Three-Dimensional Direct Imaging of Structural Relaxation Near the Colloidal Glass Transition (original) (raw)
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Correlations between short- and long-time relaxation in colloidal supercooled liquids and glasses
Physical Review E, 2019
Spatiotemporal dynamics of short-and long-time structural relaxation are measured experimentally as a function of packing fraction, φ, in quasi-two-dimensional colloidal supercooled liquids and glasses. The relaxation times associated with long-time dynamic heterogeneity and short-time intracage motion are found to be strongly correlated and to grow by orders of magnitude with increasing φ towards dynamic arrest. We find that clusters of fast particles on the two timescales often overlap, and, interestingly, the distribution of minimum-spatial-separation between closest non-overlapping clusters across the two timescales is revealed to be exponential with a decay length that increases with φ. In total, the experimental observations suggest short-time relaxation events are very often precursors to heterogeneous relaxation at longer timescales in glassy materials.
2011
We analyze data from confocal microscopy experiments of a colloidal suspension to validate predictions of rapid sporadic events responsible for structural relaxation in a glassy sample. The trajectories of several thousand colloidal particles are analyzed, confirming the existence of rapid sporadic events responsible for the structural relaxation of significant regions of the sample, and complementing prior observations of dynamical heterogeneity. The emergence of relatively compact clusters of mobility allows the dynamics to transition between the large periods of local confinement within its potential energy surface, in good agreement with the picture envisioned long ago by Adam and Gibbs and Goldstein. PACS numbers: 61.20.Ne (Structure of simple liquids), 82.70.Dd (Colloids)
Microscopic structural relaxation in a sheared supercooled colloidal liquid
2010
The rheology of dense amorphous materials under large shear strain is not fully understood, partly due to the difficulty of directly viewing the microscopic details of such materials. We use a colloidal suspension to simulate amorphous materials, and study the shear-induced structural relaxation with fast confocal microscopy. We quantify the plastic rearrangements of the particles in several ways. Each of these measures of plasticity reveals spatially heterogeneous dynamics, with localized regions where many particles are strongly rearranging by these measures. We examine the shapes of these regions and find them to be essentially isotropic, with no alignment in any particular direction. Furthermore, individual particles are equally likely to move in any direction, other than the overall bias imposed by the strain.
Contribution of Slow Clusters to the Bulk Elasticity Near the Colloidal Glass Transition
Physical Review Letters, 2006
We use confocal microscopy to visualize individual particles near the colloidal glass transition. We identify the most slowly-relaxing particles and show that they form spatially correlated clusters that percolate across the sample. In supercooled fluids, the largest cluster spans the system on short time scales but breaks up on longer time scales. In contrast, in glasses, a percolating cluster exists on all accessible time scales. Using molecular dynamics simulation, we show that these clusters make the dominant contribution to the bulk elasticity of the sample.
The physics of the colloidal glass transition
2012
As one increases the concentration of a colloidal suspension, the system exhibits a dramatic increase in viscosity. Beyond a certain concentration, the system is said to be a colloidal glass; structurally, the system resembles a liquid, yet motions within the suspension are slow enough that it can be considered essentially frozen. For several decades, colloids have served as a valuable model system for understanding the glass transition in molecular systems. The spatial and temporal scales involved allow these systems to be studied by a wide variety of experimental techniques. The focus of this review is the current state of understanding of the colloidal glass transition, with an emphasis on experimental observations. A brief introduction is given to important experimental techniques used to study the glass transition in colloids. We describe features of colloidal systems near and in glassy states, including increases in viscosity and relaxation times, dynamical heterogeneity and ageing, among others. We also compare and contrast the glass transition in colloids to that in molecular liquids. Other glassy systems are briefly discussed, as well as recently developed synthesis techniques that will keep these systems rich with interesting physics for years to come. 18 4.3. Shear of colloidal glasses 20 5. Other soft glassy materials 21 5.1. Soft colloids, sticky particles, emulsions and foams 21 5.2. Future directions: anisotropic colloidal particles 22 6. Conclusion 23 Acknowledgments 24 References 24
Physical Review Letters, 2010
We identify the pattern of microscopic dynamical relaxation for a two dimensional glass forming liquid. On short timescales, bursts of irreversible particle motion, called cage jumps, aggregate into clusters. On larger time scales, clusters aggregate both spatially and temporally into avalanches. This propagation of mobility, or dynamic facilitation, takes place along the soft regions of the systems, which have been identified by computing isoconfigurational Debye-Waller maps. Our results characterize the way in which dynamical heterogeneity evolves in moderately supercooled liquids and reveal that it is astonishingly similar to the one found for dense glassy granular media.
Physical Review E, 2018
Superposition approaches have generally been proposed to create a dynamic rheological map to access colloidal glassy dynamics beyond experimental time windows. However, the validity of the superposition approaches in colloids near the glass transition is questionable owing to the well-known emergence of a β-relaxation process there. Here, we employ a time-concentration superposition (TCS) approach, analogous to time-temperature superposition (TTS) and TCS approaches in molecular systems, utilizing a combination of macroscopic rheological experiments and microscopic Brownian dynamics (BD) simulations, where concentration jumps are performed by a sudden growth of particle size (soft PS-PNIPAM particles in experiment and nearly hard spheres in simulation) at a fixed number of particles. We have examined whether a characteristic master curve can be obtained through horizontal and vertical shifting of the dynamic data, finding that TCS does not hold for either the experimental or simulation systems. We identify the origin of this breakdown as not only the emergence of a strong β-relaxation process but also its overlap with the α-relaxation in both the experimental soft-sphere and the simulated near hard-sphere colloids near to the glass transition concentration. Further understanding of the lack of validity of TCS results from analysis of both experimental and simulation data in the framework of the Baumgaertel-Schausberger-Winter (BSW) relaxation spectrum which provide a means to determine the concentration dependences of both the α-and the β-relaxations, which seem to follow TCS themselves.
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 (
Structural and microscopic relaxations in a colloidal glass
Soft Matter, 2015
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 À2. The length-scale dependence of structural relaxation time changes from diffusive, characterized by a Q À2-dependence in the early stages of aging, to a Q À1-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.
Dynamical facilitation governs glassy dynamics in suspensions of colloidal ellipsoids
One of the greatest challenges in contemporary condensed matter physics is to ascertain whether the formation of glasses from liquids is fundamentally thermodynamic or dynamic in origin. While the thermodynamic paradigm has dominated theoretical research for decades, the purely kinetic perspective of the dynamical facilitation (DF) theory has attained prominence in recent times.