Excitation Chains at the Glass Transition

The excitation-chain theory of the glass transition, proposed in an earlier publication, predicts diverging, super-Arrhenius relaxation times and, {\it via} a similarly diverging length scale, suggests a way of understanding the relations between dynamic and thermodynamic properties of glass-forming liquids. I argue here that critically large excitation chains play a role roughly analogous to that played by critical clusters in the droplet model of vapor condensation. The chains necessarily induce spatial heterogeneities in the equilibrium states of glassy systems; and these heterogeneities may be related to stretched-exponential relaxation. Unlike a first-order condensation point in a vapor, the glass transition is not a conventional phase transformation, and may not be a thermodynamic transition at all.Comment: 4 pages, no figure

Effects of Secondary Task Demands on Drivers’ Responses to Events During Driving: Surrogate Methods and Issues

OBJECTIVES (1) To examine effects of secondary task demands on event detection and response during driving. (2) To examine the validity of surrogate methods which might be used in a laboratory environment to evaluate these effects early in the development process for advanced information systems. (3) To identify issues which limit and/or require careful interpretation in the application of event detection results to product considerations. (4) To identify issues which are at the forefront of scientific knowledge in this area, and in need of further research. METHODS Analytic methods were used to compare data derived from prior studies—and/or to make comparisons across studies. Linear mixed models, correlation, and other statistical methods were used, as appropriate. Results were drawn from several sources, and hence reflected multiple methodologies. One of the main sources of data for this work was the CAMP Driver Workload Metrics project (2006) (which was a collaborative project between GM, Ford, Toyota, Nissan, and USDOT). Three types of methods were compared: on-road, test track, and laboratory methods. In the onroad and test-track methods, 108 participants and 69 participants (respectively) drove an instrumented vehicle that was the middle car in a platoon of three vehicles. (That is, the test participants followed a lead vehicle—and also had a vehicle following them). Drivers engaged in 22 different secondary tasks while driving on public roads (or a test track), and while also responding to three different types of visual events that could potentially occur during a task. These event types included: the illumination of a CHMSL-like light on the lead vehicle, a deceleration of the lead vehicle (without brake lights), and a turn signal illumination on the follow-vehicle. In the laboratory, methods included a specially-developed variation of the Sternberg task, and two Peripheral Detection Tasks. Data were collected from 57 participants. Data from these methods are reported, compared with each other, and compared with data from other sources, where methods permitted that. Dependent variables were primarily “percent missed events”; key independent variables included task type and workload level. RESULTS Data showed that visual-manual and auditory-vocal tasks differed in their effects on event detection during driving, with visual-manual tasks (as a class) interfering to a greater extent with event detection. The effects of task demands on event detection could not, however, be fully accounted for by glance-related measurements, suggesting that there are other variables involved, such as an attentional component to event detection. Furthermore, the modifiedSternberg method emerged as a laboratory surrogate with promising predictive validity (as well as repeatability) for evaluating a task’s effects on event detection/response during driving. However, several issues emerged related to how the results of this assessment method can be interpreted and applied. These issues relate to practical matters (of task length, and task type), and also to theoretical matters (e.g., regarding how to change task design to mitigate interference with event detection). CONCLUSIONS Drivers’ responses to events while driving (particularly unexpected events) are an important element of driving performance. Early in the development of advanced information systems, it would be desirable to have a valid way to evaluate how task demands may affect event detection (so that system task designs could be modified and improved, if necessary). However, it is particularly difficult to study event detection experimentally. Nearly all existing methods of evaluating event detection and response (other than naturalistic driving studies) must be classified as surrogate methods. And most of the methods examined here used techniques that did not really qualify as presenting “completely unexpected” events. Nonetheless, with this caveat, the method which offered the most promise was that which was sensitive both to a task’s loading on visual processing and also its effects on central attention (e.g., to a cognitive/memory load). That method was the modified-Sternberg method. Issues related to interpretation and application suggested that discriminating high from low workload tasks may be a problem when Task Types are separated. However, conclusions about grouping Task Types together are offered. Further, the findings had implications for strategies that might be used during system development to improve event detection performance for a task. Additional research is needed, not only to verify the kinds of strategies for task development that would in fact lead to improved event detection performance, but also to examine issues surrounding development of methods for presenting unexpected events, and to establish a deeper understanding of processes that underlie event detection and response, especially in naturalistic driving REFERENCES Angell, L. S., Auflick, L. L., Austria, P. A., Kochhar, D. S. & Tijerina, L. (2006). “Driver workload metrics project: Task 2 final report,” submitted to U.S. Dept. of Transportation, FHA, & NHTSA under Cooperative Agreement Number DTFH61-01-X-00014

The viscous slowing down of supercooled liquids as a temperature-controlled superArrhenius activated process: a description in terms of frustration-limited domains

We propose that the salient feature to be explained about the glass transition of supercooled liquids is the temperature-controlled superArrhenius activated nature of the viscous slowing down, more strikingly seen in weakly-bonded, fragile systems. In the light of this observation, the relevance of simple models of spherically interacting particles and that of models based on free-volume congested dynamics are questioned. Finally, we discuss how the main aspects of the phenomenology of supercooled liquids, including the crossover from Arrhenius to superArrhenius activated behavior and the heterogeneous character of the $\alpha$ relaxation, can be described by an approach based on frustration-limited domains.Comment: 13 pages, 4 figures, accepted in J. Phys.: Condensed Matter, proceedings of the Trieste workshop on "Unifying Concepts in Glass Physics

The relationship between fragility, configurational entropy and the potential energy landscape of glass forming liquids

Glass is a microscopically disordered, solid form of matter that results when a fluid is cooled or compressed in such a fashion that it does not crystallise. Almost all types of materials are capable of glass formation -- polymers, metal alloys, and molten salts, to name a few. Given such diversity, organising principles which systematise data concerning glass formation are invaluable. One such principle is the classification of glass formers according to their fragility\cite{fragility}. Fragility measures the rapidity with which a liquid's properties such as viscosity change as the glassy state is approached. Although the relationship between features of the energy landscape of a glass former, its configurational entropy and fragility have been analysed previously (e. g.,\cite{speedyfr}), an understanding of the origins of fragility in these features is far from being well established. Results for a model liquid, whose fragility depends on its bulk density, are presented in this letter. Analysis of the relationship between fragility and quantitative measures of the energy landscape (the complicated dependence of energy on configuration) reveal that the fragility depends on changes in the vibrational properties of individual energy basins, in addition to the total number of such basins present, and their spread in energy. A thermodynamic expression for fragility is derived, which is in quantitative agreement with {\it kinetic} fragilities obtained from the liquid's diffusivity.Comment: 8 pages, 3 figure

Molecular Dynamics Simulations of a Pressure-induced Glass Transition

We simulate the compression of a two-component Lennard-Jones liquid at a variety of constant temperatures using a molecular dynamics algorithm in an isobaric-isothermal ensemble. The viscosity of the liquid increases with pressure, undergoing a broadened transition into a structurally arrested, amorphous state. This transition, like the more familiar one induced by cooling, is correlated with a significant increase in icosahedral ordering. In fact, the structure of the final state, as measured by an analysis of the bonding, is essentially the same in the glassy, frozen state whether produced by squeezing or by cooling under pressure. We have computed an effective hard-sphere packing fraction at the transition, defining the transition pressure or temperature by a cutoff in the diffusion constant, analogous to the traditional laboratory definition of the glass transition by an arbitrary, low cutoff in viscosity. The packing fraction at this transition point is not constant, but is consistently higher for runs compressed at higher temperature. We show that this is because the transition point defined by a constant cutoff in the diffusion constant is not the same as the point of structural arrest, at which further changes in pressure induce no further structural changes, but that the two alternate descriptions may be reconciled by using a thermally activated cutoff for the diffusion constant. This enables estimation of the characteristic activation energy for diffusion at the point of structural arrest.Comment: Latex using Revtex macro

Liquid-liquid phase transition in Stillinger-Weber silicon

It was recently demonstrated that the Stillinger-Weber silicon undergoes a liquid-liquid first-order phase transition deep into the supercooled region (Sastry and Angell, Nature Materials 2, 739 (2003)). Here we study the effects of perturbations on this phase transition. We show that the order of the liquid-liquid transition changes with negative pressure. We also find that the liquid-liquid transition disappears when the three-body term of the potential is strengthened by as little as 5 %. This implies that the details of the potential could affect strongly the nature and even the existence of the liquid-liquid phase.Comment: 13 page

The Dimensions of Driver Performance during Secondary Manual Tasks

This analysis identified the underlying dimensions of driver performance using data obtained from drivers engaged in secondary manual tasks. Randomly chosen subjects balanced for age and gender used one of five advanced navigation and communication systems while driving on a closed roadway. Fifteen driver performance variables were averaged and standardized across subjects for 79 tasks. There were high correlations between all variables. Principal Component Analysis (PCA) found that the vector of loadings defining the first principal component (PC1) was positive for all 15 variables, accounting for 61 percent of the total variation across all tasks. It is interpreted as overall driver demand. PC2 loaded with one sign on event detection and response variables, but opposing sign on visual-manual workload variables. It identified tasks making drivers more inattentive to outside events than expected, given a task’s visual-manual workload, and accounted for 17 percent of total variation. It is interpreted as low-workloadbut-high-inattentiveness. PC3 had loadings of opposing sign for peripheral vs. central event variables (5 percent of total variation). It is interpreted as peripheral insensitivity. The first three components together accounted for 83 percent of total variation, which is deemed substantial. Thus most of the information available through the 15 original variables can be summarized by only three PC variables. Because the vectors of loadings defining the components are orthogonal to each other as defined by PCA, no single variable by itself can capture all the important variations in driver performance during secondary manual tasks. A multivariate design and analysis is required

Molecular Dynamics Computer Simulation of the Dynamics of Supercooled Silica

We present the results of a large scale computer simulation of supercooled silica. We find that at high temperatures the diffusion constants show a non-Arrhenius temperature dependence whereas at low temperature this dependence is also compatible with an Arrhenius law. We demonstrate that at low temperatures the intermediate scattering function shows a two-step relaxation behavior and that it obeys the time temperature superposition principle. We also discuss the wave-vector dependence of the nonergodicity parameter and the time and temperature dependence of the non-Gaussian parameter.Comment: 5 pages, Latex, 6 postscript figure

On the static length of relaxation and the origin of dynamic heterogeneity in fragile glass-forming liquids

The most puzzling aspect of the glass transition observed in laboratory is an apparent decoupling of dynamics from structure. In this paper we recount the implication of various theories of glass transition for the static correlation length in an attempt to reconcile the dynamic and static lengths associate with the glass problem. We argue that a more recent characterization of the static relaxation length based on the bond ordering scenario, as the typical length over which the energy fluctuations are correlated, is more consistent with, and indeed in perfect agreement with the typical linear size of the dynamically heterogeneous domains observed in deeply supercooled liquids. The correlated relaxation of bonds in terms of energy is therefore identified as the physical origin of the observed dynamic heterogeneity.Comment: 6 pages, 1 figur