Friederike Schmid: Research


Lipid Membranes

All living things depend on membranes. Their basic structure is provided by lipid bilayers, which self-assemble spontaneously in water due to the amphiphilic character of lipid molecules - they contain both hydrophilic and hydrophobic units. In our group, we are interested in generic properties of such amphiphilic bilayers.

We have established a coarse-grained lipid model, which reproduces the main phases and phase transitions of phospholipid membranes at temperatures close to room temperature. As particular highlights, we have (i) recovered and investigated the mysterious modulated "ripple phase" in one-component membranes, which had intrigued researchers for many decades, and (ii) discovered and investigated nanoscale structures, examples of so-called "lipid rafts", in multicomponent membranes. Rafts are small domains in biomembranes which are believed to play a role for many cellular functions (see review article). We found that ripple states and nanoscale rafts are stabilized by very similar mechanisms: A propensity for global phase separation, which is suppressed by elastic interactions in the membrane. This is analyzed by computer simulations and elastic theories.

The same approach is used to study lipid-mediated interaction mechanisms membrane proteins. In the past, we have focused on a comparison between analytical predictions and simulation data for "proteins" that can be represented by stiff inclusions (see Figure). Currently, we investigate flexible amyloid-like peptides and their interaction with membranes. For more information, please contact Friederike Schmid.

Hybrid Field-based Simulation Methods for Polymers

The so-called 'self-consistent field' (SCF) theory is one of the most successful density functional theories fo inhomogeneous polymer systems, which allows to calculate the local structure of dense blends at an almost quantitative level (see review article).

We develop new hybrid simulation schemes for such systems that combine particle- and field-based representations of polymers, thus allowing to treat large parts of a system at the field level and zoom into certain areas in space with adaptive resolutions. Furthermore, we develop methods to combine different kinetic descriptions of polymeric fluids (diffusive Langevin and hydrodynamic Lattice-Boltzmann fields) in a consistent way. For more information, please contact
Friederike Schmid .

Self-Assembling Block Copolymers and Polymer Brushes

Melts of one or more kinds of polymers exhibit a wealth of diverse phases whose geometric properties make them interesting systems not only for condensed matter research, but for industrial applications, as well. Specifically, block copolymers made of chemically incompatible monomers (say, A and B) exhibit microphase separation, thus forming regular nanoscale patterns of varying complexity. In solvent, they self-assemble to nanoparticles or vesicles which can be used, e.g., as nanocontainers.

Among other, we study the influence of curvature on structure formation and pattern orientation in thin films and membranes, and we are interested in the effect of crosslinking for the stabilization of ordered structures. Furthermore, we use dynamic self-consistent field theory to study the kinetics of structure formation, e.g., in solutions containing amphiphilic block copolymers.

Another important application for polymers is to attach them to surfaces, thus modifying the surface properties. We are interested in the effect of polydispersity on the structure of such ''polymer brushes'', and on strategies to design smart surfaces that can be used as sensors and switches. For more information, please contact Friederike Schmid .

Memory Effects in Colloidal Systems

In soft matter, the separation of time scales is often incomplete and memory effects become important. We develop coarse-graining strategies for such situations, using the example of colloidal dispersions. We develop methods to reconstruct memory kernels in simple and complex fluids (e.g., electrolyte fluids). Our goal is to construct implicit solvent models that include memory effects and can be used for equilibrium and non-equilibrium simulations. In this context, we also develop algorithms for the efficient simulation of coupled generalized Langevin equations. For more information, please contact Friederike Schmid .

Interplay of Electrostatic and Hydrodynamic Interactions in Complex Fluids

The structure and dynamics of nano-objects (polymers, colloids) in solution is to a large extent governed by their interaction with the solvent. We aim at developing efficient methods for simulating nano-objects (polymers, colloids) that are dispersed in complex fluids, at equilibrium and nonequilibrium.

In particular, we are interested in studying electrolyte solvents, where the interplay of electrostatic interactions and hydrodynamic flows gives rise to a wealth of intriguing phenomena on a wide range of time and length scales. Physical problems of interest are the electrophoresis of charged polyelectrolytes or colloids in microchannels with different geometries and wall structurings, or the dielectrophoresis of polyelectrolytes or colloids in alternating electric fields. For more information, please contact Friederike Schmid .

Transport and Properties of Blood Proteins

Transport of nutrients to peripheral tissues and healing of damaged blood vessels are among the most important functions of blood. These functions involve the action of a series of proteins some of which are found in large amounts in the blood circulation. Fibrinogen is a multiprotein complex which, when activated, aggregate to form fibrin, a net-shaped molecular formation which is fundamental for the coagulation of blood following, i.e, a wound or when an extraneous body comes into contact with blood (i.e., graft implants). Thus, adsorption of fibrinogen on material surfaces play an important role in viability of those materials for implants.

In collaboration with experimental groups in the field, we use atomistic molecular dynamics simulations to characterize the adsorption process of fibrinogen on material surfaces. Another important molecule in the blood is albumin, which mediate transport of lipids and other molecules in blood. Albumin is a multidomain protein which provides several binding sites used to bind a range of different target molecules. Target molecules (lipids, drugs, etc.) bind to albumin which act as a transporter, and are then released where needed by blood circulation. Here we use molecular dynamics simulations to study the binding modes of several lipids to Albumin and the kinetics of lipid release/uptake. If you are interested, please contact Friederike Schmid or Giovanni Settanni.

Statistical Physics of Molecular Recognition

Selective interactions between biomolecules play an essential role in biological systems. Without selective recognition of antigens by corresponding antibodies, for example, the immune system could not work efficiently. One of the most salient features of molecular recognition is the fact that biomolecules often discriminate very accurately between many different but structurally similar interaction partners which are also present in a heterogeneous biological system.

Our studies aim at an understanding of the basic and universal mechanisms of recognition processes between biomolecules in an heterogeneous environment. In order to identify and investigate these basics mechanisms we develop idealised coarse-grained models. These models neglect those details which are particular for a specific system and are thus constructed to represent generic types of recognition processes. The thermostatic and dynamical properties of the models are then analysed with numerical and analytical methods from statistical physics. For more information, please contact  Friederike Schmid .