Mario Heidernätsch

Characterizing N-dimensional anisotropic Brownian motion by the distribution of diffusivities

Anisotropic diffusion processes emerge in various fields such as transport in biological tissue and diffusion in liquid crystals. In such systems, the motion is described by a diffusion tensor. For a proper characterization of processes with more than one diffusion coefficient, an average description by the mean squared displacement is often not sufficient. Hence, in this paper, we use the distribution of diffusivities to study diffusion in a homogeneous anisotropic environment. We derive analytical expressions of the distribution and relate its properties to an anisotropy measure based on the mean diffusivity and the asymptotic decay of the distribution. Both quantities are easy to determine from experimental data and reveal the existence of more than one diffusion coefficient, which allows the distinction between isotropic and anisotropic processes. We further discuss the influence on the analysis of projected trajectories, which are typically accessible in experiments. For the experimentally most relevant cases of two- and three-dimensional anisotropic diffusion, we derive specific expressions, determine the diffusion tensor, characterize the anisotropy, and demonstrate the applicability for simulated trajectories.

Optical investigation of diffusion of single Ag markers in confined water films

Silver (Ag) clusters with sub-nanometer diameter show as a kind of marker fluorescence emission in the visible range and allow besides optical single-cluster tracking for spectroscopy of single clusters revealing a vibronic coupling of the Ag clusters to the interface. Fluorescence intensity intermittence (blinking) and laser power-dependent photobleaching are observed. The spatial diffusion of monodisperse silver nanoclusters explores ultrathin nanostructured water films on silicon dioxide (SiO2). These heterogeneous diffusion dynamics depend strongly on the thickness of wetting with ultrathin water films and the chemical composition of the SiO2 interface. Thickness-dependent structural changes of the water film have significant influence on diffusion dynamics. We present a numerical model which explains the heterogeneous diffusion across the layers of water with varying viscosity. Our results do not only demonstrate the feasibility to use Ag nano markers to explore nanostructures but also reveal the complexity of nanostructured liquids at solid interfaces.