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Schlagberger, Xaver (2006): Polymer Dynamics in External Fields. Dissertation, LMU München: Fakultät für Physik



The dynamics of single semiflexible polymers in solution under the influence of an external field are investigated with Brownian Dynamics simulations. Hydrodynamic interactions are included on the Rotne-Prager level and proof to be essential. Model equations are used to derive scaling laws. The work consists of five projects that are distinct but closely related to each other: In the first project, a neutral semiflexible particle is moved by centrifugal or gravitational forces relative to quiescent fluid. A coupling between hydrodynamic interactions and flexibility leads - depending on the elastic parameters - to a rod orientation perpendicular to the external field. This coupling is also investigated for a filament that is rotated at one end by some external torque (second project). Above a critical torque the filament folds itself around the rotational axis, with important consequences for the propulsion with a nano-machine. The third project deals with flexible polymers in an ultracentrifuge where a novel compactification and unfolding scenario is predicted: The established theories on sedimentation use the preaveraging approximation of the hydrodynamic interactions and cannot explain the polymer configurations at high fields consisting of a dense head and a long tail, which make a new efficient separation technique possible. In the forth project, the diffusion of charged semiflexible polymers under different salt conditions is treated. Ions are included explicitly and not on a mean-field level. The theory of electrolyte friction for spherical objects is qualitatively extended to semiflexible polymers. A heuristic formula for the diffusion constant over the whole range of persistence lengths is proposed. In the final project, the hydrodynamic orientation mechanism found in the first project is suggested as a possible source of anomalous electric birefringence which is observed for rod-like polymers. It is compared with the competing parallel induced dipole orientation. The dependence of the polarizability on rod length, salt and polymer concentration is clarified.