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International Helmholtz Research School
of Biophysics and Soft Matter

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Research Interests

Prof. Dr. Peter Lang

Interaction of colloidal particles with a wall

In order to understand the phenomena described in the sections below on a microscopic level, it is necessary to obtain a quantitative picture of the interaction potential of the colloidal particles with the interface. Using, Total Internal Reflection Microscopy (TIRM) it is possible to measure e. g. electrostatic interaction potentials, van der Waals potentials and depletion forces, as well as thermophoretic forces and fluctuation forces. We used TIRM to measure the depletion potentials for a variety of systems. Besides exploring possibilities to tune depletion interaction potentials by flow fields, currently we are investigating new methods to extract dynamic information from TIRM data and the potential of TIRM as a micro-rheology tool.

Interaction of colloidal particles with a wallEffect of flow on the depletion interaction mediated by colloidal discs.
Copyright: Lang, P.; FZ Jülich

Dynamics at interfaces

The dynamics of colloids in solution is affected by the presence of a solid interface by the mutual hydrodynamic interaction between the particles and the wall. It was theoretically known for a long time that the translational diffusion coefficient of spheres parallel to a nearby interface is different from that in the direction perpendicular to the interface. However, only recently we were able to provide a qualitative experimental verification of these predictions using evanescent wave dynamic light scattering (EWDLS). Similarly, we confirmed that the rotational diffusion of colloids is slowed down and anisotropic, if they come close to an interface. Currently we are investigating the influence particle concentration, long range interactions and shear fields on the wall hydrodynamic effect. Further we plan to study model systems mimicking the dynamics of membrane proteins in lipid layers.

Dynamics at interfacesCreation of an evanescent wave for EWDLS experiments with a penetration depth of κ-1 of the order of several hundred nano-meters. Left: sample cell; Right: schematic view, not drawn to scale
Copyright: P. Lang and Y. Liu, FZ Jülich