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Hans Joachim Schöpe, Gary Bryant* and William van Megen*
*Department of Applied Physics, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne 3001, Australia A complete understanding of the solidification process (nucleation, crystal growth, ripening, vitrification) is one of the long-standing problems in condensed matter physics. The use of colloidal model systems provides an ideal controlled experimental system to reduce this lack of knowledge because the particle movement is much slower than in atomic systems and the crystallisation process can be monitored using optical investigations. A significant difference between the particles in a suspension and the atoms of a (one-component melt) is that colloidal particles always have a particle size distribution. Here we present systematic measurements of the solidification kinetics and of the phase behaviour as a function of the volume fraction as well as a function of the of the particle size distribution in a nominally one component hard sphere colloidal model system. Using a special Bragg scattering spectrometer it is possible to get meanfull data at the earliest stage of crystal nucleation. Interistingly we observe a two step behaviour in the nucleation kinetics: The nucleation rate directly after stopping the shear melting process is surprisingly independent of the particle size distribution and has a weak dependence on the undercooling. The nucleation rate density at later times shows the expected dependence on the undercooling but an unexpected dependence on the polydispersity. This investigation also shows that polydispersity has a significant influence on nucleation, the growth velocity and the morphology of the resulting sample. These studies also indicate that in addition to the rate of crystallization, the crystal structure and the propensity to vitrify are also very sensitive to small variations in the particle size distribution. References 1. Hans Joachim Schöpe, Gary Bryant and William van Megen, PRL 96, 175701 (2006) |
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