Crystallization of hard-sphere colloids -- deviations from classical nucleation theory
The creation of three-dimensional ordered colloidal crystals, for application in a range of nanotechnologies, has been a goal for many researchers in the past few years. The main difficulty in creating macroscopic sized crystals of densely packed colloidal particles is that colloidal particles always have a range of particle sizes - ie, they are polydisperse. This paper studied the crystallization kinetics of a hard-sphere colloid with a well defined Gaussian polydispersity. The authors find that crystallization occurs in two stages, and does not follow the simple classical nucleation picture. The paper discusses the implications of these results for research into colloidal crystals as poss…
Small changes in particle-size distribution dramatically delay and enhance nucleation in hard sphere colloidal suspensions.
We present hard-sphere crystallization kinetics for three samples with small differences in polydispersity. We show that an increase in polydispersity of 1% is sufficient to cause dramatic changes in the crystallization kinetics: Crystallization is delayed by almost one decade in time and quantitative and qualitative changes in the crystallization scenario are observed. Surprisingly the nucleation rate density is enhanced by almost a factor of 10. We interpret these results in terms of polydispersity limited growth, where local fractionation processes lead to a delayed but faster nucleation.
Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition
We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to…
Two-Step Crystallization Kinetics in Colloidal Hard-Sphere Systems
The crystallization kinetics of colloidal hard spheres was studied using a special Bragg spectrometer with high sensitivity. In contrast with the classical scenario we observe a two-step nucleation process: the number of crystallites increases slowly at early times, followed by a dramatic reduction at intermediate times, prior to undergoing a rapid increase at late times. We explain these results in terms of a polydispersity limited growth of crystallites, where the crystallization at early times is governed by local fractionation processes, leading to a long delay prior to final crystallization.