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Elijah Flenner

Publications and source records attributed to Elijah Flenner.

At least 37 records · Page 2Linked to original sources
arXiv2019

Perspective: Nonequilibrium glassy dynamics in dense systems of active particles

Despite the diversity of materials designated as active matter, virtually all active systems undergo a form of dynamic arrest when crowding and activity compete, reminiscent of the dynamic arrest observed in colloidal and molecular fluids undergoing a glass transition. We present a short perspective on recent and ongoing efforts to understand how activity competes with other physical interactions in dense systems. We first review recent experimental work on active materials that uncovered both classic signatures of glassy dynamics and intriguing novel phenomena at large density. We introduce a minimal model of self-propelled particles where the competition between interparticle interactions, crowding, and self-propulsion can be studied in great detail. We discuss more complex models that include some additional, material-specific ingredients. We end with some general perspectives on dense active materials, suggesting directions for future research, in particular for theoretical work.

cond-mat.soft↗
arXiv2019

Low-frequency vibrational modes of stable glasses

We numerically study the evolution of the vibrational density of states $D(ω)$ of zero-temperature glasses when their kinetic stability is varied over an extremely broad range, ranging from poorly annealed glasses obtained by instantaneous quenches from above the onset temperature, to ultrastable glasses obtained by quenching systems thermalised below the experimental glass temperature. The low-frequency part of the density of states splits between extended and quasi-localized modes. Extended modes exhibit a boson peak crossing over to Debye behaviour ($D(ω) \sim ω^2$) at low-frequency, with a strong correlation between the two regimes. Quasi-localized modes instead obey $D(ω) \sim ω^4$, irrespective of the glass stability. However, the prefactor of this quartic law becomes smaller in more stable glasses, and the corresponding modes become more localized and sparser. Our work is the first numerical observation of quasi-localized modes in a regime relevant to experiments, and it establishes a direct connection between glass stability and soft vibrational motion in amorphous solids.

cond-mat.soft↗
arXiv2018

Efficient swap algorithms for molecular dynamics simulations of equilibrium supercooled liquids

It was recently demonstrated that a simple Monte Carlo (MC) algorithm involving the swap of particle pairs dramatically accelerates the equilibrium sampling of simulated supercooled liquids. We propose two numerical schemes integrating the efficiency of particle swaps into equilibrium molecular dynamics (MD) simulations. We first develop a hybrid MD/MC scheme combining molecular dynamics with the original swap Monte Carlo. We implement this hybrid method in LAMMPS, a software package employed by a large community of users. Secondly, we define a continuous time version of the swap algorithm where both the positions and diameters of the particles evolve via Hamilton's equations of motion. For both algorithms, we discuss in detail various technical issues as well as the optimisation of simulation parameters. We compare the numerical efficiency of all available swap algorithms and discuss their relative merits.

cond-mat.stat-mech↗
arXiv2018

Viscoelastic shear stress relaxation in two-dimensional glass forming liquids

Translational dynamics of two-dimensional glass forming fluids is strongly influenced by soft, long-wavelength fluctuations first recognized by D. Mermin and H. Wagner. As a result of these fluctuations, characteristic features of glassy dynamics, such as plateaus in the mean squared displacement and the self-intermediate scattering function, are absent in two dimensions. In contrast, Mermin-Wagner fluctuations do not influence orientational relaxation and well developed plateaus are observed in orientational correlation functions. It has been suggested that by monitoring translational motion of particles relative to that of their neighbors, one can recover characteristic features of glassy dynamics and thus disentangle the Mermin-Wagner fluctuations from the two-dimensional glass transition. Here we use molecular dynamics simulations to study viscoelastic relaxation in two and three dimensions. We find different behavior of the dynamic modulus below the onset of slow dynamics (determined by the orientational or cage-relative correlation functions) in two and three dimensions. The dynamic modulus for two-dimensional supercooled fluids is more stretched than for three-dimensional supercooled fluids and it does not exhibit a plateau, which implies the absence of glassy viscoelastic relaxation. At lower temperatures, the two-dimensional dynamic modulus starts exhibiting an intermediate time plateau and decays similarly to the three-dimensional dynamic modulus. The differences in the glassy behavior of two- and three-dimensional glass forming fluids parallel differences in the ordering scenarios in two and three dimensions.

cond-mat.soft↗
arXiv2017

Origin of ultrastability in vapor-deposited glasses

Glass films created by vapor-depositing molecules onto a substrate can exhibit properties similar to those of ordinary glasses aged for thousands of years. It is believed that enhanced surface mobility is the mechanism that allows vapor deposition to create such exceptional glasses, but it is unclear how this effect is related to the final state of the film. Here we use molecular dynamics simulations to model vapor deposition and an efficient Monte Carlo algorithm to determine the deposition rate needed to create ultra-stable glassy films. We obtain a scaling relation that quantitatively captures the efficiency gain of vapor deposition over bulk annealing, and demonstrates that surface relaxation plays the same role in the formation of vapor-deposited glasses as bulk relaxation does in ordinary glass formation.

cond-mat.soft↗
arXiv2017

How active forces influence nonequilibrium glass transitions

Dense assemblies of self-propelled particles undergo a nonequilibrium form of glassy dynamics. Physical intuition suggests that increasing departure from equilibrium due to active forces fluidifies a glassy system. We falsify this belief by devising a model of self-propelled particles where increasing departure from equilibrium can both enhance or depress glassy dynamics, depending on the chosen state point. We analyze a number of static and dynamic observables and suggest that the location of the nonequilibrium glass transition is primarily controlled by the evolution of two-point static density correlations due to active forces. The dependence of the density correlations on the active forces varies non-trivially with the details of the system, and is difficult to predict theoretically. Our results emphasize the need to develop an accurate liquid state theory for nonequilibrium systems.

cond-mat.soft↗
arXiv2016

Kinetic stability and energetics of simulated glasses created by constant pressure cooling

We use computer simulations to study the cooling rate dependence of the stability and energetics of model glasses created at constant pressure conditions and compare the results with glasses formed at constant volume conditions. To examine the stability, we determine the time it takes for a glass cooled and reheated at constant pressure to transform back into a liquid, $t_{\mathrm{trans}}$, and calculate the stability ratio $S = t_{\mathrm{trans}}/τ_α$, where $τ_α$ is the equilibrium relaxation time of the liquid. We find that, for slow enough cooling rates, cooling and reheating at constant pressure results in a larger stability ratio $S$ than for cooling and reheating at constant volume. We also compare the energetics of glasses obtained by cooling while maintaining constant pressure with those of glasses created by cooling from the same state point while maintaining constant volume. We find that cooling at constant pressure results in glasses with lower average potential energy and average inherent structure energy. We note that in model simulations of the vapor deposition process glasses are created under constant pressure conditions, and thus they should be compared to glasses obtained by constant pressure cooling.

cond-mat.soft↗
arXiv2016

The nonequilibrium glassy dynamics of self-propelled particles

We study the glassy dynamics taking place in dense assemblies of athermal active particles that are driven solely by a nonequilibrium self-propulsion mechanism. Active forces are modeled as an Ornstein-Uhlenbeck stochastic process, characterized by a persistence time and an effective temperature, and particles interact via a Lennard-Jones potential that yields well-studied glassy behavior in the Brownian limit, obtained as the persistence time vanishes. By increasing the persistence time, the system departs more strongly from thermal equilibrium and we provide a comprehensive numerical analysis of the structure and dynamics of the resulting active fluid. Finite persistence times profoundly affect the static structure of the fluid and give rise to nonequilibrium velocity correlations that are absent in thermal systems. Despite these nonequilibrium features, for any value of the persistence time we observe a nonequilibrium glass transition as the effective temperature is decreased. Surprisingly, increasing departure from thermal equilibrium is found to promote (rather than suppress) the glassy dynamics. Overall, our results suggest that with increasing persistence time, microscopic properties of the active fluid change quantitatively, but the broad features of the nonequilibrium glassy dynamics observed with decreasing the effective temperature remain qualitatively similar to those of thermal glass-formers.

cond-mat.soft↗
arXiv2016

Dynamic heterogeneity in two-dimensional supercooled liquids: comparison of bond-breaking and bond-orientational correlations

We compare the spatial correlations of bond-breaking events and bond-orientational relaxation in a model two-dimensional liquid undergoing Newtonian dynamics. We find that the relaxation time of the bond-breaking correlation function is much longer than the relaxation time of the bond-orientational correlation function and self-intermediate scattering function. However, the relaxation time of the bond-orientational correlation function increases faster with decreasing temperature than the relaxation time of the bond-breaking correlation function and the self-intermediate scattering function. Moreover, the dynamic correlation length that characterizes the size of correlated bond-orientational relaxation grows faster with decreasing temperature than the dynamic correlation length that characterizes the size of correlated bond-breaking events. We also examine the ensemble-dependent and ensemble-independent dynamic susceptibilities for both bond-breaking correlations and bond-orientational correlations. We find that for both correlations, the ensemble-dependent and ensemble-independent susceptibilities exhibit a maximum at nearly the same time, and this maximum occurs at a time slightly shorter than the peak position of the dynamic correlation length.

cond-mat.stat-mech↗
arXiv2015

Reduced strength and extent of dynamic heterogeneity in a strong glass former as compared to fragile glass formers

We examined dynamic heterogeneity in a model tetrahedral network glass-forming liquid. We used four-point correlation functions to extract dynamic correlation lengths xi_4^a(t) and susceptibilities chi_4^a(t) corresponding to structural relaxation on two length scales a. One length scale corresponds to structural relaxation at nearest neighbor distances and the other corresponds to relaxation of the tetrahedral structure. We find that the dynamic correlation length xi_4^{a} grows much slower with increasing relaxation time than for model fragile glass formers. We also find that chi_4^a ~ (xi_4^a)^z for a range of temperatures, but z < 3 at the lowest temperatures examined in this study. However, we do find evidence that the temperature where Stokes-Einstein violation begins marks a temperature where there is a change in the character of dynamically heterogeneous regions. Throughout the paper, we contrast the structure and dynamics of a strong glass former with that of a representative fragile glass former.

cond-mat.soft↗
arXiv2015

Glassy dynamics of athermal self-propelled particles: Computer simulations and a nonequilibrium microscopic theory

We combine computer simulations and analytical theory to investigate the glassy dynamics in dense assemblies of athermal particles evolving under the sole influence of self-propulsion. The simulations reveal that when the persistence time of the self-propelled particles is increased, the local structure becomes more pronounced whereas the long-time dynamics first accelerates and then slows down. These seemingly contradictory evolutions are explained by constructing a nonequilibrium mode-coupling-like theory for interacting self-propelled particles. To predict the collective dynamics the theory needs the steady state structure factor and the steady state correlations of the local velocities. It yields nontrivial predictions for the glassy dynamics of self-propelled particles in qualitative agreement with the simulations.

cond-mat.soft↗
arXiv2015

Fundamental differences between glassy dynamics in two and three dimensions

The two-dimensional freezing transition is very different from its three-dimensional counterpart. In contrast, the glass transition is usually assumed to have similar characteristics in two and three dimensions. Using computer simulations we show that glassy dynamics in supercooled two- and three-dimensional fluids are fundamentally different. Specifically, transient localization of particles upon approaching the glass transition is absent in two dimensions, whereas it is very pronounced in three dimensions. Moreover, the temperature dependence of the relaxation time of orientational correlations is decoupled from that of the translational relaxation time in two dimensions but not in three dimensions. Lastly, the relationships between the characteristic size of dynamically heterogeneous regions and the relaxation time are very different in two and three dimensions. These results strongly suggest that the glass transition in two dimensions is different than in three dimensions.

cond-mat.soft↗
arXiv2015

Cooling-rate dependence of kinetic and mechanical stability of simulated glasses

Recently, ultrastable glasses have been created through vapor deposition. Subsequently, computer simulation algorithms have been proposed that mimic the vapor deposition process and result in simulated glasses with increased stability. In addition, random pinning has been used to generate very stable glassy configurations without the need for lengthy annealing or special algorithms inspired by vapor deposition. Kinetic and mechanical stability of experimental ultrastable glasses is compared to those of experimental glasses formed by cooling. We provide the basis for a similar comparison for simulated stable glasses: we analyze the kinetic and mechanical stability of simulated glasses formed by cooling at a constant rate by examining the transformation time to a liquid upon rapid re-heating, the inherent structure energies, and the shear modulus. The kinetic and structural stability increases slowly with decreasing cooling rate. The methods outlined here can be used to assess kinetic and mechanical stability of simulated glasses generated by using specialized algorithms.

cond-mat.soft↗
arXiv2014

Long-range spatial correlations of particle displacements and the emergence of elasticity

We examine correlations of transverse particle displacements and their relationship to the shear modulus of a glass and the viscosity of a fluid. To this end we use computer simulations to calculate a correlation function of the displacements, $S_4(q;t)$, which is similar to functions used to study heterogeneous dynamics in glass-forming fluids. We show that in the glass the shear modulus can be obtained from the long-time, small-q limit of $S_4(q;t)$. By using scaling arguments, we argue that a four-point correlation length $ξ_4(t)$ grows linearly in time in a glass and grows as $\sqrt{t}$ at long times in a fluid, and we verify these results by analyzing $S_4(q;t)$ obtained from simulations. For a viscoelastic fluid, the simulation results suggest that the crossover to the long-time $\sqrt{t}$ growth of $ξ_4(t)$ occurs at a characteristic decay time of the shear stress autocorrelation function. Using this observation, we show that the amplitude of the long-time $\sqrt{t}$ growth is proportional to $\sqrtη$ where $η$ is the viscosity of the fluid.

cond-mat.stat-mech↗
arXiv2013

Universal features of dynamic heterogeneity in supercooled liquids

A few years ago it was showed that some systems that have very similar local structure, as quantified by the pair correlation function, exhibit vastly different slowing down upon supercooling [L. Berthier and G. Tarjus, Phys. Rev. Lett. 103, 170601 (2009); U.R. Pedersen, T.B. Schroder and J.C. Dyre, Phys. Rev. Lett. 105, 157801 (2010)]. Recently, a more subtle structural quantity, the so-called "point-to-set" length, was found to reliably correlate with the average dynamics [G.M. Hocky, T.E. Markland and D.R. Reichman, Phys. Rev. Lett. 108, 225506 (2012)]. Here we use computer simulations to examine the behavior of fluctuations around the average dynamics, i.e., dynamic heterogeneity. We study five model glass-forming liquids: three model liquids used in previous works and two additional model liquids with finite range interactions. Some of these systems have very similar local structure but vastly different dynamics. We show that for all these systems the spatial extent and the anisotropy of dynamic heterogeneity correlate very well with the average dynamics.

cond-mat.soft↗
arXiv2013

Breakdown of a renormalized perturbation expansion around mode-coupling theory of the glass transition

We analyze a renormalized perturbation expansion around the mode-coupling theory of the glass transition. We focus on the long-time limit of the irreducible memory function. We discuss a renormalized diagrammatic expansion for this function and re-sum two infinite classes of diagrams. We show that the resulting contributions to the irreducible memory function diverge at the mode-coupling transition. A further re-summation of ladder diagrams constructed by iterating these divergent contributions gives a finite result which cancels the mode-coupling theory's expression for the irreducible memory function.

cond-mat.stat-mech↗
arXiv2012

Dynamic heterogeneities above and below the mode-coupling temperature. Evidence of a dynamic crossover

We examine dynamic heterogeneities in a model glass-forming fluid, a binary harmonic sphere mixture, above and below the mode-coupling temperature T_c. We calculate the ensemble independent susceptibility chi_4(tau_alpha) and the dynamic correlation length xi_4(tau_alpha) at the alpha-relaxation time tau_alpha. We also examine in detail the temperature dependence of tau_alpha and the diffusion coefficient D. For higher temperatures we find that the standard Stokes-Einstein relationship, D ~ tau_alpha^{-1}, holds, but at lower temperatures a fractional Stokes-Einstein relationship, D ~ tau_alpha^{-sigma} with sigma = 0.65, emerges. By examining the relationships between tau_alpha, D, chi_4(tau_alpha), and xi_4(tau_alpha) we determine that the emergence of the fractional Stokes-Einstein relationship is accompanied by a dynamic crossover from tau_alpha ~ e^{k_2 xi_4} at higher temperatures to tau_alpha ~ e^{k_1 xi_4^{3/2}} at lower temperatures.

cond-mat.stat-mech↗