Tumbling, stretching and cross-stream migration of polymers in rectilinear shear flow from dissipative particle dynamics simulations

Abstract

Solutions of flexible polymer chains with harmonic bonds undergoing rectilinear flow in slit pores are investigated via dissipative particle dynamics (DPD) simulations. We found that when DPD with low Schmidt number (Sc∼1) is used, the polymer chains tend to migrate across the streamlines towards the walls. However, a cross-stream migration towards the centerline is observed when DPD with relatively high values of Schmidt number (Sc∼10) is used. The effect of chain length and Weissenberg number, defined as Wi=Γ̇τrel, where Γ̇ and τrel are the shear rate and polymer longest relaxation time, respectively, are investigated. The polymer chains exhibit a large number of orientational and extensional fluctuations, with the distributions of both latitude and azimuthal angles exhibiting power-law decays in agreement with experiments, theory and previous simulations. The polymer chains exhibit tumbling kinetics characterized by an exponential distribution of tumbling times. The characteristic time scale is proportional to the longest relaxation time of the polymer chains at equilibrium. The power spectral density of the extension, while monotonically decaying for large chain length or large Weissenberg number, exhibits a shallow peak for short chains, implying that shear flow induces nearly repetitive tumbling of the polymer chains. The time scale corresponding to the peak of the extension power spectral density is also proportional to the longest chain relaxation time.