Large Amplitude Oscillatory Extension (LAOE) of dilute polymer solutions

TL;DR

This paper introduces a microfluidic method for studying the nonlinear extensional rheology of dilute polymer solutions under large amplitude oscillatory extension, revealing deviations from Newtonian behavior and identifying conditions for nonlinearities.

Contribution

It presents a novel experimental framework combining microfluidics and oscillatory flows to analyze nonlinear extensional rheology of complex fluids, validated by simulations.

Findings

Newtonian fluids show linear pressure-strain relationships.

Dilute polymer solutions exhibit excess pressure drops and nonlinear deviations.

Identifies critical conditions and unique Lissajous curves for nonlinear behavior.

Abstract

This study presents an experimental framework for large amplitude oscillatory extension (LAOE) to investigate nonlinear material properties of complex fluids. Using a microfluidic optimized shape cross-slot extensional rheometer, we generate approximately homogeneous planar extensional flows driven by programmable syringe pumps operating in oscillatory or pulsatile sinusoidal modes. Micro-particle image velocimetry and simultaneous pressure drop measurements are employed to analyze the time-dependent flow field and elastic stress response. For Newtonian fluids, a linear relationship between the applied strain rate and pressure drop is observed across a wide range of oscillation amplitudes and frequencies. In contrast, dilute polymer solutions exhibit significant deviations, with excess pressure drops and divergence between average strain rates along extension and compression axes during the LAOE cycle. By spanning a broad range of Weissenberg and Deborah numbers, we identify unique Lissajous curves and critical conditions for the onset of nonlinearities under oscillatory extension. Numerical simulations, assuming homogeneous flow, underpin the experimental findings, validating the robustness of our microfluidic approach. This study demonstrates the utility of oscillatory extensional flows for probing the nonlinear rheological behavior of soft materials, offering quantitative insights into their extensional properties under nonlinear flow conditions.