Time-resolved Ultra-small-angle X-ray Scattering and Bragg-microscopy of Binary Mixtures of Charged Colloidal Silica Spheres

Kurzfassung

In order to develop a deeper understanding of the solidification of metals from their melts, colloidal model systems were established which facilitate a much simpler observation and analysis of the processes involved, due to their nanoscopic spatial scale and their decelerated temporal dynamics. The samples studied consist of aqueous suspensions of high X-ray scattering contrast, spherical silica particles of distinct diameter in the nanometer range which exhibit a tunable interaction via a screened Coulomb potential. Upon confinement their mutual repulsion due to negatively charged silanol surface groups favours crystallization into body-centered cubic colloidal crystals which may be molten by applied shear. Previous experiments focused on the phase behaviour and the nucleation kinetics of single-component charged colloidal silica spheres. Here we report on corresponding experiments particularly devoted to binary mixtures, thus allowing for systematic variation of size (and thereby charge) as well as composition ratios. A set of three mono-, bi- and polydisperse samples where investigated by means of time-resolved ultra-small angle X-ray scattering (BW4 @ HASYLAB, DESY, Hamburg) in order to determine the phase behaviour and nucleation kinetics for homo- and heterogeneously nucleated crystals in dependence of particle number density and sodium hydroxide concentration. In addition, time-resolved in-house Bragg-microscopy together with static light scattering were used to complement the synchrotron measurements. As a first result a re-entrant crystallization was observed while steadily increasing the amount of large particles in a suspension of smaller ones (size ratio dL/dS ~ 2). Further results show a sensitive dependence of the nucleation kinetics, regarding crystal growth and size, on the composition.