Theory & Simulation

Molecular-dynamics simulations are used to study microscopic processes in model metallic melts, using effective interaction potentials that are calibrated against available experimental data. The simulation then allows to explain the trends that are observed in experiment, for example upon changing the composition of the molten alloy, and to rationalize these in terms of the relation between dynamics and liquid structure. The origin of the underlying microscopic processes, thermodynamic or kinetic, as well as entropic or energetic, can thus be addressed. Various empirical material laws, such as those connecting self- and interdiffusion, of diffusion and viscosity, can be tested to understand the region of respectively the limits of their validity. Within MD simulations, we also address the effect of flow on the solidification of metallic melts, revealing a delicate interplay between thermodynamic driving forces and kinetic processes in the flowing melt.

Theoretical descriptions are based on the mode-coupling theory of the glass transition (MCT). Combined with MD simulation, this theoretical modeling gives key insights into the generic aspects of liquid dynamics. MCT describes the dynamics of a liquid based on information about its static structure; we combine the theory with experimentally measured static-structure information to provide "first-principles" predictions of the mass-transport processes in realistic model systems for multi-component alloys. The theory is also used to describe the dynamics of soft-matter systems under flow, providing a microscopic description of the non-linear rheology of colloidal suspensions or of agitated granular matter. These systems are prone to strong nonlinear-response effects that cause qualitatively novel phenomena as compared to the near-equilibrium linear response.

In order to understand how these microscopic phenomena affect the macroscopic material properties, we combine microscopic theory with meso- and macro-scale simulation techniques. In particular in glass-forming melts and suspensions showing pronounced visco-elastic response, memory effects become important. These give rise to material properties that depend on the processing history of the material, and that need to be captured by the material laws that enter the continuum-mechanics equations and that we derive from microscopic principles. A prominent example are frozen-in residual stresses in glassy solids that are caused by the flow prior to solidification and that dramatically change the toughness of the final material. To achieve such a multi-scale description of material properties starting from the microscopic equations of motion, is a major numerical challenge in computational fluid dynamics.

Specifically, the institute’s theoretical and simulation work covers the following topics:

• Relation between liquid structure and dynamics; influence of strong fields on the dynamics
• Solid-liquid interfaces and their growth kinetics
• Multi-scale theory and simulation of dense fluids