Translocation of a Polymer through a Nanopore across a Viscosity Gradient

TL;DR

This study investigates how viscosity gradients across a nanopore influence polymer translocation, revealing an intrinsic bias that depends on simulation methods and highlighting complex scaling behaviors.

Contribution

It introduces a computational analysis of polymer translocation across viscosity gradients, comparing simulation algorithms and proposing a simple force model for bias estimation.

Findings

Bias towards low viscosity in Langevin Dynamics simulations

Bias towards high viscosity in Brownian Dynamics simulations

Scaling saturation of translocation bias with polymer length

Abstract

The translocation of a polymer through a pore in a membrane separating fluids of different viscosities is studied via several computational approaches. Starting with the polymer halfway, we find that as a viscosity difference across the pore is introduced, translocation will predominately occur towards one side of the membrane. These results suggest an intrinsic pumping mechanism for translocation across cell walls which could arise whenever the fluid across the membrane is inhomogeneous. Somewhat surprisingly, the sign of the preferred direction of translocation is found to be strongly dependent on the simulation algorithm: for Langevin Dynamics (LD) simulations, a bias towards the low viscosity side is found while for Brownian Dynamics (BD), a bias towards the high viscosity is found. Examining the translocation dynamics in detail across a wide range of viscosity gradients and developing a simple force model to estimate the magnitude of the bias, the LD results are demonstrated to be more physically realistic. The LD results are also compared to those generated from a simple, one dimensional random walk model of translocation to investigate the role of the internal degrees of freedom of the polymer and the entropic barrier. To conclude, the scaling of the results across different polymer lengths demonstrates the saturation of the preferential direction with polymer length and the non-trivial location of the maximum in the exponent corresponding to the scaling of the translocation time with polymer length.