Brownian dynamics simulations of isolated polymer molecules in shear flow near adsorbing and nonadsorbing surfaces

Abstract

We use Brownian dynamics methods to simulate the interactions of polymers with nonadsorbing and adsorbing solid surfaces at equilibrium and during flow under very dilute conditions. Near a nonadsorbing surface, under uniform shearing flow, we find there is a depletion layer near the wall whose thickness decreases with increasing Weissenberg number, because of the compression of the chain in the shear gradient direction due to finite extensibility, in qualitative agreement with a scaling law derived by Hur et al. (2000). We also demonstrate how the presence of the wall interferes with the molecular tumbling in shear flow. For an irreversibly adsorbing wall, the adsorbed molecule is more stretched than an equivalent unadsorbed molecule in the bulk at the same shear rate. This results from a propensity for the “beads” representing the polymer conformation to affix to the surface sequentially starting at one end, due in part to the tendency in a random-walk polymer for a free end to lie at the periphery, and in part to an increased likelihood of initial contact between polymer and wall during fluctuations that expand the coil in the direction perpendicular to the wall.