Colloidal Glasses
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Colloidal Glasses Wilson C.K. Poon Abstract This article reviews recent advances in understanding amorphous glassy states in dense colloidal suspensions with or without short-range interparticle attractions. Experiments, theory, and simulation show that two kinds of glassy states are possible, dominated respectively by repulsion and attraction. Under suitable conditions, a small change in the interparticle potential can lead to a transition between these two kinds of colloidal glasses that entails sharp changes in material properties such as the shear modulus. This may provide novel routes for fine-tuning the properties of industrial pastes and slurries. Keywords: colloids, glasses, metastability, nonergodicity, rheology, sticky hard spheres.
Introduction Understanding the vitreous, or glassy, state of matter is one of the “grand challenges” of 21st century materials science. Glasses in traditional atomic, smallmolecule, and polymeric materials are well known. Vitrification is also important in biology as a means of preserving life under extreme conditions (e.g., in seeds). In all of these cases, we are speaking of structural glasses involving frozen-in disorder in the spatial arrangement of constituent atoms and molecules. There are also “glasses” in which the frozen-in disorder is not primarily structural, for example, “spin glasses,” in which the disorder is of a magnetic kind. Despite much commonality among them, it is unknown whether a unified description of all of these glasses is possible (e.g., how fundamental is the difference between annealed and quenched disorder in structural and spin glasses, respectively?) The study of structural colloidal glasses did not really get off the ground until the mid-1980s, but it has blossomed tremendously since then. Some of the results have corroborated and clarified our existing understanding of glasses in noncolloidal systems. In other cases, novel phenomena that may well be unique to colloids have been discovered, challenging any conceptual framework that seeks a generic understanding of glasses. In this article, I will review this research in colloidal glasses.
Glass Transition Due to Crowding Although there had been speculation about a hard-sphere glass transition since at least Bernal’s pioneering work on sphere packing in the 1960s, it was the mode cou-
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pling theory (MCT) calculations of Götze and collaborators in the 1980s1 and the experiments of Pusey and van Megen2,3 that put this phenomenon on a firm footing. The equilibrium phase behavior of N hard spheres of radius R in volume V depends only on the volume fraction,
4 R3NV. 3
(1)
Below 0.494, the lowest free-energy state is a fluid, a disorderly arrangement of particles that can individually explore all of V (the system is “ergodic”). From 0.545 up to close-packing ( 0.74), the equilibrium state is crystalline. Fluid and crystal coexist for 0.494 0.545. Pusey and van Megen found2 that suspensions of poly(methyl methacrylate) (PMMA) particles, stabilized by a thin layer of gr
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