Glass transition of soft colloids (original) (raw)

Soft Colloids as a Realization of Mean-Field Glass Formers

arXiv: Soft Condensed Matter, 2015

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q, t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

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.

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

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.

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.

Amorphous and crystalline states of ultrasoft colloids: a molecular dynamics study

Rheologica Acta, 2007

In this work, we study the temperature-induced development of "dynamically arrested" states in dense suspensions of "soft colloids" (multi-arm star polymers and/or block-copolymers micelles) by means of molecular dynamics (MD) simulations. Temperature increase in marginal solvents results in "soft sphere" swelling, dynamical arrest, and eventually crystallization. However, two distinct "dynamically arrested" states were found, one almost amorphous ("glassy") and one with a considerable degree of crystallinity, yet lower than that of the fully equilibrated crystal. It is remarkable that even that latter state permitted self-diffusion in the timescale of the simulations, an effect that underlies the importance of the "ultra-soft" nature of inter-particle potential. The "number of connections" criterion for crystallinity proved to be very successful in identifying the ultimate thermodynamic trend from the very early stages of the α-relaxation.

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 (

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.

Validity of the Stokes-Einstein Relation in Soft Colloids up to the Glass Transition

Physical Review Letters, 2015

, second revised version) We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q, t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

Density-Temperature-Softness Scaling of the Dynamics of Glass-Forming Soft-Sphere Liquids

Physical Review Letters, 2011

The principle of dynamic equivalence between soft-sphere and hard-sphere fluids [Phys. Rev. E \textbf{68}, 011405 (2003)] is employed to describe the interplay of the effects of varying the density n, the temperature T, and the softness (characterized by a softness parameter {\nu}^{-1}) on the dynamics of glass-forming soft-sphere liquids in terms of simple scaling rules. The main prediction is that the dynamic parameters of these systems, such as the {\alpha}-relaxation time and the long-time self-diffusion coefficient, depend on n, T, and {\nu} only through the reduced density n^\ast \equiv n{\sigma}^{3}_{HS}(T, {\nu}),where the effective hard-sphere diameter {\sigma}_{HS}(T, {\nu}) is determined, for example, by the Andersen-Weeks-Chandler condition for soft-sphere-hard-sphere structural equivalence. A number of scaling properties observed in recent simulations involving glass-forming fluids with repulsive short range interactions are found to be a direct manifestation of this general dynamic equivalence principle. The self-consistent generalized Langevin equation (SCGLE) theory of colloid dynamics is shown to accurately capture these scaling rules