Direct observation of DNA dynamics in semi-dilute solutions in extensional flow

TL;DR

This study uses single molecule techniques to observe DNA dynamics in semi-dilute solutions under extensional flow, revealing unique transient behaviors and scaling laws that differ from dilute solutions, advancing understanding of polymer flow behavior.

Contribution

The paper provides the first detailed molecular-level insights into semi-dilute DNA solutions under extensional flow, highlighting unique transient conformations and a power-law relaxation time scaling.

Findings

Polymer relaxation time scales with concentration as a power law with exponent 0.56.

Transient stretching behavior differs from dilute solutions, with reduced maximum stretch.

Unique molecular conformations during transient stretching indicate different pathways due to intermolecular interactions.

Abstract

The dynamic behavior of semi-dilute polymer solutions is governed by an interplay between solvent quality, concentration, molecular weight, and flow type. Semi-dilute solutions are characterized by large fluctuations in concentration, wherein polymer coils interpenetrate but may not be topologically entangled at equilibrium. In non-equilibrium flows, it is generally thought that polymer chains can self-entangle in semi-dilute solutions, thereby leading to entanglements in solutions that are nominally unentangled at equilibrium. Despite recent progress, we still lack a complete molecular-level understanding of these dynamics. In this work, we use single molecule techniques to study the dynamics of semi-dilute solutions of DNA in planar extensional flow, including polymer relaxation from high stretch, transient stretching dynamics in step-strain experiments, and steady-state stretching in flow. Our results are consistent with a power-law scaling of the longest polymer relaxation time in semi-dilute solutions, revealing an effective excluded volume exponent $\nu$ = 0.56. We further studied the non-equilibrium stretching dynamics in extensional flow, with results showing a decrease in transient polymer stretch at moderate Weissenberg number and a milder coil-to-stretch transition in semi-dilute solutions compared to dilute solutions. Interestingly, a unique set of molecular conformations during the transient stretching process for single polymers in semi-dilute solutions is observed, which suggests transient stretching pathways for polymer chains in semi-dilute solutions are qualitatively different than dilute solutions due to intermolecular interactions. Taken together, this work provides a molecular framework for understanding the non-equilibrium stretching dynamics of semi-dilute solutions in strong flows.