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 ...

PACS. 82.70 -Disperse systems. PACS. 64.75 -Solubility, segregation, and mixing.Abstract. -A new treatment of the phase behaviour of a colloid + nonadsorbing polymer mixture is described. The calculated phase diagrams show marked polymer partitioning between coexisting phases, an effect not considered in the usual effective-potential approaches to this problem. We also predict that under certain conditions an area of three-phase coexistence should appear in the phase diagram.Introduction. -Phase separation in colloidal suspensions, induced by the addition of nonadsorbing polymer, is a phenomenon of fundamental interest and considerable technological importance. A theoretical explanation was first advanced by Asakura and Oosawa [1], and also independently by Vrij [2], based on the exclusion of polymer from the region between two colloid particles when their surface-surface separation becomes smaller than the diameter of a free polymer coil. The resulting imbalance in osmotic pressure gives rise to an effective attractive «depletion» force between the colloid particles [3,4]. At high enough concentration of polymer this depletion force causes the suspension to separate into colloid-poor and colloid-rich phases. In the latter the particles can, depending on conditions (see below), be in either liquidlike or crystalline spatial arrangements.To predict the phase diagram of a colloid + polymer mixture, most workers to date have adopted an approach in which the depletion potential (an effective pair potential) is added to the parent interparticle potential; thermodynamic perturbation theory is then used to calculate phase stability boundaries [5,6]. Although experimental studies [6,7] show qualitative agreement with the predictions of these calculations, an important reservation

When a simple alcohol such as methanol or ethanol is mixed with water, the entropy of the system increases far less than expected for an ideal solution of randomly mixed molecules. This well-known effect has been attributed to hydrophobic headgroups creating ice-like or clathrate-like structures in the surrounding water, although experimental support for this hypothesis is scarce. In fact, an increasing amount of experimental and theoretical work suggests that the hydrophobic headgroups of alcohol molecules in aqueous solution cluster together. However, a consistent description of the details of this self-association is lacking. Here we use neutron diffraction with isotope substitution to probe the molecular-scale structure of a concentrated alcohol water mixture (7:3 molar ratio). Our data indicate that most of the water molecules exist as small hydrogen-bonded strings and clusters in a 'fluid' of close-packed methyl groups, with water clusters bridging neighbouring methanol hydroxyl groups through hydrogen bonding. This behaviour suggests that the anomalous thermodynamics of water alcohol systems arises from incomplete mixing at the molecular level and from retention of remnants of the three-dimensional hydrogen-bonded network structure of bulk water.

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 addition of non-adsorbing polymer to a colloidal suspension induces an interparticle ‘depletion’ attraction whose range and depth can be ‘tuned’ independently by altering the polymer’s molecular weight and concentration respectively. Over the past decade, one particularly simple experimental realization of such a mixture has been studied in considerable detail: nearly-hard-sphere particles of poly(methyl methacrylate) and random-coil polystyrene dispersed in simple hydrocarbon solvents (mainly cis-decalin). The simplicity of the system has enabled rather detailed comparison of experimental findings with theory and simulation. Here I review the current understanding of the equilibrium phase behaviour, structure, phase transition kinetics, and metastability of this model colloid–polymer mixture. These findings form a useful reference point for understanding more complex mixtures. Moreover, in some cases, insights gained from studying this model system have relevance beyond soft-condensed-matter physics, e.g. in understanding the liquid state, in controlling protein crystallization, and in elucidating the nature of glasses.

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Recent large-scale computer simulations suggest that it may be possible to create a new class of soft solids, called 'bijels', by stabilizing and arresting the bicontinuous interface in a binary liquid demixing via spinodal decomposition using particles that are neutrally wetted by both liquids. The interfacial layer of particles is expected to be semi-permeable; hence, if realized, these new materials would have many potential applications, for example, as micro-reaction media. However, the creation of bijels in the laboratory faces serious obstacles. In general, fast quench rates are necessary to bypass nucleation, so that only samples with limited thickness can be produced, which destroys the three-dimensionality of the putative bicontinuous network. Moreover, even a small degree of unequal wettability of the particles by the two liquids can lead to ill-characterized, 'lumpy' interfacial layers and therefore irreproducible material properties. Here, we report a reproducible protocol for creating three-dimensional samples of bijel in which the interfaces are stabilized by essentially a single layer of particles. We demonstrate how to tune the mean interfacial separation in these bijels, and show that mechanically, they indeed behave as soft solids. These characteristics and their tunability will be of great value for microfluidic applications.

Colloidal particles partially coated with platinum and dispersed in H2O2 solution are often used as model self-propelled colloids. Most current data suggest that neutral self-diffusiophoresis propels these particles. However, several studies have shown strong ionic effects in this and related systems, such as a reduction of propulsion speed by salt. We investigate these ionic effects in Pt-coated polystyrene colloids, and find here that the direction of propulsion can be reversed by addition of an ionic surfactant, and that although adding pH neutral salts reduces the propulsion speed, adding the strong base NaOH has little effect. We use these data, as well as measured reaction rates, to argue against propulsion by either neutral or ionic self-diffusiophoresis, and suggest instead that the particle's propulsion mechanism may in fact bear close resemblance to that operative in bimetallic swimmers.