Single-Polymer Friction Force Microscopy of dsDNA Interacting with a Nanoporous Membrane

Surface-grafted polymers can reduce friction between solids in liquids by compensating the normal load with osmotic pressure, but they can also contribute to friction when fluctuating polymers entangle with the sliding counter face. We have measured forces acting on a single fluctuating double-stranded DNA polymer, which is attached to the tip of an atomic force microscope and interacts intermittently with nanometer-scale methylated pores of a self-assembled polystyrene-block-poly(4-vinylpyridine) membrane. Rare binding of the polymer into the pores is followed by a stretching of the polymer between the laterally moving tip and the surface and by a force-induced detachment. We present results for the velocity dependence of detachment forces and of attachment frequency and discuss them in terms of rare excursions of the polymer beyond its equilibrium configuration.