Ergodic to non-ergodic transition monitored by scattered light intensity statistics

Many soft materials, such as gels or glasses, exhibit both a fast and a very slow relaxation behavior, often related to thermally activated processes restoring ergodicity. Pusey and Van Megen (Physica A 157 (1989) 705), have elaborated a theory that allows the usage of standard light scattering techniques to treat systems that are dynamically arrested, or non-ergodic, over the experimental time-scale. This theory concerning the distribution of intensity scattered by non-ergodic media is here extended, by taking into account second order temporal coherence of scattered radiation. The time-integrated intensity distribution function so obtained allows to distinguish between fast and slow contributions when the two time scales are not (or not yet) completely separated. Thus, by simple and quick static light scattering measurements one can follow an ergodic-to-non-ergodic transition. We present an experiment on gelation kinetics of sucrose-pectin systems, which illustrates the quality of the method, and show how the gel network is formed out of a homogeneous solution. (C) 2004 Elsevier B.V. All rights reserved.