Forced translocation of a polymer: dynamical scaling vs. MD-simulation

Abstract: We suggest a theoretical description of the force-induced translocation dynamics of a polymer chain through a nanopore. Our consideration is based on the tensile (Pincus) blob picture of a pulled chain and the notion of propagating front of tensile force along the chain backbone, suggested recently by T. Sakaue. The driving force is associated with a chemical potential gradient that acts on each chain segment inside the pore. Depending on its strength, different regimes of polymer motion (named after the typical chain conformation, "trumpet", "stem-trumpet", etc.) occur. Assuming that the local driving and drag forces are equal (i.e., in a quasi-static approximation), we derive an equation of motion for the tensile front position $X(t)$. We show that the scaling law for the average translocation time $<\tau>$ changes from $<\tau> \sim N^{2\nu}/f^{1/\nu}$ to $<\tau> \sim N^{1+\nu}/f$ (for the free-draining case) as the dimensionless force ${\widetilde f}{R} = a N^{\nu}f /T$ (where $a$, $N$, $\nu$, $f$, $T$ are the Kuhn segment length, the chain length, the Flory exponent, the driving force, and the temperature, respectively) increases. These and other predictions are tested by Molecular Dynamics (MD) simulation. Data from our computer experiment indicates indeed that the translocation scaling exponent $\alpha$ grows with the pulling force ${\widetilde f}{R}$) albeit the observed exponent $\alpha$ stays systematically smaller than the theoretically predicted value. This might be associated with fluctuations which are neglected in the quasi-static approximation.