Poornachandra Sekhar
Entropic transport in confined media
Supervisor: Prof. Dr. Peter Hänggi [Theoretical physics I]
Date of oral examination: 07/11/2008
107 pages, english , OPUS (Online-Publikations-Server) der Universitätsbibliothek Augsburg
With this work we study biased, diffusive transport of Brownian particles through narrow, spatially periodic structures in which the motion is constrained in lateral directions. In such structures entropic effects may play a dominant role. Under certain conditions the higher dimensional (2D or 3D) dynamics can be approximated by an effective one-dimensional motion of the particle in the longitudinal direction. This reduced one-dimensional kinetic equation contains an entropic potential determined by the varying extension of the eliminated channel direction, and a correction to the diffusion constant that introduces a position dependent diffusion. The constrained dynamics yields a scaling regime for the particle current and the effective diffusion coefficient in terms of the ratio between the work done to the particles and available thermal energy. We show that transport in the presence of entropic barriers exhibits peculiar characteristics which makes it distinctly different from that occurring through energy barriers. The average particle current decreases upon increasing the strength of thermal noise, contrary to the behavior usually found in purely energetic systems. We observed an enhancement in the effective diffusion coefficient whose maximum always exceeds the bulk diffusion constant. The one-dimensional kinetic description has been applied to geometries with different degrees of confinement. The diffusion process is evaluated and the optimal regions for the transport in these confined systems are identified. Since the applicability of the one-dimensional kinetic description varies upon the confinement of the geometry, we analyze the regime of validity both by means of analytical estimates and the comparisons with numerical results for the full two dimensional stochastic dynamics.