Optical tweezers microrheology maps the dynamics of strain-induced local inhomogeneities in entangled polymers

TL;DR

This paper introduces optical tweezers microrheology (OTM) protocols and modeling techniques to map strain-induced inhomogeneities in entangled polymers, revealing Rouse-like elastic retraction as the dominant nonlinear relaxation mechanism.

Contribution

The study develops new OTM protocols and analytical models to characterize inhomogeneity fields, demonstrating that stress relaxation is dominated by elastic retraction rather than disentanglement.

Findings

Post-strain homogenization does not affect intrinsic stress relaxation.

Rouse-like elastic retraction dominates nonlinear stress relaxation.

New mLAOS protocols enable precise nonlinear microrheology.

Abstract

Optical tweezers microrheology (OTM) offers a powerful approach to probe the nonlinear response of complex soft matter systems, such as networks of entangled polymers, over wide-ranging spatiotemporal scales. OTM can also uniquely characterize the microstructural dynamics that lead to the intriguing nonlinear rheological properties that these systems exhibit. However, the strain in OTM measurements, applied by optically forcing a micro-probe through the material, induces network inhomogeneities in and around the strain path, and the resultant flow field complicates the measured response of the system. Through a robust set of custom-designed OTM protocols, coupled with modeling and analytical calculations, we characterize the time-varying inhomogeneity fields induced by OTM measurements. We show that post-strain homogenization does not interfere with the intrinsic stress relaxation dynamics of the system, rather it manifests as an independent component in the stress decay, even in highly nonlinear regimes such as with the microrheological-LAOS (mLAOS) protocols we introduce. Our specific results show that Rouse-like elastic retraction, rather than disentanglement and disengagement, dominates the nonlinear stress relaxation of entangled polymers at micro- and meso- scales. Thus, our study opens up possibilities of performing precision nonlinear microrheological measurements, such as mLAOS, on a wide range of complex macromolecular systems.