Understanding the molecular basis of rheological properties is crucial from both experimental and theoretical perspectives. Slit rheometry is commonly employed to measure the viscosity of fluids. This study utilized molecular dynamics simulations to investigate isothermal contraction flow at the nanoscale. Short linear polyethylene chains were uniformly extruded by a constant-speed piston from a reservoir through an abrupt contraction slit into the surrounding vacuum. Overall, die swelling and die wetting phenomena were observed. Molecular chains were stretched within the slit, while those outside the slit shrunk. Notably, the velocity profile within the slit varied with wall slip at different extrusion velocities. The relationship between the apparent shear viscosity and shear rate exhibited two primary characteristics: the first-Newtonian plateau and the shear thinning slope. Therefore, this molecular simulation method effectively demonstrates the general non-Newtonian behavior of macroscopic polymer fluids.
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