In the past few years, novel theoretical and experimental developments have significantly advanced our understanding of slow dynamics in viscous liquids, gels, glasses, and other dense, disordered materials. While challenges to complete our fundamental understanding of many of these systems remain, an additional intriguing slow and complex dynamical behavior has emerged also in a range of technological applications, driven by mechanisms as different as packing, energy, bonding, mesoscopic ordering, and free-energy fluxes. In this respect, the wide range of materials that exhibit slow dynamics coupled with the different scientific questions and standpoints from which these systems are probed, from fundamental to applied, makes the study of slow dynamics a broad and growing area within chemical physics. The papers in this special issue represent the aspects of the state-of-the-art of experimental, theoretical, and computer simulation work that tackles these questions, exploring the chemical physics of the relevant disordered systems.
Particular topics of interest include the dynamics in supercooled liquids and glasses (including ultrastable glasses, the vibrational properties of glasses, orientational glasses), the dynamics of dense soft materials, jamming, aging and rejuvenation, confined liquids, dense active matter, single molecule/particle investigations of slow dynamics, deformation and rheology, polyamorphism, algorithmic advances, and slow dynamics in biological systems.
SUMMARY OF AREAS COVERED BY THE CONTRIBUTIONS TO THIS SPECIAL ISSUE
Collective processes and heterogeneous dynamics in theories and model glass formers. Among the long-standing challenges of slow dynamics, and in particular vitrification, heterogeneous dynamics and collective relaxation phenomena are central. The theoretical work in this special issue includes dynamic density functional-like theory for density fluctuations,1 the single-saddle model for β-relaxation,2 and spatiotemporal correlations of the local stress tensor.3 Computer simulations consider the evolution of structure and dynamics of hard (hyper-) spheres across a range of dimensions,4 collective dynamics in a mean field-like particulate model,5 facilitation in a trap model,6 and alternative methods of entropy calculation.7 Computer simulations are combined with elastically collective nonlinear Langevin equation to investigate the actuated relaxation in monoatomic and polymeric systems.8 The experimental work reported here concerns investigations of excitations (short-lengthscale relaxation events) in quasi-2d colloidal systems9 and of heterogeneous dynamics at multiple lengthscales.10
Fundamental understanding of aging and glasses. Aging of glasses is often thought of in terms of a “fictive” temperature, with little emphasis on density. Niss et al. introduce a scaling conjecture that explicitly includes density in aging glass formers,11 and a model for aging is developed around the concept of “inner clocks.”12 Aging dynamics of far-from-equilibrium polymer glasses in computer simulations were found to violate the well-known Tool–Narayanaswamy–Moynihan model.13
Confinement, surfaces, and films. The effect of confinement upon the glass transition has long been a particularly challenging problem. In this issue, dynamics in polymer thin films are studied with NMR,14 fluorescent probe molecules,15 and computer simulations.16 Other systems include ionic liquids,17 organic semiconductors (studied with surface grating decay),18 and aqueous ethylene glycol (studied with NMR).19 Vapor deposited films with massively varying dielectric constants are investigated, and this property is given a structural interpretation.20 Quasi-2d confined colloids are often used as model 2d systems, and the validity of this approach is investigated here.21
Molecular processes. The recent developments in molecular systems reported in this special issue include experimental work in the design of a homologous series of molecular glass formers (based on triarylbenzene)22 and intramolecular relaxations in stable glasses.23 X-ray scattering and diffraction are used to study the structural consequences of cooling rate in itraconazole24 and intermolecular correlations in CS2.25 NMR has been used to study the reorientational susceptibility in organophosphates,26 and dielectric spectroscopy has been used to probe the dynamics of the isotropic and smectic phases of the itraconazole–glycerol system.27 Computer simulations have been used to investigate isochronal scaling in Johari–Goldstein β-relaxations in metallic glasses.28 Water is a particular favorite, with the vestiges of the putative liquid–liquid transition (LLT) studied with oxygen NMR29 and nuclear quantum effects in a related LLT.30 The dynamics of ice nucleation are studied with computer simulations31 and Raman spectroscopy.32 The structure of deeply supercooled D2O was studied with pulsed laser heating,33 while simulations have been used to tackle the dynamic structure factor of supercooled water.33,34
Elasticity, rheology, and memory. In experiments on soft materials, Williams et al. use a colloidal model system to probe the rheological behavior in an extreme confinement,35 while other investigations tackle the delayed elastic behavior in foams.36 For molecular systems, a rheological model is demonstrated to predict the α relaxation of glass-forming liquids.37 A number of papers in this special issue used cyclic shear in computer simulations to investigate the role of local structure38 and local mechanical properties assessed via the Debye–Waller factor.39 Other simulations with similar protocols investigated the brittle-to-ductile behavior in 2d40 and shear-induced rejuvenation.41
Gels and associating systems. A number of papers on gels considered colloids gels, from the perspective of time–composition superposition,42 local structure43 and modeling of the colloidal interactions,44 and unexpected mechanical behavior following quenching.45 The microrheology of a thermosensitive polymer system was also investigated.46 This topic also includes ferromagnetic nematics47 and slow dynamics in wormlike micellar networks.48
Active systems. The explosion in studies on active mater is here represented by a group of works combining the Fokker–Planck equation and dynamical mean-field theory to study active matter in infinite dimension,49 computer simulations used to unravel the non-monotonic dynamical behavior in active glass formers,50 and investigations of the role of multiple lengthscales in the interaction potential of active Brownian particles.51
New techniques, probes, and algorithms. The challenge of studying systems with slow dynamics has long inspired new and innovative methods. Computer simulation work reported here makes use of the overlap between pairs of configurations to obtain a local estimate of the configurational entropy.52 Cui and Fichthorn report the development of a new method based on hyperdynamics to accelerate molecular dynamics simulations.53 In experiments on molecular systems, molecular rotors are used to probe the local viscosity of a polymer glass.54 Developments in experimental methods for soft matter systems also include a time-Laplace method to extract relaxation times from non-stationary dynamic light scattering,55 using a microgel material to control the volume fraction of colloidal systems in situ,56 and a fluorescence imaging technique to infer forces between particles in a colloidal gel.57
The works collected here constitute a multifaceted snapshot of the range of scientific questions, materials, experimental and computational techniques, and theoretical concepts that the research on slow dynamics in the chemical physics community currently encompasses. They interestingly capture how this research field has recently evolved and provide hints of new directions to emerge in the coming years.
The guest editors E.D.G., A.L., and C.P.R. would like to extend their gratitude to the authors of the many manuscripts published in this special issue for sharing their excellent research and to the anonymous referees for their invaluable support. The Flatiron Institute is a division of the Simons Foundation.
Conflict of Interest
The authors have no conflicts to disclose.