Peter Harrowell

Perspective: Supercooled liquids and glasses

Supercooled liquids and glasses are important for current and developing technologies. Here we provide perspective on recent progress in this field. The interpretation of supercooled liquid and glass properties in terms of the potential energy landscape is discussed. We explore the connections between amorphous structure, high frequency motions, molecular motion, structural relaxation, stability against crystallization, and material properties. Recent developments that may lead to new materials or new applications of existing materials are described.

Irreversible reorganization in a supercooled liquid originates from localized soft modes

A simulation establishes the relationship between structural relaxation in a supercooled liquid and the low-frequency dynamics in the underlying inherent structures.

Crystal growth kinetics exhibit a fragility-dependent decoupling from viscosity.

In this paper we establish the temperature dependence of the kinetic coefficient associated with crystal growth into the supercooled liquid for a wide range of organic and inorganic materials. We show that the kinetic coefficient for crystal growth scales with the shear viscosity eta as eta(-xi) and that the exponent depends systematically on the fragility of the liquid. The greater the fragility (i.e., deviation away from an Arrhenius temperature dependence for eta), the larger the difference 1-xi. We argue that this breakdown in scaling between the crystal growth kinetics and the viscosity is a manifestation of heterogeneous dynamics in supercooled liquids. In addition, we show that the absolute growth rate at intermediate viscosities is correlated with the entropy difference between the liquid and the crystal.

Predicting the long-time dynamic heterogeneity in a supercooled liquid on the basis of short-time heterogeneities.

We report that the local Debye-Waller factor in a simulated 2D glass-forming mixture exhibits significant spatial heterogeneities and that these short-time fluctuations provide an excellent predictor of the spatial distribution of the long-time dynamic propensities. In contrast, the potential energy per particle of the inherent structure does not correlate well with the spatially distributed dynamics.

A molecular theory of crystal nucleation from the melt

We construct a molecular theory for the homogeneous nucleation of crystals from pure liquids by combining an order parameter theory of freezing with a square gradient approximation for the nonlocal dependence of free energy on density. We show the results of our theory for the free energy, radius, and shape of crystal nuclei as a function of supercooling, and compare with the conventional nucleation theory based on the capillarity approximation. We argue that the simple picture given by the conventional approach is qualitatively reasonable, although calculated values of the liquid–solid surface free energy may be inaccurate. We point out some of the limitations of our approach and stress the need for additional structural data on supercooled liquids.

Spatiotemporal hierarchy of relaxation events, dynamical heterogeneities, and structural reorganization in a supercooled liquid.

R. Candelier, A. Widmer-Cooper, J. K. Kummerfeld, O. Dauchot, G. Biroli, P. Harrowell, and D.R. Reichman SPEC, CEA-Saclay, URA 2464 CNRS, 91 191 Gif-sur-Yvette, France Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA Department of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia Institut de Physique Théorique, CEA, IPhT, F-91191 Gif-sur-Yvette, France and CNRS, URA 2306 Columbia University, 3000 Broadway, New York, New York, 10027, USA

Geometry of slow structural fluctuations in a supercooled binary alloy.

The liquid structure of a glass-forming binary alloy is studied using molecular dynamics simulations. The analysis employs the geometrical approach of Frank and Kasper to establish that the supercooled liquid contains extended clusters characterized by the same short range order as the crystal. The steep increase in the heat capacity on cooling is directly coupled to the growing fluctuations of the Frank-Kasper clusters. The relaxation of particles in the clusters dominates the slow tail of the self-intermediate scattering function.

On the interaction between order and a moving interface: Dynamical disordering and anisotropic growth rates

The way in which the velocity of a propagating interface determines the degree of surface disorder and the anisotropy of such interfacial velocities due to a crystalline lattice are examined using steady state solutions of the time dependent Landau–Ginzburg or Cahn–Hilliard equations. In the case of an interface described by two weakly coupled order parameters a divergence of the thickness of the surface disorder at a critical interfacial velocity is described. It is demonstrated that even for two surfaces with the same surface tension the growth rates may differ significantly due to a geometric factor arising from the underlying crystal lattice.

Non‐Gaussian behavior and the dynamical complexity of particle motion in a dense two‐dimensional liquid

The single particle dynamics of a 2D liquid made up of soft disks interacting by a repulsive r−12 potential are studied using molecular dynamics simulations. We find that mean squared particle displacement 〈Δr(t)2〉 behaves diffusively, i.e., increases linearly with time, within a time interval tc very much shorter than that required for structural relaxation. The non‐Gaussian parameter α(t)=〈Δr4〉/2〈Δr2〉2−1, on the other hand, exhibits a significant peak at times considerably greater than tc and a subsequent slow decay. It is argued that the only picture of diffusion consistent with these results considers particles moving in a medium characterized by fluctuating local mobilities. This picture provides an explicit connection between structural fluctuations (as characterized by the local mobility) and single particle motion. The possibility of obtaining the width and lifetime of the distribution of local relaxation times from incoherent scattering is examined.

Relaxation dynamics and their spatial distribution in a two-dimensional glass-forming mixture

Molecular dynamics simulations are used to explore the spatial fluctuations associated with structural relaxation and particle transport in a supercooled binary mixture in two dimensions. The study includes (i) the characterization of heterogeneities in the local particle dynamics in terms of their length scale and lifetime, (ii) the relationship between local kinetics and local structure/composition, and (iii) preliminary identification of the principal collective motions involved in the long-time relaxation of the supercooled liquid.