Dynamic rheology of a supercooled polymer melt in non-uniform oscillating flows in rapidly oscillating plates

TL;DR

This study explores the complex, position-dependent rheological behavior of a supercooled polymer melt under rapid oscillations, revealing spatial variations in shear thinning and viscosity influenced by local flow conditions.

Contribution

It introduces a hybrid simulation approach to analyze non-uniform oscillating flows and uncovers detailed spatial rheological variations near oscillating plates.

Findings

Local rheology varies significantly with oscillation frequency and position.

Near the plate, viscosity becomes independent of frequency and shear thinning is enhanced.

Non-linear effects include intra-cycle shear thickening and suppression of higher harmonics.

Abstract

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.