Structural and microscopic relaxations in a colloidal glass

Abstract

The aging dynamics of a colloidal glass has been studied by multiangle dynamic light scattering, neutron spin echo, X-ray photon correlation spectroscopy and molecular dynamics simulations. The two relaxation processes, microscopic (fast) and structural (slow), have been investigated in an unprecedentedly wide range of time and length scales covering both ergodic and nonergodic regimes. The microscopic relaxation time remains diffusive at all length scales across the glass transition scaling with wavevector Q as Q(-2). The length-scale dependence of structural relaxation time changes from diffusive, characterized by a Q(-2)-dependence in the early stages of aging, to a Q(-1)-dependence in the full aging regime which marks a discontinuous hopping dynamics. Both regimes are associated with a stretched behaviour of the correlation functions. We expect these findings to provide a general description of both relaxations across the glass transition.

Relaxation is a Two-Step Process for Metallic Glasses

“…In these works the fast process is independent on both T and time, and is due to microscopic PRL 118, 225901 (2017) Selected for a Viewpoint in Physics P H Y S I C A L R E V I E W L E T T E R S week ending 2 JUNE 2017 interactions between a particle and the cage created by its nearest neighbors. This fast process reflects a diffusive particle motion and it is usually described by a single exponential decay (thus with a shape exponent ¼ 1) [21]. In contrast with those works, here we probe the stress response in the deep nonequilibrium glassy state, and we observe a completely different additional decoupling between two dynamical processes.…”

“…This behavior changes when T is decreased 20 − 30 K below T g (open symbols). There, we do observe the formation of a shoulder for intermediate times (∼30 min) which becomes more and more prominent upon cooling, resulting in an unexpectedly well-defined two-step decay in the deep glass.The relaxation spectra observed here, phenomenologically bear a striking resemblance to that usually observed in the density fluctuations in Lennard-Jones glasses [2,17], molecular supercooled liquids [18], and colloidal suspensions [19][20][21]. In these works the fast process is independent on both T and time, and is due to microscopic PRL 118, 225901 (2017) …”

Non-diffusive dynamics in a colloidal glass: Aging versus rejuvenation

“…4. Previous studies agree on the existence of a typical dual waiting time dependence of the structural relaxation time τ [2,3,37,5,13,27]: an exponential waiting time dependence τ (t w ) ∼ e tw at small t w , in the cage forming regime [37], and a waiting time power law dependence τ (t w ) ∼ t α w with α ∼ 1 at larger t w , in the full aging regime [37]. While in the cage forming regime the dynamical behaviour is diffusive (τ (Q) ∼ Q −2 ) with stretched (β <1) intensity correlation functions, in the full aging regime the dynamics is no more diffusive but characterized by a τ (Q) ∼ Q −1 dependence with three different behaviours of the correlation functions depending on the history of the sample.…”

“…It has been attributed to the relaxation of internal stresses [2,39,40]. In between the cage forming and full aging regimes MD simulations [27] find, in spontaneously aged sample, stretched exponential curves and a relaxation time well described as τ (Q) ∼ Q −n with 1 < n < 2.…”

“…Once dispersed in water Laponite ages towards different arrested states like equilibrium gel and glass depending on ionic strength and particle concentrations [17]. In this work we focus on the glass state at concentration C w = 3.0 % largely investigated experimentally through different complementary techniques such as Dynamic Light Scattering [3,18,5,19], Rheology [20,21,22], Small Angle X-ray Scattering [23,24,25], Small Angle Neutron Scattering [26], X-ray Photon Correlation Spectroscopy [4,13,15,27] and Neutron Spin Echo [27]. The waiting time (t w ) (time scale) and Q (spatial scale) dependence of the structural relaxation process of spontaneously aged and rejuvenated samples are studied.…”