Single Polymer Relaxation Dynamics in Entangled Solutions

We directly observe the relaxation dynamics of single DNA molecules in semi-dilute entangled solutions using fluorescence microscopy and microfluidics. Our results reveal the emergence of multiple relaxation modes upon increasing concentration. Fluorescently labeled tracer polymers are dissolved in a background of semi-dilute entangled λ-DNA solutions (c_e 〈c〈c**), and the relaxation dynamics are directly observed. Single polymer relaxation trajectories are shown to exhibit either a single-mode or double-mode exponential decay, which starkly contrasts relaxation behavior from dilute (c«c*) and semi-dilute unentangled solutions (c*〈c〈c_e). As polymer concentration is increased from 3.6 c* to 19.1 c*, the fraction of molecules that exhibit single-mode exponential decay behavior decreases, while the fraction of double-mode exponential trajectories increases. We further explore the power law scalings of these relaxation times with concentration. We find the slow double-mode relaxation time is consistent with blob theory for semi-dilute entangled solutions in a good solvent, whereas the single-mode relaxation time and the fast double-mode relaxation time are found to have the same but weaker power law dependence on concentration.