Multiscale Simulations of Soft and Hard Matter

Colloidal suspensions usually consist of colloidal particles of size 1 nm to 1 μm which are surrounded by a solvent, typically water. In these systems the kinetic energy of the colloids at room temperature is about the strength of their interaction energy. Hence, these systems or materials are considered as soft and deformable and known as “soft matter”. Molecular dynamics (MD) simulation of suspensions have to address on the one hand the colloid dynamics due to the presence of a solvent and on the other hand various types of interactions depending on the shape and effective charge of the colloidal particles. Furthermore, in case of a explicit solvent its dynamics has to be correctly modeled. However, the explicit computation of the dynamics of solvent particles represented, for example as water molecules, by means of MD simulations is not feasible, even on modern supercomputers. This is simply due to the enormous amount (1023) of molecules. In order to overcome this limitation, we use a combination of MD simulations for the dynamics of the colloids and a discretized continuum solver for the dynamics of the solvent. Besides the complexity and richness of static and dynamic properties of colloidal suspensions, many of them also show a variety of different fluid and solid phases. The possibility of forming solid structures makes colloidal suspensions an ideal toy model to investigate crystallization and nucleation phenomena [133, 132]. For example, spherical colloids form either basic body centered cubic (BCC) or face centered cubic (FCC) crystal structures [56]. A major advantage of using colloidal suspensions as a toy model is that it is possible to gain insight into the nature of crystallization processes in general, because the time and length scales are more readily accessible than in, for instance, metal melts. The temperature and conductance of metal melts makes tracking of dynamic phenomena experimentally challenging. The use of colloidal suspensions as model systems for metal solidification is based on Pusey’s paradigm [141], which states that colloidal systems and atomic systems show comparable static properties, for example the structure factor. From an experimental point of view it is nowadays possible to generate highly monodisperse suspensions with well-defined interaction strength [137, 124]. Despite recent advances in controlling the shape and type of interactions of the colloids, colloidal suspensions always contain a solvent by default, which induces long-range hydrodynamic interactions (HIs) into the system, in contrast to atomic systems. But, as crystallization processes are typically taking place on much larger time and length