675 research outputs found
An Exploratory Study of Web-Based Electronic Commerce Applications
This study explores the use of the Web in electronic commerce (EC) applications and proposes a framework for classifying such applications. The framework is based on three widely used models of organizations that identify application categories. The ability of each of these models to classify thirty common EC applications was then examined. The Value Chain model provided the best schema and is used as a basis for the proposed framework. This new framework was evaluated with the observed EC applications from a random sample of Fortune 500 firms. The results of this classification process are presented along with an industry analysis of the observations. The analysis suggests that adoption of particular EC applications can be explained both by the nature of the industry and the nature of the application. The framework and tests suggest fruitful avenues for future research. In particular, testing a broader set of firms and refinement of the methodology would be advantageous
Glass Transition Phenomena Semiannual Status Report
Multiple glass transitions, heat capacities, and equation of state properties of polymer system
Statistical mechanics far from equilibrium: prediction and test for a sheared system
We report the complete statistical treatment of a system of particles
interacting via Newtonian forces in continuous boundary-driven flow, far from
equilibrium. By numerically time-stepping the force-balance equations of a
model fluid we measure occupancies and transition rates in simulation. The
high-shear-rate simulation data verify the invariant quantities predicted by
our statistical theory, thus demonstrating that a class of non-equilibrium
steady states of matter, namely sheared complex fluids, is amenable to
statistical treatment from first principles.Comment: 4 pages plus a 3-page pdf supplemen
Active nematics on a substrate: giant number fluctuations and long-time tails
We construct the equations of motion for the coupled dynamics of order
parameter and concentration for the nematic phase of driven particles on a
solid surface, and show that they imply (i) giant number fluctuations, with a
standard deviation proportional to the mean and (ii) long-time tails in the autocorrelation of the particle velocities in dimensions
despite the absence of a hydrodynamic velocity field. Our predictions can be
tested in experiments on aggregates of amoeboid cells as well as on layers of
agitated granular matter.Comment: Submitted to Europhys Lett 26 Aug 200
A Dynamic Renormalization Group Study of Active Nematics
We carry out a systematic construction of the coarse-grained dynamical
equation of motion for the orientational order parameter for a two-dimensional
active nematic, that is a nonequilibrium steady state with uniaxial, apolar
orientational order. Using the dynamical renormalization group, we show that
the leading nonlinearities in this equation are marginally \textit{irrelevant}.
We discover a special limit of parameters in which the equation of motion for
the angle field of bears a close relation to the 2d stochastic Burgers
equation. We find nevertheless that, unlike for the Burgers problem, the
nonlinearity is marginally irrelevant even in this special limit, as a result
of of a hidden fluctuation-dissipation relation. 2d active nematics therefore
have quasi-long-range order, just like their equilibrium counterpartsComment: 31 pages 6 figure
Hydrodynamic fluctuations and instabilities in ordered suspensions of self-propelled particles
We construct the hydrodynamic equations for {\em suspensions} of
self-propelled particles (SPPs) with spontaneous orientational order, and make
a number of striking, testable predictions:(i) SPP suspensions with the
symmetry of a true {\em nematic} are {\em always} absolutely unstable at long
wavelengths.(ii) SPP suspensions with {\em polar}, i.e., head-tail {\em
asymmetric}, order support novel propagating modes at long wavelengths,
coupling orientation, flow, and concentration. (iii) In a wavenumber regime
accessible only in low Reynolds number systems such as bacteria, polar-ordered
suspensions are invariably convectively unstable.(iv) The variance in the
number N of particles, divided by the mean , diverges as in
polar-ordered SPP suspensions.Comment: submitted to Phys Rev Let
Constraining the cosmic radiation density due to lepton number with Big Bang Nucleosynthesis
The cosmic energy density in the form of radiation before and during Big Bang
Nucleosynthesis (BBN) is typically parameterized in terms of the effective
number of neutrinos N_eff. This quantity, in case of no extra degrees of
freedom, depends upon the chemical potential and the temperature characterizing
the three active neutrino distributions, as well as by their possible
non-thermal features. In the present analysis we determine the upper bounds
that BBN places on N_eff from primordial neutrino--antineutrino asymmetries,
with a careful treatment of the dynamics of neutrino oscillations. We consider
quite a wide range for the total lepton number in the neutrino sector, eta_nu=
eta_{nu_e}+eta_{nu_mu}+eta_{nu_tau} and the initial electron neutrino asymmetry
eta_{nu_e}^in, solving the corresponding kinetic equations which rule the
dynamics of neutrino (antineutrino) distributions in phase space due to
collisions, pair processes and flavor oscillations. New bounds on both the
total lepton number in the neutrino sector and the nu_e -bar{nu}_e asymmetry at
the onset of BBN are obtained fully exploiting the time evolution of neutrino
distributions, as well as the most recent determinations of primordial 2H/H
density ratio and 4He mass fraction. Note that taking the baryon fraction as
measured by WMAP, the 2H/H abundance plays a relevant role in constraining the
allowed regions in the eta_nu -eta_{nu_e}^in plane. These bounds fix the
maximum contribution of neutrinos with primordial asymmetries to N_eff as a
function of the mixing parameter theta_13, and point out the upper bound N_eff
< 3.4. Comparing these results with the forthcoming measurement of N_eff by the
Planck satellite will likely provide insight on the nature of the radiation
content of the universe.Comment: 17 pages, 9 figures, version to be published in JCA
Nonequilibrium statistical mechanics of shear flow: invariant quantities and current relations
In modeling nonequilibrium systems one usually starts with a definition of
the microscopic dynamics, e.g., in terms of transition rates, and then derives
the resulting macroscopic behavior. We address the inverse question for a class
of steady state systems, namely complex fluids under continuous shear flow: how
does an externally imposed shear current affect the microscopic dynamics of the
fluid? The answer can be formulated in the form of invariant quantities, exact
relations for the transition rates in the nonequilibrium steady state, as
discussed in a recent letter [A. Baule and R. M. L. Evans, Phys. Rev. Lett.
101, 240601 (2008)]. Here, we present a more pedagogical account of the
invariant quantities and the theory underlying them, known as the
nonequilibrium counterpart to detailed balance (NCDB). Furthermore, we
investigate the relationship between the transition rates and the shear current
in the steady state. We show that a fluctuation relation of the
Gallavotti-Cohen type holds for systems satisfying NCDB.Comment: 24 pages, 11 figure
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