The macroscopic properties of polymeric materials arise from the collective dynamics and interactions of individual polymer molecules. Yet these molecular-scale dynamics are generally hidden in conventional measurements, particularly far from equilibrium where polymeric liquids exhibit strongly nonlinear rheological behavior. Single-molecule fluorescence microscopy provides a powerful approach for directly observing molecular conformations and dynamics, revealing heterogeneous behavior and molecular subpopulations that are obscured by ensemble-averaged measurements.
Charles Schroeder III,
Princeton University
In this talk, I will describe our recent work using single-molecule techniques and automated flow control to investigate the nonequilibrium dynamics of entangled polymer solutions. I will first discuss how single-molecule measurements provide a direct window into characterizing polymer chain relaxation, revealing heterogeneous dynamics associated with intermolecular constraints. I will then describe recent experiments that directly visualize the dynamics of individual entangled polymer chains during the startup of shear flow. Surprisingly, individual chains undergo repeated end-over-end tumbling despite the strong topological confinement imposed by surrounding molecules. By combining single-molecule imaging with bulk rheology, we find that polymer chain tumbling emerges only after the stress overshoot during shear startup, suggesting a transition from elastic loading of the entanglement network to a nonlinear dynamical state in which transverse confinement is substantially weakened. Single-molecule trajectories further reveal coexisting populations of weakly deformed chains and chains undergoing intermittent stretching and tumbling. Together, these results provide a fundamentally new molecular-scale picture of how entangled polymeric materials reorganize under strong deformation and show how single-molecule measurements can connect microscopic molecular dynamics to macroscopic material response.
Charles Schroeder is the Eugene Higgins Professor and Director of Graduate Studies in the Department of Chemical and Biological Engineering at Princeton University. He is also Associated Faculty in Princeton Materials Institute. Dr. Schroeder received his B.S. in Chemical Engineering from Carnegie Mellon University, followed by an M.S. and Ph.D. in Chemical Engineering from Stanford University under the supervision of Professor Eric Shaqfeh and Professor Steven Chu. Dr. Schroeder was an NIH K99/R00 Postdoctoral Fellow and Jane Coffin Childs Postdoctoral Fellow in the Department of Chemistry and Chemical Biology at Harvard University. Dr. Schroeder has been recognized by several awards, including a Packard Fellowship for Science and Engineering, a Camille Dreyfus Teacher-Scholar Award, an NSF CAREER Award, the Arthur B. Metzner Award from the Society of Rheology, the Dean’s Award for Excellence in Research at the University of Illinois, and an NIH Pathway to Independence Award (K99/R00). Dr. Schroeder is a Fellow of the American Association for the Advancement of Science (AAAS), the Society of Rheology (SOR), and the American Physical Society (APS).