Nonclassical pathways of crystallization in colloidal systems
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Introduction Nucleation is an important process, both for understanding the dynamics of first-order phase transitions and for practical applications, ranging from semiconductors, metals, chemicals, pharmaceutics, and the food industry, to studies of climate change. Despite a century of studies on nucleation (for a broad review, see Reference 1), the process is still not entirely understood, especially at a microscopic level. By far, the biggest problem with our classical understanding of nucleation is the small size of the crystalline nuclei involved in the transition (on the order of 10–1000 molecules in typical conditions). Such a small size casts doubt on the use of macroscopic thermodynamic properties in describing crystalline nuclei, for example, the use of surface tension in the presence of highly curved and rough interfaces of small nuclei. At the same time, it makes the observation of crystalline nuclei impractical for most atomic and molecular systems. A notable class of systems in which the crystallization process is more easily accessible is colloidal suspensions. The size and the slow dynamics of typical colloidal suspensions allow for direct single-particle-level observations with confocal microscopy experiments.2–6 Moreover, the tunability of their interactions makes them amenable to direct comparisons with simple soft matter models. For example, steric stabilization can be used to prevent colloidal particles from getting close to each other within the range of attractive forces, such
that their phase behavior can be described by the ideal hardsphere model. Colloidal systems can thus be considered as an ideal benchmark for testing our understanding of the nucleation theories. In this article, we review the recent progress in the field of colloidal crystallization, focusing on nonclassical pathways. These are identified as pathways in which different order parameters take distinct roles during the liquid-to-solid transition.
Beyond classical nucleation theory The simplest and most general understanding of nucleation is embodied in classical nucleation theory (CNT), which provides a successful framework to understand and analyze data from nucleation for a large variety of processes.1 However, its predictions are experimentally very difficult to test, especially given the exponential nature of the transition, which makes the results sensitive to small changes in experimental conditions. This difficulty is exemplified in nucleation studies of systems that, despite their simple interactions, have nucleation rates that are at odds with CNT. An example is the condensation of argon,7 where CNT predictions and experimentally measured rates differ by approximately 26 orders of magnitude. A second example involves the prototypical colloidal system, hard spheres (both poly(methyl methacrylate) and polystyrene microgel systems), where the discrepancy between predicted nucleation rates and
John Russo, Institute of Industrial Science, The University of Tokyo, Japan; and School of Mathematics, University of Bristol, UK;
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