Equilibrium Dynamics of Dense Colloids and Polymers: X-ray Photon Correlation Spectroscopy Studies

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

This is an experimental condensed matter physics project that will investigate the dynamical properties of colloids, polymers and related fluids, commonly termed soft mattes. While the static structure of soft matter has been extensively examined, the dynamical properties on the most relevant length scales (10-100 nm) have been much more difficult to study. This project will yield new insight into the collective dynamics of soft materials, with particular emphasis on concentrated and entangled systems. To achieve the scattering wave vectors required, the emerging technique of x-ray photon correlation spectroscopy (XPCS) will be employed. The role of hydrodynamic interactions and entanglements in the dynamics of isotropic suspensions of colloidal rods will also be studied. Dynam-ical critical behavior near the colloidal liquid-colloidal gas critical point in colloid/polymer mixtures is another topic of interest. Understanding how a divergent viscosity, predicted for this dynamic universality class, affects the dynamics on length scales comparable to the inter-particle separation will improve our understanding of protein crystallization. The dynamics of entangled polymer systems will be investigated and compared to relevant theories. Importantly, these investigations will provide quantitative tests of the reptation model of polymer dynamics in the previously-inaccessible regime of long times, short distances, and flexible poly-mers. Students trained under this project will contribute to the pool of talent needed to fully utilize this nation's major investment in synchrotron-radiation-based research. In addition, the further development of x-ray photon correlation spectroscopy under this program will contribute to the scientific case for a new generation of even-more-powerful x-ray sources. %%% Complex fluids, such as fine dispersions of solid particles within a fluid (colloidal dispersions), blends of long-chain polymers, and mixtures of oil, water and soap are an important class of materials, commonly termed "soft matter". While the static structure of such soft materials has been studied extensively, the molecular motions of complex fluids have been much more difficult to study. In this project, a novel synchrotron-based x-ray scatter-ing method - x-ray photon correlation spectroscopy - will be used to examine very slow motions on small distances, and should yield new insight into the thermal movements within colloidal and polymeric systems. Comparisons between the results obtained and available theories will provide a critical test of the theories. The insights gained may be practically important for improved understanding and control of such diverse phenomena as protein crystallization or the flow of polymeric fluids. Students trained to carry out cutting-edge x-ray scattering research under this program will contribute to the pool of talent needed to make use of this nation's major investment in synchrotron-radiation-based research. The graduates will be well-prepared to carry out academic, industrial, or governmental research. The further development of x-ray photon correlation spectroscopy under this program will contribute to the scientific case for a new generation of even-more-powerful x-ray sources.