A theoretical perspective is provided on the glass transition in molecular liquids at thermal equilibrium, on the spatially heterogeneous and aging dynamics of disordered materials, and on the rheology of soft glassy materials. We start with a broad introduction to the field and emphasize its connections with other subjects and its relevance. The important role played by computer simulations in studying and understanding the dynamics of systems close to the glass transition at the molecular level is given. The recent progress on the subject of the spatially heterogeneous dynamics that characterizes structural relaxation in materials with slow dynamics is reviewed. The main theoretical approaches are presented describing the glass transition in supercooled liquids, focusing on theories that have a microscopic, statistical mechanics basis. We describe both successes and failures and critically assess the current status of each of these approaches. The physics of aging dynamics in disordered materials and the rheology of soft glassy materials are then discussed, and recent theoretical progress is described. For each section, an extensive overview is given of the most recent advances, but we also describe in some detail the important open problems that will occupy a central place in this field in the coming years.

Theoretical perspective on the glass transition and amorphous materials

Understanding glass formation is a challenge, because the existence of a true glass state, distinct from liquid and solid, remains elusive: Glasses are liquids that have become too viscous to flow. An old idea, as yet unproven experimentally, is that the dynamics becomes sluggish as the glass transition approaches, because increasingly larger regions of the material have to move simultaneously to allow flow. We introduce new multipoint dynamical susceptibilities to estimate quantitatively the size of these regions and provide direct experimental evidence that the glass formation of molecular liquids and colloidal suspensions is accompanied by growing dynamic correlation length scales.

https://science.sciencemag.org/content/sci/310/5755/1797.full.pdf

Direct Experimental Evidence of a Growing Length Scale Accompanying the Glass Transition

Foods make up some of the most complex examples of soft condensed matter (SCM) with which we interact daily. Their complexity arises from several factors: the intricacy of components, the different aggregation states in which foods are encountered, and the multitude of relevant characteristic time and length scales. Because foodstuffs are governed by the rules of SCM physics but with all the complications related to real systems, the experimental and theoretical approaches of SCM physics have deepened our comprehension of their nature and behaviour, but many questions remain. In this review we discuss the current understanding of food science, by considering established SCM methods as well as emerging techniques and theoretical approaches. With their complexity, heterogeneity and multitude of states, foods provide SCM physics with a challenge of remarkable importance.

Understanding foods as soft materials

The field of microrheology is concerned with how materials store and dissipate mechanical energy as a function of length scale. Recent developments in the theory and instrumentation of the microrheology of complex fluids are reviewed. Equal emphasis is given to the physical phenomena probed, advances in instrumentation, and specific experimental systems in which this field has already had an impact. The inversion of the compliance data, measurement of sample heterogeneity, high frequency viscoelasticity, effects of shear flow, single molecule experiments, surface viscoelasticity and time evolution studies are considered. The techniques highlighted include particle tracking microrheology, diffusing wave spectroscopy, laser tracking, magnetic tweezers and atomic force microscopy. Specific examples of complex fluid systems are chosen from the fields of polymers, colloids and biological assemblies.

https://iopscience.iop.org/article/10.1088/0034-4885/68/3/R04/pdf

Microrheology of complex fluids

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.

/pdf/the-physics-of-the-colloidal-glass-transition-41jncqk0al.pdf

The Physics of the Colloidal Glass Transition

We introduce a new scheme for investigating temporally heterogeneous dynamics, which is termed time-resolved correlation (TRC). TRC is applied to data obtained by diffusing wave spectroscopy probing the slow dynamics of a strongly aggregated colloidal gel. Other examples of TRC data, collected for different jammed materials in single and multiple scattering, are provided to demonstrate the wide range of applicability of this method. In all cases we find evidence that the slow dynamics results from a series of discrete steps rather than from a continuous motion, suggesting temporal heterogeneities to be a general feature of slow dynamics in jammed systems.

/pdf/time-resolved-correlation-a-new-tool-for-studying-temporally-1375irgev1.pdf

Time-resolved correlation: a new tool for studying temporally heterogeneous dynamics

Time resolved correlation (TRC) is a recently introduced light scattering technique that allows one to detect and quantify dynamic heterogeneities. The technique is based on the analysis of the temporal evolution of the speckle pattern generated by the light scattered by a sample, which is quantified by ${c}_{I}(t,\ensuremath{\tau})$, the degree of correlation between speckle images recorded at time $t$ and $t+\ensuremath{\tau}$. Heterogeneous dynamics results in significant fluctuations of ${c}_{I}(t,\ensuremath{\tau})$ with time $t$. We describe how to optimize TRC measurements and how to detect and avoid possible artifacts. The statistical properties of the fluctuations of ${c}_{I}$ are analyzed by studying their variance, probability distribution function, and time autocorrelation function. We show that these quantities are affected by a noise contribution due to the finite number $N$ of detected speckles. We propose and demonstrate a method to correct for the noise contribution, based on a $N\ensuremath{\rightarrow}\ensuremath{\infty}$ extrapolation scheme. Examples from both homogeneous and heterogeneous dynamics are provided. Connections with recent numerical and analytical works on heterogeneous glassy dynamics are briefly discussed.

Time-resolved-correlation measurements of temporally heterogeneous dynamics.

We use time-resolved X-Photon Correlation Spectroscopy to investigate the slow dynamics of colloidal gels made of moderately attractive carbon black particles. We show that the slow dynamics is temporally heterogeneous and quantify its fluctuations by measuring the variance $\chi$ of the instantaneous intensity correlation function. The amplitude of dynamical fluctuations has a non-monotonic dependence on scattering vector $q$, in stark contrast with recent experiments on strongly attractive colloidal gels [Duri and Cipelletti, \textit{Europhys. Lett.} \textbf{76}, 972 (2006)]. We propose a simple scaling argument for the $q$-dependence of fluctuations in glassy systems that rationalizes these findings.

https://arxiv.org/pdf/cond-mat/0702646.pdf

Investigation of q-dependent dynamical heterogeneity in a colloidal gel by x-ray photon correlation spectroscopy.

We show by means of experiments, theory, and simulations that the slow dynamics of coarsening systems displays dynamic heterogeneity similar to that observed in glass-forming systems. We measure dynamic heterogeneity via novel multipoint functions which quantify the emergence of dynamic, as opposed to static, correlations of fluctuations. Experiments are performed on a coarsening foam using time-resolved correlation, a recently introduced light scattering method. Theoretically we study the Ising model, and present exact results in one dimension, and numerical results in two dimensions. For all systems the same dynamic scaling of fluctuations with domain size is observed.

/pdf/heterogeneous-dynamics-of-coarsening-systems-508hmgw3z4.pdf

Heterogeneous dynamics of coarsening systems.

We investigate the slow dynamics of strongly aggregated particle gels by diffusing wave spectroscopy (DWS) using a charge coupled device (CCD) camera as a multispeckle detector. The evolution of the slow dynamics after the gel-point is followed using two different techniques, the multispeckle method and a novel correlation scheme, time resolved correlation (TRC), which allows us to obtain time resolved information on the dynamics of the gel. We find that the intensity autocorrelation function measured by multispeckle DWS exhibits an ultraslow relaxation, whose characteristic time strongly depends on the age of the gel. An analysis of the TRC data reveals that the slow dynamics is intermittent. We propose that the intermittency is due to rare disruptions or formations of bonds affecting the elastic backbone of the network, which lead to sudden long range rearrangements of the particle position and thus to a sudden change of the speckle pattern.

Hugo Bissig

Papers

Time-resolved correlation: a new tool for studying temporally heterogeneous dynamics

Time-resolved-correlation measurements of temporally heterogeneous dynamics.

Investigation of q-dependent dynamical heterogeneity in a colloidal gel by x-ray photon correlation spectroscopy.

Heterogeneous dynamics of coarsening systems.

Intermittent dynamics and hyper-aging in dense colloidal gels