Department of Physics, Engineering Physics & Astronomy

Department of Physics, Engineering Physics & Astronomy
Department of Physics, Engineering Physics & Astronomy

X-ray Photon Correlation Spectroscopy Studies of Slow Dynamics in Soft Glassy Materials

James Harden
University of Ottawa

Date: Friday, January 27, 2012
Time: 1:30 PM
Location: Stirling A


Disordered soft solids, such as foams, gels, concentrated emulsions, and dense colloidal suspensions, exhibit apparently universal features that are characteristic of out-of-equilibrium systems. In particular, these materials often display non-diffusive slow dynamics and a protracted evolution of their dynamical properties that bears strong resemblance to the phenomenon of aging seen in molecular glasses and glassy polymers, but also have features peculiar to soft systems. In an effort to understand the essential dynamical behaviour of soft disordered materials at the nanoscale, we have conducted X-ray photon correlation spectroscopy (XPCS) studies on a variety of soft solids and complex fluids. The combination of large wave vectors and long time scales accessible with XPCS makes the technique uniquely well suited for elucidating the nanoscale motions in such glassy materials.

In this talk, I will provide an overview of XPCS studies for several characteristic soft solids and complex fluids, including synthetic clay gels, concentrated nano-emulsions, and nano-particle networks. For each of these disordered soft solids, we observed the same generic slow dynamics characterized by an intermediate scattering function that follows a "compressed" exponential form, g1(Q,t) ~ exp[-(t/τ)^β], with β ≈ 1.5 and τ ~ Q-1. Such behaviour contrasts with glassy diffusion, for which correlation functions are stretched (β < 1) and τ ~ Q-2. We argue that the dynamical evolution of such disordered soft solids, while displaying signatures of aging, cannot be directly related to traditional aging phenomena in glasses. Rather, these dynamical features can be explained in terms of strain from random, highly localized stress relaxation events.

Refreshments will be available before the talk.