Aging and ultra-slow equilibration in concentrated colloidal hard spheres (original) (raw)
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Ageing and ultra-slow equilibration in concentrated colloidal hard spheres
Journal of Physics: Condensed Matter, 2005
We study the dynamic behaviour of concentrated colloidal hard spheres using Time Resolved Correlation, a light scattering technique that can detect the slow evolution of the dynamics in out-of-equilibrium systems. Surprisingly, equilibrium is reached a very long time after sample initialization, the non-stationary regime lasting up to three orders of magnitude more than the relaxation time of the system. Before reaching equilibrium, the system displays unusual aging behaviour. The intermediate scattering function decays faster than exponentially and its relaxation time evolves non-monotonically with sample age.
Dynamic light scattering measurements in the activated regime of dense colloidal hard spheres
Journal of Statistical Mechanics: Theory and Experiment, 2009
We use dynamic light scattering and numerical simulations to study the approach to equilibrium and the equilibrium dynamics of systems of colloidal hard spheres over a broad range of densities, from dilute systems up to very concentrated suspensions undergoing glassy dynamics. We discuss several experimental issues (sedimentation, thermal control, non-equilibrium ageing effects, dynamic heterogeneity) arising when very large relaxation times are measured. When analyzed over more than seven decades in time, we find that the equilibrium relaxation time, τ α , of our system is described by the algebraic divergence predicted by mode-coupling theory over a window of about three decades. At higher density, τ α increases exponentially with distance to a critical volume fraction ϕ 0 , which is much larger than the mode-coupling singularity. This is reminiscent of the behavior of molecular glass-formers in the activated regime. We compare these results to previous work, carefully discussing crystallization and size polydispersity effects. Our results suggest the absence of a genuine algebraic divergence of τ α in colloidal hard spheres.
iii Pa'Mi Chaparrita Que siempre ilumina mi camino en la oscuridad v Abstract This thesis reports on a comprehensive experimental study of the collective dynamics of colloidal hard sphere suspensions. The main quantity measured is the coherent Intermediate Scattering Function (ISF) using a range of techniques based on Dynamic Light Scattering (DLS). The collective dynamics are measured as a function of scattering vector for volume fractions spanning from dilute samples, through the fluid phase and the metastable region, up until deep in the glass region. This work describes two major explorations: (i) the effect of volume of fraction on the q-dependency of the collective dynamics; and (ii) a study of the ageing processes in colloidal glasses. The present work is unique in the application of several advanced experimental techniques, and in the level of averaging that has been carried out, enabling a more sophisticated analysis than has previously been possible. This includes the characterization of non-Fickian processes and the determination of the current-current correlation function (CCCF) in the metastable fluid, and the quantitative characterization of the ageing process in the hard sphere glass. In addition, by combining aspects of the coherent and incoherent ISFs, this work also allows the expression of the collective dynamics in terms of the single particle displacement. The results show a dynamical change at the freezing point (φ f ), which exposes the incapacity of the system to dissipate thermal energy via diffusing momentum currents, i.e. viscous flow. The structural impediments responsible for this, associated with dynamical heterogeneities, begin at the structure factor peak, and spread to other spatial modes as the volume fraction increases. Above the glass transition (φ g ), structural relaxation becomes arrested at all spatial modes probed, i.e. flow is arrested. It is found that, following the quench, samples above the glass volume fraction approach some final "ideal" glass in an algebraic manner. However, although the long time dynamics exhibit ageing, the non-ergodicity factor, a measure of the average structure of the sample, does not show any significant ageing. This dynamical ageing process, decoupled from changes in the average structure, is identified with irreversible exchange of particles.
Temporal heterogeneity of the slow dynamics of a colloidal paste
AIP Conference Proceedings, 2004
We investigate the slow dynamics of a soft glass, a concentrated suspension of polydisperse colloidal particles, by using multispeckle Diffusing Wave Spectroscopy (DWS). Two distinct regimes are observed: for small sample age, t w , the dynamics smoothly slows down, as revealed by a nearly linear increase of the characteristic relaxation time, τ s , measured by DWS. At longer ages, the dynamics is quasi stationary, but τ s exhibit anomalously large fluctuations in time. The time scale of the dynamical fluctuations is found to be slightly shorter than the average relaxation time. The variance of the intensity correlation function is maximum for time delays comparable to the average relaxation time, in striking analogy with recent simulations of glass-forming liquids.
Aging in short-ranged attractive colloids: a numerical study
The Journal of chemical physics, 2004
We study the aging dynamics in a model for dense simple liquids, in which particles interact through a hard-core repulsion complemented by a short-ranged attractive potential, of the kind found in colloidal suspensions. In this system, at large packing fractions, kinetically arrested disordered states can be created both on cooling (attractive glass) and on heating (repulsive glass). The possibility of having two distinct glasses, at the same packing fraction, with two different dynamics offers the unique possibility of comparing-within the same model-the differences in aging dynamics. We find that, while the aging dynamics of the repulsive glass is similar to the one observed in atomic and molecular systems, the aging dynamics of the attractive glass shows novel unexpected features.
Physical Review Letters, 2012
The spatial fluctuations of the dynamics of a colloidal system composed of nanoparticles are probed by a novel experimental setup, which combines homodyne and heterodyne dynamic light scattering focused onto a micron-sized volume via a microscope objective. The technique is used to measure the four-point susceptibility of an aging colloidal suspension, revealing a breakdown of the Gaussian approximation for the correlation function of the scattered electromagnetic field. The deviation from the Gaussian approximation increases with waiting time as the system evolves toward an arrested phase, signaling the gradual emergence of higher-order nontrivial dynamic correlations.
Physical Review E, 2005
Self-intermediate scattering functions ͑ISFs͒ are measured by dynamic light scattering for the colloidal fluid of hard spheres for both equilibrium and nonequilibrium ͑undercooled͒ conditions, i.e., for volume fractions below and above the known freezing transition of the hard-sphere system. The delay time m where the mean-squared displacement, or the low wave-vector limit of the ISF, exhibits its maximum stretching is identified as a characteristic of the non-Markovian process͑es͒ and is used to separate the ISF into fast ͑ Ͻ m ͒ and slow ͑ Ͼ m ͒ contributions. Each of these contributions exposes qualitative differences in the dynamics of the particles between the equilibrium and nonequilibrium colloidal fluids. These changes in the relaxation scenario signal the colloidal fluid's awareness of its traversal of the freezing volume fraction.
Probing the Equilibrium Dynamics of Colloidal Hard Spheres above the Mode-Coupling Glass Transition
Physical Review Letters, 2009
We use dynamic light scattering and computer simulations to study equilibrium dynamics and dynamic heterogeneity in concentrated suspensions of colloidal hard spheres. Our study covers an unprecedented density range and spans seven decades in structural relaxation time, , including equilibrium measurements above ' c , the location of the glass transition deduced from fitting our data to mode-coupling theory. Instead of falling out of equilibrium, the system remains ergodic above ' c and enters a new dynamical regime where increases with a functional form that was not anticipated by previous experiments, while the amplitude of dynamic heterogeneity grows slower than a power law with , as found in molecular glass formers close to the glass transition.
Mesoscopic Model of Aging in Colloids
Physics Procedia, 2014
A model of dense hard sphere colloids building on simple notions of particle mobility and spatial coherence is presented and shown to reproduce results of experiments and simulations for key quantities such as the intermediate scattering function, the particle mean-square displacement and the χ 4 mobility correlation function. All results are explained by two emerging and interrelated dynamical properties: i) a rate of intermittent events, quakes, which decreases as the inverse of the system age t, leading to μ q (t w , t) ∝ log(t/t w) as the average number of quakes occurring between the 'waiting time' t w and the current time t; ii) a length scale characterizing correlated domains, which increases linearly in log t.
The Journal of Chemical Physics, 2011
Intermediate Scattering Functions (ISF's) are measured for colloidal hard sphere systems using both Dynamic Light Scattering (DLS) and X-ray Photon Correlation Spectroscopy (XPCS). We compare the techniques, and discuss the advantages and disadvantages of each. Both techniques agree in the overlapping range of scattering vectors. We investigate the scaling behaviour found by Segre and Pusey 1 but challenged by Lurio et al. 2. We observe a scaling behaviour over several decades in time but not in the long time regime. Moreover, we do not observe long time diffusive regimes at scattering vectors away from the peak of the structure factor and so question the existence of a long time diffusion coefficients at these scattering vectors.