Professor Laura Kaufman
Department of Chemistry
Columbia University
"Revisiting Rotational-Translational Decoupling in Supercooled Systems on a Molecule-by-Molecule Basis"
Supercooled liquids and rubbery polymers are metastable systems that display unusual behaviors consistent with the presence of spatially heterogeneous dynamics, i.e. dynamics that vary across space and time. We measure rotations of single fluorescent probes in high molecular weight polystyrene near its glass transition temperature to characterize the time scales over which heterogeneities persist in this system. Additionally, aiming to resolve long-standing questions regarding the origins of a phenomenon known as rotational-translational decoupling, we also combine rotational and translational measurements of such probes in polystyrene. Rotational-translational decoupling, in which translational motion is apparently enhanced over rotational motion in violation of Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) predictions, has been posited to result from ensemble averaging in the context of spatially heterogeneous dynamics. I will describe ensemble and single molecule experiments that were performed in parallel to elucidate the origins of this phenomenon. Ensemble results and single molecule measurements both show a high degree of decoupling, with the most significant decoupling seen for particularly mobile molecules with anisotropic trajectories, providing support for anomalous diffusion as a critical driver of rotational-translational decoupling. Simulations of increasing complexity suggest that dynamic heterogeneity in the system under study is correlated; such that molecules exhibiting fast, (slow) dynamics maintain those dynamics for short (long) times. Taken together, the experiments and simulations reveal that rotational-translational decoupling exists at the single molecule level, is driven by changes in dynamics that occur over a range of timescales, and is a process in which exchange frequency is correlated with spatiotemporally local dynamics.
Laura Kaufman
Laura J. Kaufman leads a laboratory that is highly interdisciplinary and focused on the dynamics of complex, crowded systems. In particular, the laboratory studies heterogeneous dynamics in supercooled liquids with single molecule imaging, exciton diffusion in conjugated molecules at the single molecule and aggregate levels with single molecule spectroscopy, the mechanical properties and structure of biopolymer gels using rheology and microscopy, and cancer cell invasion in tissue approximations of tailored architecture. Laura graduated from Columbia University and earned her Ph.D. in Chemistry from the University of California, Berkeley in 2002. There she helped develop multi-dimensional Raman spectroscopy in the laboratory of Professor Graham R. Fleming under the expert guidance of our very own Department Head, David A. Blank. She went on to do postdoctoral work at Harvard University with Professors X. Sunney Xie and David A. Weitz, where she used CARS microscopy to study colloidal glasses and cell migration in three-dimensional environments.
331 Smith Hall