Experimental evidence for the intricate free-energy landscape for a soft glassy system (original) (raw)
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Physical review letters, 2007
In the free-energy landscape picture of glassy systems, their slow dynamics is due to a complicated free-energy landscape with many local minima. We show that for a colloidal glassy material multiple paths can be taken through the free-energy landscape. The evolution of the nonergodicity parameter shows two distinct master curves that we identify as gels and glasses. We show that for a range of colloid concentrations, the transition to nonergodicity can occur in either direction (gel or glass), accompanied by "hesitations" between the two. Thus, colloidal gels and glasses are merely global free-energy minima in the same free-energy landscape, and the paths leading to these minima can be complicated.
Signatures of Dynamical Heterogeneity in the Structure of Glassy Free-Energy Minima
Physical Review Letters, 2008
From numerical minimization of a model free energy functional for a system of hard spheres, we show that the width of the local peaks of the time-averaged density field at a glassy free-energy minimum exhibits large spatial variation, similar to that of the "local Debye-Waller factor" in simulations of dynamical heterogeneity. Molecular dynamics simulations starting from a particle configuration generated from the density distribution at a glassy free-energy minimum show similar spatial heterogeneity in the degree of localization, implying a direct connection between dynamical heterogeneity and the structure of glassy free energy minima.
Glassy dynamics and dynamical heterogeneity in colloids
Concentrated colloidal suspensions are a well-tested model system which has a glass transition. Colloids are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high; the particles are roughly analogous to individual molecules in a traditional glass. Because the particle size can be large (100 nm-1000 nm), these samples can be studied with a variety of optical techniques including microscopy and dynamic light scattering. Here we review the phenomena associated with the colloidal glass transition, and in particular discuss observations of spatial and temporally heterogeneous dynamics within colloidal samples near the glass transition. Although this Chapter focuses primarily on results from hard-sphere-like colloidal particles, we also discuss other colloidal systems with attractive or soft repulsive interactions. 0.1 Colloidal hard spheres as a model system for the glass transition 0.1.1 The hard sphere colloidal glass transition When some materials are rapidly cooled, they form an amorphous solid known as a glass. This transition to a disordered solid is the glass transition (Götze and Sjogren, 1992; Stillinger, 1995; Ediger et al., 1996; Angell et al., 2000). As the temperature of a molecular glass-forming material is decreased the viscosity rises smoothly but rapidly, with little apparent change in the microscopic structure (Ernst et al., 1991; van Blaaderen and Wiltzius, 1995). Glass formation may result from dense regions of well-packed molecules or a decreasing probability of finding mobile regions. As no structural mechanisms for this transition have been found, many explanations rely on dynamic mechanisms. Some theoretical explanations focus on the idea of dynamical heterogeneities (Götze and Sjogren, 1992; Sillescu, 1999; Ediger, 2000; Adam and Gibbs, 1965). The underlying concept is that, for any molecule to move, all molecules within a surrounding region must "cooperate" in their movement. As the glass transition is approached the sizes of these regions grow, causing the rise in macroscopic viscosity (Adam and Gibbs, 1965). The microscopic length scale characterizing the size of these regions could potentially diverge, helping explain the macroscopic viscosity divergence. However, it is also possible that these regions could grow but not be directly connected to the viscosity divergence. Additionally, it is not completely clear if the viscosity itself diverges or simply becomes too large to measure (Hecksher et al., 2008). While the existence of dynamical heterogeneities in glassy systems has been confirmed in a wide variety of systems, the details of this conceptual picture remain in debate (
Free-energy landscape of simple liquids near the glass transition
Journal of Physics: Condensed Matter, 2000
Properties of the free energy landscape in phase space of a dense hard sphere system characterized by a discretized free energy functional of the Ramakrishnan-Yussouff form are investigated numerically. A considerable number of glassy local minima of the free energy are located and the distribution of an appropriately defined "overlap" between minima is calculated. The process of transition from the basin of attraction of a minimum to that of another one is studied using a new "microcanonical" Monte Carlo procedure, leading to a determination of the effective height of free energy barriers that separate different glassy minima. The general appearance of the free energy landscape resembles that of a putting green: deep minima separated by a fairly flat structure. The growth of the effective free-energy barriers with increasing density is consistent with the Vogel-Fulcher law, and this growth is primarily driven by an entropic mechanism.
Energy barriers, entropy barriers, and non-Arrhenius behavior in a minimal glassy model
Physical review. E, 2016
We study glassy dynamics using a simulation of three soft Brownian particles confined to a two-dimensional circular region. If the circular region is large, the disks freely rearrange, but rearrangements are rarer for smaller system sizes. We directly measure a one-dimensional free-energy landscape characterizing the dynamics. This landscape has two local minima corresponding to the two distinct disk configurations, separated by a free-energy barrier that governs the rearrangement rate. We study several different interaction potentials and demonstrate that the free-energy barrier is composed of a potential-energy barrier and an entropic barrier. The heights of both of these barriers depend on temperature and system size, demonstrating how non-Arrhenius behavior can arise close to the glass transition.
Non-equilibrium transitions in colloidal glasses and gels
2016
In this thesis, we study the structure dynamics and mechanical response of colloidal glasses and gels in the presence of applied stresses and aging. We follow the dynamics and three-dimensional structures of the glass and gel in real time using confocal microscopy and X-ray scattering. The main purpose of this study is to motivate a new conceptual approach in terms of a dynamic first-order transition in response to applied stress or even upon aging of the glass. This topic is poorly understood due to the difficulty of investigating these driven systems over a wide range of length and time scales, and the lack of theoretical models to describe the non-affine displacements of the particles under shear, and of an appropriate dynamic order parameter to describe dynamic states in a disordered glassy system. Recent development of X-ray scattering and confocal microscopy, and a new conceptual idea of a dynamical phase transition allows us to investigate this challenging topic. Here, we can...
Slow dynamics in glassy soft matter
Journal of Physics: Condensed Matter, 2005
Measuring, characterizing and modelling the slow dynamics of glassy soft matter is a great challenge, with an impact that ranges from industrial applications to fundamental issues in modern statistical physics, such as the glass transition and the description of out-of-equilibrium systems. Although our understanding of these phenomena is still far from complete, recent simulations and novel theoretical approaches and experimental methods have shed new light on the dynamics of soft glassy materials. In this paper, we review the work of the last few years, with an emphasis on experiments in four distinct and yet related areas: the existence of two different glass states (attractive and repulsive), the dynamics of systems very far from equilibrium, the effect of an external perturbation on glassy materials, and dynamical heterogeneity.
Entropic origin of the growth of relaxation times in simple glassy liquids
Physical Review E, 1998
Transitions between "glassy" local minima of a model freeenergy functional for a dense hard-sphere system are studied numerically using a "microcanonical" Monte Carlo method that enables us to obtain the transition probability as a function of the free energy and the Monte Carlo "time". The growth of the height of the effective free energy barrier with density is found to be consistent with a Vogel-Fulcher law. The dependence of the transition probability on time indicates that this growth is primarily due to entropic effects arising from the difficulty of finding low-free-energy saddle points connecting glassy minima. 64.70.Pf,64.60.Ak,64.60.Cn