Lab for Microscopic Modelling of Materials Properties and Processes

The laboratory aims at a theoretical understanding of general properties of complex fluids and solids.

Complex fluids (e.g. electrolyte solutions, mixtures of multiple components, fused salts, room temperature ionic liquids, liquid crystals and colloidal suspensions) and solids are described by a large number of relevant degrees of freedom of their constituting particles, e.g. their position, orientation and conformation.

A particular emphasis of this laboratory is on the interplay of various length scales, which range from the microscopic size of single fluid molecules and atoms composing a solid via the Debye length of electrolyte solutions or the electron plasma in the conduction band of a solid and the bulk correlation length of near-critical solvent mixtures up to the mesoscopic size of colloidal particles.

Due to the multi-particle character of complex fluids and solids methods of statistical physics (in particular density functional theory) and computer simulations are applied.

The focus is on the following research topics:

Phase behaviour

Complex fluids and solids exhibit a rich phase behaviour, e.g. separation of two fluid phases, multiple solid phases, multi-critical behaviour in mixtures of several components and mesophases in liquid crystalline materials. The aim is to relate the numerous phases and phase transitions between them with the microscopic degrees of freedom of the constituting particles.

Microscopic structure of homogeneous systems

Competing length and time scales of complex fluids and solids lead to non-trivial structures in homogeneous systems, e.g. the formation of microheterogeneities due to antagonistic salts in solvent mixtures and the alternating charge structure in complex ionic fluids. As the microscopic structure of homogeneous systems is experimentally available by means of scattering methods, it offers a test for applied mathematical models.

Interfacial structures

The interfacial structure of complex fluids and solids is determined by molecular sizes on short distances, by the bulk correlation length on intermediate distances and, in the case of ionic fluids and solids, by the Debye length on large distances. This hierarchy of length scales leads to numerous interfacial phenomena, e.g. wetting, interfacial tension and the 'solvation force'.

Wetting phenomena

Wetting phenomena of a fluid in contact with a solid substrate, e.g. complete or partial wetting, 'prewetting', first order and continuous wetting transitions as well as critical adsorption, occur due to fine details of all interactions in the fluid and in between the fluid and the solid substrate. An understanding of these interactions is essential for the application of wetting phenomena, e.g. of the electrowetting effect in ionic complex fluids.

Effective interactions

The mesoscopic description of complex fluids and solids, within which microscopic details, e.g. the properties of the solvent of colloidal suspensions, are represented by materials constants, requires expressions for the effective interactions of the remaining degrees of freedom, e.g. the screened Coulomb interaction in electrolyte solutions and the electron plasma in the conduction band of a solid as well as the 'solvation force' in between colloidal particles. This calls for the construction of systematic and efficient mathematical methods.

Dynamic processes

Dynamic processes in complex fluids and solids, e.g. the fluctuations in a uniform equilibrium phase, the relaxation of an interface into the thermodynamic equilibrium and the structure formation dynamics far from thermodynamic equilibrium, frequently exhibit non-linear and non-local character due to the coupling of numerous degrees of freedom. These phenomena lead to fundamental questions concerning the understanding of systems in thermodynamic non-equilibrium.