Homogeneous and Heterogeneous Crystallization of Charged Colloidal Particles

Crystallization of macroions happens in many applications like protein purification, photonic crystals or structure determination. However, the mechanism of crystallization in these systems is only poorly understood. We thus study homogeneous nucleation and crystallization of charged spherical particles using molecular dynamics (MD) computer simulations with the software package ESPResSo. Nucleation, i.e. crystallization from a homogeneous bulk liquid, has a high energy barrier, which is overcome by rare spontaneous fluctuations. Thus nucleation is a rare event, which can only be studied in computer simulations using special sampling techniques. We apply Forward Flux Sampling (FFS) embedded in our Flexible Rare Event Sampling Harness System to run simulations as close as possible to the coexistence line. Here the supersaturation of the system is low and the crystallization process can be investigated step-by-step. Based on investigations of crystallization pathways of our system, we found that the crystallization can follow a mechanism with both a smooth or stepwise ordering process, depending on the position in the phase diagram. To study the later stage of crystallization, we use a system of particles confined between two planar walls. In this system, we study the influence of hydrodynamic interactions (HIs) on the crystallization dynamics. To model long-range hydrodynamic interactions, we couple the particle dynamics computed on the CPU with a Lattice- Boltzmann (LB) fluid computed on the GPU. Our results show a significant effect of the HIs on the crystallization dynamics. In order to obtain a deeper insight of the underlying effects, we measured the particle diffusion nearby the crystal front and the corresponding van Hove correlation function.