Direct visualization of continuous simple shear in non-Newtonian polymeric fluids

Area of Science:

• Rheology
• Polymer Physics
• Fluid Dynamics

Background:

• Simple-shear flow of entangled polymer solutions is typically assumed to exhibit linear velocity profiles.
• Understanding these profiles is crucial for explaining phenomena like stress overshoot in polymer processing.

Purpose of the Study:

• To provide direct experimental evidence of nonlinear velocity profiles in entangled polymer solutions during startup shear flow.
• To investigate the origins of deviations from assumed linear velocity variations.

Main Methods:

• Particle tracking velocimetry (PTV) was employed to measure the velocity field.
• Experiments involved applying startup shear to a confined polymer solution sample.

Main Results:

• Direct evidence of nonlinear velocity profiles was observed, deviating from initial linearity.
• A shear rate gradient developed in the final state, indicating a non-uniform velocity distribution.
• The deviation is attributed to polymer chain disentanglement and orientation under high shear.

Conclusions:

• Entangled polymers exhibit nonlinear velocity profiles during shear flow, contrary to common assumptions.
• The ability of polymer chains to disentangle and orient leads to anisotropic constraints, causing velocity profile nonlinearity.
• This discovery provides new insights into the fundamental mechanisms underlying stress overshoot and polymer rheology.