  The dynamic rheology of a polymer melt composed of short chains with ten
beads between rapidly oscillating plates is investigated for various
oscillation frequencies by using the hybrid simulation of the molecular
dynamics and computational fluid dynamics. In the rapidly oscillating plates,
non-uniform boundary layer flows are generated over the plate due to inertia of
the fluid, and the local rheological properties of the melt are spatially
varied according to the local flow fields. Far from the plate, the local strain
decreases as the oscillation frequency increases such that the dynamic rheology
deviates from the linear moduli at the low oscillation frequencies rather than
high oscillation frequencies. On the contrary, near the plate, the local strain
rate increases with the oscillation frequency such that the shear thinning is
enhanced at high oscillation frequencies. In close vicinity to the plate, the
dynamic viscosity is mostly independent of the oscillation frequency, and the
shear thinning behavior becomes similar to that observed in steady shear flows.
Non-linear rheological properties are also investigated by the spectrum
analysis and the Lissajous-Bowditch curve. It is found that the fractional
amplitude of the higher harmonics to the linear harmonics is suppressed within
the boundary layer due to the non-slip boundary on the oscillating plate. We
also find that the melt exhibits inter-cycle shear thinning between different
positions but exhibits intra-cycle shear thickening at a fixed position in the
vicinity of the plate.
