2,561 research outputs found
Molecular structural order and anomalies in liquid silica
The present investigation examines the relationship between structural order,
diffusivity anomalies, and density anomalies in liquid silica by means of
molecular dynamics simulations. We use previously defined orientational and
translational order parameters to quantify local structural order in atomic
configurations. Extensive simulations are performed at different state points
to measure structural order, diffusivity, and thermodynamic properties. It is
found that silica shares many trends recently reported for water [J. R.
Errington and P. G. Debenedetti, Nature 409, 318 (2001)]. At intermediate
densities, the distribution of local orientational order is bimodal. At fixed
temperature, order parameter extrema occur upon compression: a maximum in
orientational order followed by a minimum in translational order. Unlike water,
however, silica's translational order parameter minimum is broad, and there is
no range of thermodynamic conditions where both parameters are strictly
coupled. Furthermore, the temperature-density regime where both structural
order parameters decrease upon isothermal compression (the structurally
anomalous regime) does not encompass the region of diffusivity anomalies, as
was the case for water.Comment: 30 pages, 8 figure
Subdiffusion and cage effect in a sheared granular material
We investigate experimentally the diffusion properties of a bidimensional
bidisperse dry granular material under quasistatic cyclic shear.The comparison
of these properties with results obtained both in computer simulations of hard
spheres systems and Lenard-Jones liquids and experiments on colloidal systems
near the glass transition demonstrates a strong analogy between the behaviour
of granular matter and these systems. More specifically, we study in detail the
cage dynamics responsible for the subdiffusion in the slow relaxation regime,
and obtain the values of relevant time and length scales.Comment: 4 pages, 6 figures, submitted to PR
Two-Gaussian excitations model for the glass transition
We develop a modified "two-state" model with Gaussian widths for the site
energies of both ground and excited states, consistent with expectations for a
disordered system. The thermodynamic properties of the system are analyzed in
configuration space and found to bridge the gap between simple two state models
("logarithmic" model in configuration space) and the random energy model
("Gaussian" model in configuration space). The Kauzmann singularity given by
the random energy model remains for very fragile liquids but is suppressed or
eliminated for stronger liquids. The sharp form of constant volume heat
capacity found by recent simulations for binary mixed Lennard Jones and soft
sphere systems is reproduced by the model, as is the excess entropy and heat
capacity of a variety of laboratory systems, strong and fragile. The ideal
glass in all cases has a narrow Gaussian, almost invariant among molecular and
atomic glassformers, while the excited state Gaussian depends on the system and
its width plays a role in the thermodynamic fragility. The model predicts the
existence of first-order phase transition for fragile liquids.Comment: 12 pages, 12 figure
Don't Distract Me When I'm Media Multitasking: Toward a Theory for Raising Advertising Recall and Recognition
Media multitasking, such as using handheld devices like smartphones and tablets while watching TV, has become prevalent but its effect on the recall and recognition of advertising subject to limited academic research. We contend that the context in which multitasking takes place affects consumer memory for advertising delivered via the primary activity (e.g., watching television). Specifically, we identify the importance of the degree of (a) congruence between the primary and second screen activity and (b) social accountability of second screen activities. We test our typology empirically by examining the determinants of next day recall and recognition for billboard advertisers (perimeter board advertisements) of a televised football (soccer) match. In line with our theory, in most cases media multitasking leads to worse recall and recognition, however, in situations where there is congruence between primary and second screen activities and secondary activities have a higher level of social accountability attached to them, then advertising recall and recognition improves
Effects of compression on the vibrational modes of marginally jammed solids
Glasses have a large excess of low-frequency vibrational modes in comparison
with most crystalline solids. We show that such a feature is a necessary
consequence of the weak connectivity of the solid, and that the frequency of
modes in excess is very sensitive to the pressure. We analyze in particular two
systems whose density D(w) of vibrational modes of angular frequency w display
scaling behaviors with the packing fraction: (i) simulations of jammed packings
of particles interacting through finite-range, purely repulsive potentials,
comprised of weakly compressed spheres at zero temperature and (ii) a system
with the same network of contacts, but where the force between any particles in
contact (and therefore the total pressure) is set to zero. We account in the
two cases for the observed a) convergence of D(w) toward a non-zero constant as
w goes to 0, b) appearance of a low-frequency cutoff w*, and c) power-law
increase of w* with compression. Differences between these two systems occur at
lower frequency. The density of states of the modified system displays an
abrupt plateau that appears at w*, below which we expect the system to behave
as a normal, continuous, elastic body. In the unmodified system, the pressure
lowers the frequency of the modes in excess. The requirement of stability
despite the destabilizing effect of pressure yields a lower bound on the number
of extra contact per particle dz: dz > p^(1/2), which generalizes the Maxwell
criterion for rigidity when pressure is present. This scaling behavior is
observed in the simulations. We finally discuss how the cooling procedure can
affect the microscopic structure and the density of normal modes.Comment: 13 pages, 8 figure
Effect of dynamic stall on the aerodynamics of vertical-axis wind turbines
Accurate simulations of the aerodynamic performance of vertical-axis wind turbines pose a significant challenge for computational fluid dynamics methods. The aerodynamic interaction between the blades of the rotor and the wake that is produced by the blades requires a high-fidelity representation of the convection of vorticity within the wake. In addition, the cyclic motion of the blades induces large variations in the angle of attack on the blades that can manifest as dynamic stall. The present paper describes the application of a numerical model that is based on the vorticity transport formulation of the Navier–Stokes equations, to the prediction of the aerodynamics of a verticalaxis wind turbine that consists of three curved rotor blades that are twisted helically around the rotational axis of the rotor. The predicted variation of the power coefficient with tip speed ratio compares very favorably with experimental measurements. It is demonstrated that helical blade twist reduces the oscillation of the power coefficient that is an inherent feature of turbines with non-twisted blade configurations
Inhomogeneous elastic response of silica glass
Using large scale molecular dynamics simulations we investigate the
properties of the {\em non-affine} displacement field induced by macroscopic
uniaxial deformation of amorphous silica,a strong glass according to Angell's
classification. We demonstrate the existence of a length scale
characterizing the correlations of this field (corresponding to a volume of
about 1000 atoms), and compare its structure to the one observed in a standard
fragile model glass. The "Boson-peak'' anomaly of the density of states can be
traced back in both cases to elastic inhomogeneities on wavelengths smaller
than , where classical continuum elasticity becomes simply unapplicable
Fast Simulation of Facilitated Spin Models
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 maximise 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 d=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.Comment: 8 pages, 4 figure
Liquid Limits: The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids
The liquid-gas spinodal and the glass transition define ultimate boundaries
beyond which substances cannot exist as (stable or metastable) liquids. The
relation between these limits is analyzed {\it via} computer simulations of a
model liquid. The results obtained indicate that the liquid - gas spinodal and
the glass transition lines intersect at a finite temperature, implying a glass
- gas mechanical instability locus at low temperatures. The glass transition
lines obtained by thermodynamic and dynamic criteria agree very well with each
other.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
The Proliferation of Special Accounting Items: A Threat to Corporate Credibility
Betty L. Brewer, DBA, CFP, is associate professor of finance, Department of Business and Economics, North Carolina A&T State University, Greensboro, NC 27411.
Robert J. Angell, DBA, is professor of finance, Department of Business Administration, School of Business and Economics, North Carolina A&T State University, Greensboro, NC 27411.
R. David Mautz, Jr., Ph.D., CPA, is associate professor of accounting, Department of Accounting, School of Business and Economics, North Carolina A&T State University, Greensboro, NC 27411
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