Evolution of Particle-Scale Dynamics in an Aging Clay Suspension

Abstract

Multispeckle x-ray photon correlation spectroscopy was employed to characterize the slow dynamics of a suspension of highly charged, nanometer-sized disks. At wave vectors q corresponding to interparticle length scales, the dynamic structure factor follows a form f(q,t) approximately exp([-(t/tau)(beta)], where beta approximately 1.5. The relaxation time tau increases with the sample age t(a) approximately as tau approximately t(1.8)(a) and decreases with q as tau approximately q(-1). Such behavior is consistent with models that describe the dynamics in disordered elastic media in terms of strain from random, local structural rearrangements. The measured amplitude of f(q,t) varies with q in a manner that implies caged particle motion. The decrease in the range of this motion and an increase in suspension conductivity with increasing t(a) indicate a growth in interparticle repulsion as the mechanism for internal stress development implied by these models.

Slow dynamics, aging, and glassy rheology in soft and living matter

“…Ultra-slow dynamics and aging observed in colloidal gels [3], clay suspensions [7,8,24], compressed emulsions [2] and multilamellar vesicles [25] are understood in terms of the relaxation of internal stresses that are built into the sample at the jamming transition. The dynamical structure factors (DSFs) characterising these systems are strongly dependent on sample age (quantified in terms of the waiting time t a since sample preparation or the onset of jamming) and exhibit non-diffusive, faster-than-exponential forms.…”

“…For isotropic, nonergodic materials in which a time average and ensemble average are not equivalent, such multispeckle spectroscopy [56] provides a key advantage in that the ensemble average may be obtained by an average over pixels corresponding to the same wave vector. For these reasons, multispeckle spectroscopy has proved to be an excellent tool to probe materials with slow and spatially heterogeneous dynamics [2,3,7,24].…”

“…X-ray photon correlation spectroscopy (XPCS) uses synchrotron radiation to probe very high values of q that are inaccessible by PCS methods involving visible light [53,54]. For example, XPCS has been employed in [24] to study the particle scale dynamics in an aging clay suspension, where each particle is a disc approximately 15 nm in radius and 1 nm in thickness. The dynamics of multiply scattering samples can be accessed with diffusing wave spectroscopy (DWS) [55].…”

“…For dense materials such as glasses, sandpiles and foams, Liu and Nagel [22] suggest a 'jamming' phase diagram that considers the possibility of unjamming the system (i.e driving it towards ergodicity) by increasing its temperature, by applying appropriately large shear stresses, or by decreasing the density of its constituents. Universal non-diffusive slow dynamics and aging have been observed in a variety of jammed soft materials, for example, in colloidal suspensions [2,3,7,8,23,24], concentrated emulsions [2], micellar polycrystals [2] and lamellar gels [2,25]. Glassy dynamics also occur in biologically relevant systems.…”

“…We next focus on the SGR theory [29,30] that quantifies the flow and deformation behaviour of soft glassy materials (SGMs). Section 3 reviews experimental work on the slow dynamics and aging in soft and living matter, followed by observations of anomalous rheology in many of these systems, with some emphasis on our recent work on the aging behaviour in synthetic colloidal laponite clay suspensions [24] and the observation of soft glassy rheology in a thermotropic smectic liquid crystal confined within the pores of a compliant colloidal gel [35]. This is followed by a brief summary and concluding remarks in section 4.…”