Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels) [1][2][3][4][5][6][7] The emerging glass paradigm has been success-fully applied to complex soft-matter systems, such as colloid-polymer systemss and concentrated protein solutions 9 . However, intriguing problems like the frequent occurrence of cluster phases remain 10-13 . Here we report small-angle scattering and confocal microscopy investigations of two model systems: pro-tein solutions and colloid-polymer mixtures. We demonstrate that in both systems, a combination of shortrange attraction and long-range repulsion results in the formation of small equili-brium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspension [12][13][14][15][16][17] .A number of globular proteins have been shown to exhibit the major characteristics of colloids that interact via a short-range attractive potential. At high ionic strength, where the salt screens electrostatic repulsions, these short-range attractions increasingly dominate with decreasing temperature. This leads to a metastable liquid-liquid phase separation and related critical phenomena [18][19][20] . In agreement with predictions from modecoupling theory 9 , there is also-evidence for a glass or gel transition at low-particle volume fractions and high interparticle attractions. Such a scenario obviously affects the ability to form the high quality crystals required for protein crystallography 15 .Using two apparently quite different model systems, we demonstrate the generality of this emerging description of the effect of a short-range attraction combined with either a hard or soft repulsion on the phase behaviour of a wide range of colloidal suspensions.We first investigated solutions of the globular protein lysozyme (molecular mass 14.4 kDa, radius R m ≈1.7 nm) [17][18][19] .Using small-angle X-ray (SAXS) and neutron (SANS) scattering, we studied spatial correlations in concentrated solutions at low ionic strength, where the long-range repulsive electrostatic potential is only weakly screened. We then compared these findings with confocal microscopy results using colloid-polymer mixtures, a popular model system with easily tunable interactions. Here we used spherical colloidal particles interacting with a long-range repulsion resulting from a modest charge 21 and a short-range attraction induced by a polymer-mediated 'depletion ...

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...

In this work we study the kinetics of coagulation of monodisperse spherical colloids in aqueous suspension at the early stage of coagulation. We have performed the measurements on a multiangle static and dynamic light scattering instrument using a fiber-optics-based detection system which permits simultaneous time-resolved measurements at different angles. The absolute coagulation rate constants are determined from the change of the scattering light intensity as well as from the increase of the hydrodynamic radius at different angles. The combined evaluation of static and dynamic light scattering results permits the determination of coagulation rate constants without the explicit use of light scattering form factors for the aggregates. For different electrolytes fast coagulation rate constants were estimated. Stability curves were measured as a function of ionic strength using different particle concentrations.

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