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Dive into the research topics where Lester O. Hedges is active.

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Featured researches published by Lester O. Hedges.


Science | 2013

Microscopic Evidence for Liquid-Liquid Separation in Supersaturated CaCO3 Solutions

Adam F. Wallace; Lester O. Hedges; Alejandro Fernandez-Martinez; Paolo Raiteri; Julian D. Gale; Glenn A. Waychunas; Stephen Whitelam; Jillian F. Banfield; J. J. De Yoreo

Making Crystals The initial transition from a disordered solution to the formation of nuclei that grow into crystals continues to be a puzzle. Recent experiments suggested the formation of stable ordered clusters that appear prior to the formation of the first nuclei. Wallace et al. (p. 885; see the Perspective by Myerson and Trout) used molecular dynamics to look at the potential structure and dynamics of these clusters and lattice gas simulations to explore the population dynamics of the cluster populations prior to nucleation. A liquid-liquid phase separation process was observed whereby one phase becomes more concentrated in ions and becomes the precursor for nuclei to form. The preordering seen during calcium carbonate crystallization may be due to a liquid-liquid separation process. [Also see Perspective by Myerson and Trout] Recent experimental observations of the onset of calcium carbonate (CaCO3) mineralization suggest the emergence of a population of clusters that are stable rather than unstable as predicted by classical nucleation theory. This study uses molecular dynamics simulations to probe the structure, dynamics, and energetics of hydrated CaCO3 clusters and lattice gas simulations to explore the behavior of cluster populations before nucleation. Our results predict formation of a dense liquid phase through liquid-liquid separation within the concentration range in which clusters are observed. Coalescence and solidification of nanoscale droplets results in formation of a solid phase, the structure of which is consistent with amorphous CaCO3. The presence of a liquid-liquid binodal enables a diverse set of experimental observations to be reconciled within the context of established phase-separation mechanisms.


Science | 2009

Dynamic Order-Disorder in Atomistic Models of Structural Glass Formers

Lester O. Hedges; Robert L. Jack; Juan P. Garrahan; David Chandler

The glass transition is the freezing of a liquid into a solid state without evident structural order. Although glassy materials are well characterized experimentally, the existence of a phase transition into the glass state remains controversial. Here, we present numerical evidence for the existence of a novel first-order dynamical phase transition in atomistic models of structural glass formers. In contrast to equilibrium phase transitions, which occur in configuration space, this transition occurs in trajectory space, and it is controlled by variables that drive the system out of equilibrium. Coexistence is established between an ergodic phase with finite relaxation time and a nonergodic phase of immobile molecular configurations. Thus, we connect the glass transition to a true phase transition, offering the possibility of a unified picture of glassy phenomena.


Journal of Chemical Physics | 2007

Decoupling of exchange and persistence times in atomistic models of glass formers

Lester O. Hedges; Lutz Maibaum; David Chandler; Juan P. Garrahan

With molecular dynamics simulations of a fluid mixture of classical particles interacting with pairwise additive Weeks-Chandler-Andersen potentials, we consider the time series of particle displacements and thereby determine the distributions for local persistence times and local exchange times. These basic characterizations of glassy dynamics are studied over a range of supercooled conditions and were shown to have behaviors, most notably decoupling, similar to those found in kinetically constrained lattice models of structural glasses. Implications are noted.


Journal of Chemical Physics | 2011

Limit of validity of Ostwald's rule of stages in a statistical mechanical model of crystallization.

Lester O. Hedges; Stephen Whitelam

We have only rules of thumb with which to predict how a material will crystallize, chief among which is Ostwalds rule of stages. It states that the first phase to appear upon transformation of a parent phase is the one closest to it in free energy. Although sometimes upheld, the rule is without theoretical foundation and is not universally obeyed, highlighting the need for microscopic understanding of crystallization controls. Here we study in detail the crystallization pathways of a prototypical model of patchy particles. The range of crystallization pathways it exhibits is richer than can be predicted by Ostwalds rule, but a combination of simulation and analytic theory reveals clearly how these pathways are selected by microscopic parameters. Our results suggest strategies for controlling self-assembly pathways in simulation and experiment.


Soft Matter | 2012

Patterning a surface so as to speed nucleation from solution

Lester O. Hedges; Stephen Whitelam

Motivated by the question of how to pattern a surface in order to best speed nucleation from solution, we build on the work of Page and Sear [Phys. Rev. Lett., 2006, 97, 65701] and calculate rates and free energy profiles for nucleation in the 3d Ising model in the presence of cuboidal pores. Pores of well-chosen aspect ratio can dramatically speed nucleation relative to a planar surface made of the same material, while badly chosen pores provide no such enhancement. For a given pore, the maximum nucleation rate is achieved when one of its two horizontal dimensions attains a critical length, largely irrespective of the other dimension (provided that the latter is large enough). This observation implies that patterning a surface with repeated copies of a well-chosen pore is a better strategy for speeding nucleation than e.g. scoring long grooves in it.


Physical Review Letters | 2014

Self-Assembly at a Nonequilibrium Critical Point

Stephen Whitelam; Lester O. Hedges; Jeremy D. Schmit

We use analytic theory and computer simulation to study patterns formed during the growth of two-component assemblies in two and three dimensions. We show that these patterns undergo a nonequilibrium phase transition, at a particular growth rate, between mixed and demixed arrangements of component types. This finding suggests that principles of nonequilibrium statistical mechanics can be used to predict the outcome of multicomponent self-assembly, and suggests an experimental route to the self-assembly of multicomponent structures of a qualitatively defined nature.


Soft Matter | 2013

Selective nucleation in porous media

Lester O. Hedges; Stephen Whitelam

Geometrical arguments suggest that pore-mediated nucleation happens in general in a two-step fashion, the first step being nucleation within the pore, the second being nucleation from the filled pore into solution [Page and Sear, Phys. Rev. Lett., 2006, 97, 65701]. The free energy barriers controlling the two steps of this process show opposite dependencies on pore size, implying that for given thermodynamic conditions there exists a pore size for which nucleation happens fastest. Here we show, within the two- and three-dimensional Ising lattice gas, that this preferred pore size tracks the size of the bulk critical nucleus, up to a numerical prefactor. This observation suggests a simple prescription for directing nucleation to certain locations within heterogeneous porous media.


Journal of Physics: Condensed Matter | 2007

Dynamic propensity in a kinetically constrained lattice gas

Lester O. Hedges; Juan P. Garrahan

We apply the concept of dynamic propensity to a simple kinetically constrained model of glass formers, the two-vacancy assisted triangular lattice gas, or (2)-TLG. We find that the propensity field, defined in our case as the local root-mean-square displacement averaged over the ensemble of trajectories with identical initial configurations, is a good measure of dynamical heterogeneity. This suggests a configurational origin for spatial fluctuations of the dynamics, but just as in the case of atomistic systems, we find that propensity is not correlated to any simple structural property. We show instead that certain extended clusters of particles connected to vacancies correlate well with propensity, indicating that these are the fundamental excitations of the (2)-TLG. We also discuss time correlations and the correlation between configurations within the propensity ensemble.


Journal of Statistical Mechanics: Theory and Experiment | 2005

Fast simulation of facilitated spin models

Douglas J. Ashton; Lester O. Hedges; Juan P. Garrahan

We show how to apply the absorbing Markov chain Monte Carlo algorithm of Novotny to simulate kinetically constrained models of glasses. We consider in detail one-spin facilitated models, such as the East model and its generalizations to arbitrary dimensions. We investigate how to maximize the efficiency of the algorithms, and show that simulation times can be improved on standard continuous time Monte Carlo by several orders of magnitude. We illustrate the method with equilibrium and aging results. These include a study of relaxation times in the East model for dimensions d = 1 to 13, which provides further evidence that the hierarchical relaxation in this model is present in all dimensions. We discuss how the method can be applied to other kinetically constrained models.


Soft Matter | 2015

Crystallization and arrest mechanisms of model colloids.

Thomas K. Haxton; Lester O. Hedges; Stephen Whitelam

We performed dynamic simulations of spheres with short-range attractive interactions for many values of interaction strength and range. Fast crystallization occurs in a localized region of this parameter space, but the character of crystallization pathways is not uniform within this region. Pathways range from one-step, in which a crystal nucleates directly from a gas, to two-step, in which substantial liquid-like clusters form and only subsequently become crystalline. Crystallization can fail because of slow nucleation from either gas or liquid, or because of dynamic arrest caused by strong interactions. Arrested states are characterized by the formation of networks of face-sharing tetrahedra that can be detected by a local common neighbor analysis.

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Stephen Whitelam

Lawrence Berkeley National Laboratory

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David Chandler

University of California

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Ali Javey

University of California

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Jeffrey J. Urban

Lawrence Berkeley National Laboratory

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