Semi-dilute polymer solutions are commonly encountered in printing applications for organic electronics and electroactive polymers. Despite recent progress, we still lack a complete understanding of dynamics in these systems. In this talk, I will discuss recent work from my group that effectively extends the field of single polymer dynamics to new materials, including polymers with complex topologies including rings and branched polymers. In the first project, we use single molecule techniques to study polymer dynamics in semi-dilute unentangled and entangled DNA solutions in extensional flow, including polymer relaxation from high stretch and transient stretching dynamics in flow. Interestingly, we observe a unique set of polymer chain conformations in semi-dilute solutions indicative of flow-induced topological entanglements in solutions that are nominally unentangled at equilibrium. Our results also show a decrease in transient polymer stretch and a milder coil-to-stretch transition compared to dilute solutions. Single molecule experiments are directly compared to large-scale Brownian dynamics simulations for semi-dilute solutions with intra- and intermolecular hydrodynamic interactions (HI) and excluded volume (EV), and excellent agreement is obtained using the method of successive fine graining (SFG). In a second project, I will discuss a new microfluidic method to measure normal stress and extensional viscosity that can be loosely described as passive yet non-linear microrheology. Here, we use the Stokes trap combined with particle tracking to study flow-induced particle migration in non-Newtonian solutions. Experimental results are analyzed using a second-order fluid model, which allows for measurement of extensional viscosity. Microfluidic measurements of extensional viscosity are directly compared to results from the dripping-onto-substrate (DOS) method, and good agreement is observed. In the final part of the talk, I will discuss our recent work in studying the dynamics of topologically complex polymers such as combs and stars using single molecule imaging. DNA-based comb polymers are synthesized utilizing a hybrid enzymatic-synthetic approach, followed by direct observation of the non-equilibrium dynamics of comb polymers in flow. Overall, our work aims to provide a molecular-level understanding of the role of topology and concentration on the emergent properties polymers using single molecule techniques.
Charles Schroeder is the Ray and Beverly Mentzer Faculty Scholar and Associate Professor of Chemical & Biomolecular Engineering at the University of Illinois at Urbana-Champaign. He is a member of the Center for Biophysics and Computational Biology, with affiliate status in the Department of Materials Science and Engineering, the Department of Chemistry, and the Carl Woese Institute for Genomic Biology at Illinois. Professor Schroeder received his B.S. in Chemical Engineering from Carnegie Mellon University, followed by an M.S. and Ph.D. in Chemical Engineering at Stanford University under the supervision of Professors Eric Shaqfeh and Steven Chu. Before joining the University of Illinois in 2008, he was a postdoctoral researcher at Harvard University and the University of California-Berkeley. Professor Schroeder has been the recipient of several awards, including a Packard Fellowship in Science and Engineering, a Camille Dreyfus Teacher-Scholar Award, an NSF CAREER Award, the Dean’s Award for Excellence in Research, the Arthur B. Metzner Award from the Society of Rheology, and an NIH Pathway to Independence Award (K99/R00).