Multiple Glassy States in a Simple Model System

Abstract

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...

Gels and Glasses in a Single System: Evidence for an Intricate Free-Energy Landscape of Glassy Materials

“…The study of colloids allowed for a significant contribution to elucidating the basic physics of glass transition [11,12]. In colloidal systems, as the particle volume fraction is increased, the particles become increasingly slower and for even higher volume fractions the glass transition is encountered.…”

“…This should be general for colloidal systems with attractive interactions between the particles; indeed the recent discovery of ''attractive glasses'' for spherical colloids [12] also suggests that gels and glasses are not necessarily clearly distinct states of matter, but rather metastable minima in an otherwise complicated free-energy landscape, resulting from both steric and attractive interactions, as also suggested by some simulations [18]. …”

Bond formation and slow heterogeneous dynamics in adhesive spheres with long-ranged repulsion: Quantitative test of mode coupling theory

“…Weak short ranged attractions at first melt these 'repulsion-driven glasses' because they distort and loosen the local packing. At attraction strengths somewhat higher, yet still of the order of the thermal energy, physical bonds are formed in dense systems, which leads to aggregation into 'attraction driven glasses' [3,4,5]. At attraction strengths high compared to thermal fluctuations aggregation phenomena proceed far from equilibrium at low density, resulting in tenuous solids, i.e.…”

“…Additionally, by comparing the results of MCT calculations for systems with and without small wavevector pre-peak in the structure, we highlight the importance of mesoscopic heterogeneities on attraction driven dynamic arrest. We consider a system of particles interacting with a narrow attraction and a weak long-ranged repulsion whose dynamics has been studied intensely by simulations [3,28,29,30]. At the considered density of 40% packing fraction, our system is above the percolation threshold, and exhibits an equilibrium structural pre-peak at small wavevectors for parameter ranges where the barrier suppresses phase-separation.…”

Dynamical Heterogeneity and Nonlinear Susceptibility in Supercooled Liquids with Short-Range Attraction

“…Here entropydriven crowding effects give rise to a characteristic dynamical behavior that includes a two-step nonexponential relaxation, a dramatic increase in relaxation times associated with small changes in volume fraction, and dynamical heterogeneity accompanied by a growing dynamical length scale [14]. Recently, it has been demonstrated via theory [16], simulation [17,18], and experiment [19,20] that another extreme glassy limit exists for simple spherical particles: that of the short-range attractive glassy state. Here strong short-ranged bonding between the particles can lead to extremely slow relaxation but with dramatically different dynamical characteristics.…”