocks of arrest [5]. Recent works have shown that both Wigner glasses [6], intended as arrested states formed by disconnected particles or clusters and stabilized by the electrostatic repulsion, and equilibrium gels, which occur at larger packing fractions when the clusters branch into a percolating network, can form under these conditions [7,8].To investigate the formation of multiple arrested states, colloidal clays [9, 10] have emerged as suitable candidates. The anisotropy of the particles, combined with the presence of attractive and repulsive terms in the interactions, makes the phase diagram of such colloidal systems very complex. Among these, Laponite suspensions have been widely studied not only for their appealing industrial applications [11] but also for their interesting experimental/theoretical properties [12][13][14][15][16][17][18][19][20]. In particular, Laponite displays a non-trivial aging dynamics [15] and (at least) two final arrested states, which are obtained by a simple increase of Laponite volume fraction from low (C w < 2.0%) to moderate (C w ≥ 2.0%) values, at fixed salt concentration C s = 10 −4 M [15, 17].More recently, the static properties of these two states have been investigated in detail [18], showing that they are...