Introduction to the Colloidal Glass Transition (original) (raw)
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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
Colloidal Glass Transition Observed in Confinement
Physical Review Letters - PHYS REV LETT, 2007
We study a colloidal suspension confined between two quasi-parallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.
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 (
Numerical study of the glass–glass transition in short-ranged attractive colloids
Journal of Physics: Condensed Matter, 2004
We report extensive numerical simulations in the glass region for a simple model of short-ranged attractive colloids, the square well model. We investigate the behaviour of the density autocorrelation function and of the static structure factor in the region of temperatures and packing fractions where a glass-glass transition is expected according to theoretical predictions. We strengthen our observations by studying both waiting time and history dependence of the numerical results. We provide evidence supporting the possibility that activated bond-breaking processes destabilize the attractive glass, preventing the full observation of a sharp glass-glass kinetic transition.
The nature of the colloidal ‘glass’ transition
Faraday Discussions, 2003
The dynamically arrested state of matter is discussed in the context of athermal systems, such as the hard sphere colloidal arrest. We believe that the singular dynamical behaviour near arrest expressed, for example, in how the diffusion constant vanishes may be 'universal', in a sense to be discussed in the paper. Based on this we argue the merits of studying the problem with simple lattice models. This, by analogy with the the critical point of the Ising model, should lead us to clarify the questions, and begin the program of establishing the degree of universality to be expected. We deal only with 'ideal' athermal dynamical arrest transitions, such as those found for hard sphere systems. However, it is argued that dynamically available volume (DAV) is the relevant order parameter of the transition, and that universal mechanisms may be well expressed in terms of DAV. For simple lattice models we give examples of simple laws that emerge near the dynamical arrest, emphasising the idea of a near-ideal gas of 'holes', interacting to give the power law diffusion constant scaling near the arrest. We also seek to open the discussion of the possibility of an underlying weak coupling theory of the dynamical arrest transition, based on DAV.
Spatial and temporal dynamical heterogeneities approaching the binary colloidal glass transition
2011
We study concentrated binary colloidal suspensions, a model system which has a glass transition as the volume fraction φ of particles is increased. We use confocal microscopy to directly observe particle motion within dense samples with φ ranging from 0.4 to 0.7. Our binary mixtures have a particle diameter ratio d S /d L = 1/1.3 and particle number ratio N S /N L = 1.56, which are chosen to inhibit crystallization and enable long-time observations. Near the glass transition we find that particle dynamics are heterogeneous in both space and time. The most mobile particles occur in spatially localized groups. The length scales characterizing these mobile regions grow slightly as the glass transition is approached, with the largest length scales seen being ∼ 4 small particle diameters. We also study temporal fluctuations using the dynamic susceptibility χ 4 , and find that the fluctuations grow as the glass transition is approached. Analysis of both spatial and temporal dynamical heterogeneity show that the smaller species play an important role in facilitating particle rearrangements. The glass transition in our sample occurs at φ g ≈ 0.58, with characteristic signs of aging observed for all samples with φ > φ g .
Glass transition of soft colloids
Physical Review E
We explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time τα of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard spheres. Computer simulations confirm that the growth of τα on approaching the glass transition is independent of particle softness. By contrast, softness becomes relevant at very large packing fractions when the system falls out of equilibrium. In this non-equilibrium regime, τα depends surprisingly weakly on packing fraction and time correlation functions exhibit a compressed exponential decay consistent with stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease of local order with increasing density typical of ultrasoft systems. We propose that these peculiar dynamics results from the combination of the non-equilibrium aging dynamics expected in the glassy state and the tendency of colloids interacting through soft potentials to refluidize at high packing fractions.
Physical Review Letters, 2003
We study -using molecular dynamics simulations -the temperature dependence of the dynamics in a dense short-ranged attractive colloidal glass to find evidence of the kinetic glass-glass transition predicted by the ideal Mode Coupling Theory. According to the theory, the two distinct glasses are stabilized one by excluded volume and the other by short-ranged attractive interactions. By studying the density autocorrelation functions, we discover that the short-ranged attractive glass is unstable. Indeed, activated bond-breaking processes slowly convert the attractive glass into the hard-sphere one, preempting the observation of a sharp glass-glass transition.
Microstructure and Rheology near an Attractive Colloidal Glass Transition
Physical Review Letters, 2006
Microstructure and rheological properties of a thermally reversible short-ranged attractive colloidal system are studied in the vicinity of the attractive glass transition line. At high volume fractions, the static structure factor changes very little but the low frequency shear moduli varies over several orders of magnitude across the transition. From the frequency dependence of shear moduli, fluid-attractive glass and repulsive glass-attractive glass transitions are identified.
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.