Time-resolved Crystallization of Charged Colloidal Silica Spheres

Abstract

Aqueous suspensions of charged colloidal particles have proven to be suitable model systems for metals and alloys in order to study the solidification from their respective melts. A major advantage is due to the fact that the corresponding phenomena occur on spatio-temporal scales which are much more easier to influence and monitor from an experimental point of view. Previous research focused (inter alia) on the phase behaviour and nucleation kinetics of single-component samples. Here we report on experiments particularly devoted to binary mixtures of charged colloidal silica spheres, studying their phase behaviour, nucleation kinetics, short-range order, as well as possible compound formation and/or phase transformations. For this purpose a set of distinct samples were investigated, consisting of a mono- and polydisperse reference and binary mixtures of large and small monodisperse particles (ratio of diameters dL:dS ~ 2) of varying composition. The measurements mainly involved time-resolved ultra-small-angle X-ray scattering (USAXS) experiments at the beamline BW4 at HASYLAB/DESY, Hamburg as well as time-resolved in-house Bragg-microscopy. Both methods were used in order to determine the phase behaviour and nucleation kinetics for homo- and heterogeneously nucleated colloidal crystals. Control parameters were given by the particle number density and the amount of base (NaOH) added to the suspensions. Preliminary results show a sensitive dependance of the nucleation kinetics, regarding crystal growth and size, on the composition as well as a re-entrant crystallization behaviour while steadily increasing the relative amount of large particles in a suspension of smaller ones.