Brownian dynamics simulations of flowing isolated polymer molecules in solution near surfaces

Brownian dynamics simulations have been shown to give accurate predictions of the molecular conformations and rheology of polymers (DNA in particular) in extensional and shear flow. We extend these Brownian dynamics methods to include the interactions of polymers with non-adsorbing and irreversibly adsorbing solid surfaces during flow. We develop mathematical tools and statistical analyses and apply them to the case of a simple steady shear flow (plane Couette) inside a confined geometry and to the time-dependent axisymmetric flow created by the action of a drying droplet.; Under shearing flow (plane Couette) there is a depletion layer near the wall whose thickness decreases with increasing shear rate, because of the compression of the chain in the shear gradient direction. Relative to the bulk, the molecular stretch in the direction of the flow is reduced near the wall, despite the increase in molecular alignment. We also demonstrate that the wall interferes with the molecular tumbling in shear flow.; In the case of a perfectly adsorbing wall, in the process of adsorbing, the molecule becomes more stretched than in the bulk flow at the same shear rate. We also report a propensity for the polymer to affix to the surface sequentially starting at one end, due to the tendency in a random-walk polymer for a free end to lie at the periphery. The more highly stretched molecules tend to be those that adsorbed more perfectly in sequence.; In the drying droplet flow field, the degree of stretch obtainable is substantially less than can be obtained by deposition from a simple uniform shearing flow. Statistical analysis reveals that the inefficiency of stretching in the drying droplet results from the presence of a velocity component normal to the surface, which reduces the time available for the chain to unravel sequentially as it adsorbs onto the surface.