Chi-Hang Lam

Particles in the facilitated random walk model of glass are individually trapped but travel as mobile groups, whose dominant size is set by a sequence of mobility thresholds.

Glass formers are ubiquitous, but their dynamics at deep supercooling are still under intensive debate.Free volume plays a central role in many theoretical descriptions, but earlier studies often reported only...

We study the microscopic origins of relaxation modes revealed by spectroscopic studies of glasses. Major features in calorimetric and structural relaxation spectra, including α peaks, β peaks, and excess wings, are reproduced using a distinguishable-particle lattice model. It is based on void-induced particle hops, which model stringlike particle hopping motions. Excess wings are found to be flattened variants of β peaks when particle hopping barriers admit a wide distribution. A β relaxation event is attributed to the hop of a particle which neighbors a void. An α relaxation event involves a similar process by an arbitrary particle but also includes, as a prior step, the arrival of a diffusing void to become its neighbor.

The nature of glassy dynamics and the glass transition is a long-standing problem under active debate. In the presence of a structural disorder widely believed to be an essential characteristic of structural glass, identifying and understanding key dynamical behaviors are very challenging. In this work, we demonstrate that an energetic disorder, which usually results from a structural disorder, is instead a more essential feature of glass. In particular, we develop a distinguishable-particle glassy crystal, in which particles are ordered in a face-centered cubic lattice and follow particle-dependent random interactions, leading to an energetic disorder in the particle configuration space. Molecular dynamics simulations in the presence of vacancy-induced particle diffusion show typical glassy behaviors. A unique feature of this molecular model is the knowledge of the complete set of inherent structures with easily calculable free energies, implying a well-understood potential energy landscape.

A free surface induces enhanced dynamics in glass formers. We study the dynamical enhancement of glassy films with a distinguishable-particle lattice model of glass free of elastic effects. We demonstrate that the thickness of the surface mobile layer depends on temperature differently under different definitions, although all are based on local structure relaxation rate. The rate can be fitted to a double exponential form with an exponential-of-power-law tail. Our approach and results exclude elasticity as the unique mechanism for the tail. Layer-resolved particle hopping rate, potentially a key measure for activated hopping, is also studied but it exhibits much shallower surface effects.

The physics of glass has been a significant topic of interest for decades. Dynamical facilitation is widely believed to be an important characteristic of glassy dynamics, but the precise mechanism is still under debate. We propose a lattice model of glass called the facilitated random walk (FRW). Each particle performs a continuous time random walk in the presence of its own random local kinetic constraints. The particles do not interact energetically. Instead, they interact kinetically with a hopping rate resampling rule under which motions of a particle can randomly perturb the local kinetic constraints of other particles. This dynamic interaction is reversible, following a rate restoration rule. A step-by-step reversal of the particle motions exactly restores the previous constraints, modeling randomness quenched in the configuration space of glass. The model exhibits stretched exponential relaxation and dynamical heterogeneity typical of glasses. Despite the lack of an explicit facilitation rule, the FRW shows facilitation behaviors closely analogous to those of the kinetically constrained models (KCM). The FRW is a coarse-grained version of the distinguishable particle lattice model (DPLM) and this exemplifies that compatible defect and atomistic models can complement each other in the study of glass.

We present a new approach for investigating the Markovian to non-Markovian transition in quantum aggregates strongly coupled to a vibrational bath through the analysis of linear absorption spectra. Utilizing hierarchical algebraic equations in the frequency domain, we elucidate how these spectra can effectively reveal transitions between Markovian and non-Markovian regimes, driven by the complex interplay of dissipation, aggregate–bath coupling, and intra-aggregate dipole–dipole interactions. Our results demonstrate that reduced dissipation induces spectral peak splitting, signaling the emergence of bath-induced non-Markovian effects. The spectral peak splitting can also be driven by enhanced dipole–dipole interactions, although the underlying mechanism differs from that of dissipation-induced splitting. In addition, with an increase in aggregate–bath coupling strength, initially symmetric or asymmetric peaks with varying spectral amplitudes may merge under weak dipole–dipole interactions, whereas strong dipole–dipole interactions are more likely to cause peak splitting. Moreover, we find that spectral features serve as highly sensitive indicators for distinguishing the geometric structures of aggregates while also unveiling the critical role that geometry plays in shaping non-Markovian behavior. This study not only deepens our understanding of the Markovian to non-Markovian transition but also provides a robust framework for optimizing and controlling quantum systems.

We study the calorimetric properties and structural relaxation of glassy films using a distinguishable particle lattice model (DPLM). We determine the glass transition temperature versus film thickness from the heat capacity during heating as well as from the local relaxation time. The results based on both approaches are in good agreement with the experimentally observed Keddie-Cory-Jones relation. The thus demonstrated interplay between calorimetric properties and structural relaxation is further corroborated by successfully reconstructing the simulated heat capacity during heat and cooling from the local relaxation times. Our results suggest DPLM as a useful lattice model for studying glassy films.

We experimentally manipulate the structural relaxation of colloidal glass at an atomistically controlled position. We focus a laser beam to eject a particle from the colloidal system. This creates a void that transforms rapidly into a mobile soft spot (quasivoid) composed of a few neighboring free-volume fragments. The transformation and subsequent motions of the quasivoid trigger a sequence of well-connected stringlike motions emanating precisely from the point of laser action with free volumes flowing along the string. Furthermore, the sequence can be terminated by the annihilation of the quasivoid. The observations are supported by molecular dynamics (MD) simulations.

Correction for ‘Surface mobility gradient and emergent facilitation in glassy films’ by Qiang Zhai et al., Soft Matter, 2024, https://doi.org/10.1039/D4SM00221K.