Colloids display intriguing transitions between gas, liquid, solid and liquid crystalline phases. Such phase transitions are ubiquitous in nature and have been studied for decades. However, the predictions of phase diagrams are not always realized; systems often become undercooled, supersaturated, or trapped in gel-like states. In many cases the end products strongly depend on the starting position in the phase diagram and discrepancies between predictions and actual observations are due to the intricacies of the dynamics of phase transitions. Colloid science aims to understand the underlying mechanisms of these transitions. Important advances have been made, for example, with new imaging techniques that allow direct observation of individual colloidal particles undergoing phase transitions, revealing some of the secrets of the complex pathways involved.F igure 1 illustrates three types of phase diagram. The ®rst (Fig. 1a) is a simple system of hard spheres. Introducing attractions results in three-phase equilibria, as in atomic systems such as argon (Fig. 1b). With shorter-range attractions the gas±liquid (or¯uid±¯uid) equilibrium becomes metastable (Fig. 1c). This is often observed in protein systems. One might assume equilibrium diagrams show the complete picture, with perhaps the initial distance from the phase boundary (indicative of the distance to equilibrium) controlling the speed with which the equilibrium phases are attained; however, this is the exception rather than the rule.Hard-sphere colloids suspended in a solvent provide an excellent illustration of the dif®culties involved in understanding the equilibrium states and the mechanisms by which systems evolve. Entropy considerations predict that these systems will form crystals if the volume fraction is increased. Above the`freezing' volume fraction, f f 0:494, it is entropically favourable if some spheres are in a crystal, but above the`melting' volume fraction, f m 0:545, all spheres should be in a crystal (Fig. 1a). However this is not always the situation found experimentally, either because the conditions the theory assumes (low polydispersity, for example) are not satis®ed, or the dynamics of the system have dictated a different structure (note that we cannot say that this structure is the equilibrium structureÐif entropy favours crystal formation then a crystal will form eventually, however the system may remain in the less-favoured state for a signi®cant amount of time). This crystallization process is often interpreted within the familiar framework of nucleation and growth. There has been renewed interest in this mechanism recently, both with simulations 1 and experimentally 2,3 : by monitoring the individual particles undergoing the transition it is possible to evaluate and improve the model.The classical theory predicts that the free energy cost, DG, of forming a nucleus of radius r is:where g is the surface free energy, r is the density of the bulk liquid, and Dm is the chemical potential difference between the bulk solid and bulk liquid...

We study gelation in suspensions of model colloidal particles with short-ranged attractive and long-ranged repulsive interactions by means of three-dimensional fluorescence confocal microscopy. At low packing fractions, particles form stable equilibrium clusters. Upon increasing the packing fraction the clusters grow in size and become increasingly anisotropic until finally associating into a fully connected network at gelation. We find a surprising order in the gel structure. Analysis of spatial and orientational correlations reveals that the gel is composed of dense chains of particles constructed from face-sharing tetrahedral clusters. Our findings imply that dynamical arrest occurs via cluster growth and association.

Abstract. We study the phase ordering colloids suspended in a thermotropic nematic liquid crystal below the clearing point Tni and the resulting aggregated structure. Small (150 nm) PMMA particles are dispersed in a classical liquid crystal matrix, 5CB or MBBA. With the help of confocal microscopy we show that small colloid particles densely aggregate on thin interfaces surrounding large volumes of clean nematic liquid, thus forming an open cellular structure, with the characteristic size of 10 − 100 µm inversely proportional to the colloid concentration. A simple theoretical model, based on the Landau mean-field treatment, is developed to describe the continuous phase separation and the mechanism of cellular structure formation.

SummaryBacillus anthracis, the causative agent of anthrax, is a dangerous biological weapon, as spores derived from drug-resistant strains cause infections for which antibiotic therapy is no longer effective. We sought to develop an anti-infective therapy for anthrax and targeted CapD, an enzyme that cleaves poly-g-Dglutamate capsule and generates amide bonds with peptidoglycan cross-bridges to deposit capsular material into the envelope of B. anthracis. In agreement with the model that capsule confers protection from phagocytic clearance, B. anthracis capD variants failed to deposit capsule into the envelope and displayed defects in anthrax pathogenesis. By screening chemical libraries, we identified the CapD inhibitor capsidin, 4-[(4-bromophenyl)thio]-3-(diacetylamino)benzoic acid), which covalently modifies the active-site threonine of the transpeptidase. Capsidin treatment blocked capsular assembly by B. anthracis and enabled phagocytic killing of nonencapsulated vegetative forms.

Summary The Gram-positive pathogen Bacillus anthracis grows in characteristic chains of individual, rod-shaped cells. Here, we report the cell-separating activity of BslO, a putative N-acetylglucosaminidase bearing three N-terminal S-layer homology (SLH) domains for association with the secondary cell wall polysaccharide (SCWP). Mutants with an insertional lesion in the bslO gene exhibit exaggerated chain lengths, though individual cell dimensions are unchanged. Purified BslO complements this phenotype in trans, effectively dispersing chains of bslO-deficient bacilli without lysis and localizing to the septa of vegetative cells. Compared to the extremely long chain lengths of csaB bacilli, which are incapable of binding proteins with SLH-domains to SCWP, bslO mutants demonstrate an chaining phenotype that is intermediate between wild-type and csaB. Computational simulation suggests that BslO effects a non-random distribution of B. anthracis chain lengths, implying that all septa are not equal candidates for separation.

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