Axial forces at disk surfaces in a cylindrical nanopore

Abstract

Understanding the physics of object translocation in nanopores is critical for using nanopores as sensors of molecular properties and as object size and shape sensors. Based on Poisson-Nernst-Planck and Navier-Stokes simulations we dissect three axial pressures and forces at disk edges (upper, lower and rim) - Coulomb, dielectric and fluidic. Axial Coulomb and dielectric rim forces are small and cancel each other. Upper and lower axial forces are largely controlled by the external axial electric field and interestingly by the pore wall charges that determine the amplitude and direction of axial combined force. Axial total Coulomb force (sum of its upper and lower edge components) makes the greatest contribution, but the axial total dielectric force (calculated using Maxwell stress tensor), which opposes it is surprisingly large. External ion concentration alters Coulomb and axial dielectric forces but influences only their amplitude. Axial total fluidic force is near zero (its upper and lower disk edge components are significant but cancel each other) regardless of external electric field, but pore wall charges and external fluidic pressure can alter it. Modest changes of external electric field or concentration produce axial forces comparable to those produced by large external fluidic pressures. Axial forces depend little on disk's axial position. Finally, mean axial pressures (calculated to compare forces acting on disks of different radius) are greater for larger disks.