Frictional effects in the simulation of polymers pulled around smooth and corrugated posts

摘要

This paper reports on a detailed study of solid and solvent frictional forces in Brownian dynamics simulations of a harmonic polymer chain pulled around a circular post in two dimensions. In the case of corrugated posts, a backward-driving normal-force-dependent tangential force arises implicitly, without the explicit inclusion of dissipation into the post, and we interpret this as a particular type of solid friction. Tension distributions in the polymer and velocities are determined in the steady state and compared with model predictions and empirical fits. For smooth posts, the mean field models developed are microscopic and predictive; in the case of corrugated posts they are phenomenological. A detailed study of the effective barrier heights for passage of polymer beads over the corrugation and how those barriers relate to couplings between neighboring beads helps to elucidate the reason for the breakdown of a simple mean field theory in the case of nonzero corrugation amplitude. The importance of the relationship between the corrugation period and the tension-dependent bead spacing is emphasized. Detailed analogies are made with a one-dimensional model of a particle in a tilted periodic potential for which analytic solutions for the steady state velocity can be determined. An Appendix briefly describes how similar results can be obtained using steady state Monte Carlo dynamics.