Multispeckle diffusing-wave spectroscopy: a tool to study slow relaxation and time-dependent dynamics

TL;DR

This paper introduces a multispeckle diffusing-wave spectroscopy method using a CCD camera to efficiently measure ensemble-averaged intensity autocorrelation functions, enabling real-time analysis of non-ergodic and non-stationary systems across a wide time range.

Contribution

The paper presents a novel multispeckle technique combining CCD detection and real-time correlation to study slow and evolving dynamics in complex materials.

Findings

Allows simultaneous calculation of up to 500 correlation functions

Enables real-time, snapshot measurements of non-stationary systems

Effective for studying aging in soft glassy materials

Abstract

A multispeckle technique for efficiently measuring correctly ensemble-averaged intensity autocorrelation functions of scattered light from non-ergodic and/or non-stationary systems is described. The method employs a CCD camera as a multispeckle light detector and a computer-based correlator, and permits the simultaneous calculation of up to 500 correlation functions, where each correlation function is started at a different time. The correlation functions are calculated in real time and are referenced to a unique starting time. The multispeckle nature of the CCD camera detector means that a true ensemble average is calculated; no time averaging is necessary. The technique thus provides a "snapshot" of the dynamics, making it particularly useful for non-stationary systems where the dynamics are changing with time. Delay times spanning the range from 1 ms to 1000 s are readily achieved with this method. The technique is demonstrated in the multiple scattering limit where diffusing-wave spectroscopy theory applies. The technique can also be combined with a recently-developed two-cell technique that can measure faster decay times. The combined technique can measure delay times from 10 ns to 1000 s. The method is peculiarly well suited for studying aging processes in soft glassy materials, which exhibit both short and long relaxation times, non-ergodic dynamics, and slowly-evolving transient behavior.