Multispeckle Diffusing Wave Spectroscopy Research Articles

How are colloidal suspensions that age rejuvenated by strain application?

We present here a microscopic study of the effect of shear on a dense purely repulsive colloidal suspension. We use multispeckle diffusing wave spectroscopy to monitor the transient motions of colloidal particles after being submitted to an oscillatory strain. This technique proves efficient to record the time evolution of the distribution of relaxation times. After a high oscillatory shear, we show that this distribution displays full aging behavior. Conversely, when a moderate shear is applied the distribution is modified in a non-trivial way. Whereas high shear is able to erase all the sample history and rejuvenate it, a moderate shear helps it to age. We call this phenomenon overaging. We demonstrate that overaging can be understood if the complete shape of the relaxation time distribution is taken into account. We finally report how the soft glassy rheology model accounts for this effect.

Rejuvenation and overaging in a colloidal glass under shear.

We report the modifications of the microscopic dynamics of a colloidal glass submitted to shear. We use multispeckle diffusing wave spectroscopy to monitor the evolution of the spontaneous slow relaxation processes after the samples have been submitted to various straining. We show that high shear rejuvenates the system and accelerates its dynamics, whereas moderate shear over-ages the system. We analyze these phenomena within the frame of the Bouchaud's trap model.

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

A multispeckle technique for efficiently measuring correctly ensemble-averaged intensity autocorrelation functions of scattered light from nonergodic and/or nonstationary systems is described. The method employs a charge coupled device (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 nonstationary 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 be combined with a 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, nonergodic dynamics, and slowly evolving transient behavior.

Bubble dynamics relaxation in aqueous foam probed by multispeckle diffusing-wave spectroscopy.

We study the bubble rearrangement dynamics in aqueous foam during the passage from liquidlike to solidlike behavior which follows a transient shear deformation that perturbs the bubble packing. The local dynamics is probed using multispeckle diffusing-wave spectroscopy. We show that following the perturbation the average time between rearrangements relaxes exponentially, with time elapsed since the end of the perturbation. The observed scaling of the characteristic relaxation time with perturbation amplitude and foam age is explained by a schematic coarse-grained model based on the scaling state hypothesis.