Single polymer dynamics in semi-dilute solutions: linear and ring polymers

Synthetic and biological polymers are ubiquitous in nature and modern technologies. Traditional characterization methods of polymeric materials rely on bulk level measurements that can provide useful information on material properties. However, these methods generally cannot access underlying molecular information, such as polymer conformation, distributions in molecular behavior, and the role of intermolecular interactions in non-equilibrium flows. Over the past two decades, single molecule techniques have been established to investigate molecular-level dynamics, thereby allowing direct access to polymer chain relaxation mechanisms and polymer non-linear response under a variety of flows. Despite recent progress in the field of single polymer dynamics, however, the vast majority of single molecule studies has focused on dilute solutions of linear polymers. In this thesis, we effectively extend single molecule imaging to increasingly complex polymeric systems of increasing polymer concentration and more complex chain architectures. In this way, we aim to address several fundamental questions, including how do polymer concentration and chain architecture affect dynamics at the single chain level? We address these questions using a combination of single molecule experiments and Brownian dynamics simulations. In one project, we performed a series of single molecule experiments by systematically increasing polymer concentration to the semi-dilute untangled regime. Based on these results, we obtained a scaling relation for longest polymer relaxation time as a function of concentration, and these results are compared to blob scaling theories. We further studied single polymer dynamics upon a step-strain deformation in planar extensional flow, including both transient and steady state polymer extension. Experimental data are compared to results from large-scale Brownian dynamics simulations that include intra- and intermolecular hydrodynamic interactions and excluded volume interactions, work performed in collaboration with the Prakash group at Monash University. In this way, we obtain parameter-free predictions of polymer dynamics in non-dilute flows using the method successive fine-graining. Remarkably, our results show a close comparison between experiments and simulation, which provides a solid understanding of polymer dynamics in the semi-dilute concentration regime, both near equilibrium under strong flow. In the second project, we studied the impact of circular polymer or ring polymer topology on single chain dynamics in extensional flows. Single molecule experiments revealed that ring polymers stretch differently compared to linear polymers in extensional flows in the context of the coil-stretch transition. Interestingly, we found that the ring structure exhibits a strong hydrodynamic coupling between the two strands of a stretched ring, which leads to a "slow-down" of the coil-stretch transition and a looping effect of rings under strong extensional flow. Moving beyond our work on single chain dynamics in dilute and semi-dilute solutions, we further sought to identify how molecular-scale interactions are translated into collective non-Newtonian fluid properties. In particular, we developed a new technique to directly measure normal stresses or extensional viscosity in microfluidic devices by coupling the Stokes trap with particle tracking. Here, we study the phenomenon of flow-induced particle migration to measure polymer-induced solution stresses and extensional viscosity in semi-dilute solutions of DNA and synthetic polymers. We combined the automated hydrodynamic trap, which is a home-built microfluidic hydro- dynamic trap, and a piezo-nano positioning stage to directly observe particle migration in polymer solution undergoing planar extensional flow. Experimental data was analyzed in the context of a second-order fluid model in order to determine normal stress. Finally, extensional viscosity was deduced from particle migration experiments, and these results showed favorable comparison to extensional viscosity measurements determined with the optically-detected elastocapillary self-thinning dripping-onto-substrate (ODES-DOS) extensional rheometer. Overall, this thesis aims to provide a fundamental molecular picture of polymer dynamics in the semi-dilute concentration regime and for different polymer architectures. Combining single molecule fluorescence microscopy, Brownian dynamics simulation, 3D particle tracking, and continuum-level constitutive equations, we are able to provide an informative physical picture of polymers in non-Newtonian semi-dilute polymer solutions. From a broad view, this work provides a starting point to relate macroscopic stress response to a microscopic or molecular-level interactions, thereby providing a new perspective to understand non-linear polymer properties