418 research outputs found
Maximally-fast coarsening algorithms
We present maximally-fast numerical algorithms for conserved coarsening
systems that are stable and accurate with a growing natural time-step . For non-conserved systems, only effectively finite timesteps
are accessible for similar unconditionally stable algorithms. We compare the
scaling structure obtained from our maximally-fast conserved systems directly
against the standard fixed-timestep Euler algorithm, and find that the error
scales as -- so arbitrary accuracy can be achieved.Comment: 5 pages, 3 postscript figures, Late
Silence of the Spam: Improving the CAN-SPAM Act by Including an Expanded Private Cause of Action
In the last decade, email spam has become more than just an annoyance for email users. Unsolicited messages now comprise more than 95 percent of all email sent worldwide. This costs US businesses billions of dollars in lost productivity each year. The US Congress passed the CAN-SPAM Act of 2003 to regulate the spam industry. Unfortunately, data show that spam only increased since the Act\u27s passage. Part of the reason for this failure is that the Act only authorizes the Federal Trade Commission, state attorneys general, and Internet Service Providers to bring action under its provisions. Each of these authorized entities either lacks the incentive or the resources to adequately enforce the Act, resulting in little to no reduction of spam. As a result, email recipients--not spammers--bear the cost of spam. This Note argues that the Act should incorporate an expanded private cause of action for email recipients, thereby increasing the enforcement level. This will deter spam prospectively by shifting the cost of unsolicited email from the recipient onto the sender
Scaling state of dry two-dimensional froths: universal angle deviations and structure
We characterize the late-time scaling state of dry, coarsening,
two-dimensional froths using a detailed, force-based vertex model. We find that
the slow evolution of bubbles leads to systematic deviations from 120degree
angles at three-fold vertices in the froth, with an amplitude proportional to
the vertex speed, v ~ sqrt(t), but with a side-number dependence that is
independent of time. We also find that a significant number of T1
side-switching processes occur for macroscopic bubbles in the scaling state,
though most bubble annihilations involve four-sided bubbles at microscopic
scales.Comment: 7 pages, 7 figure
Non-equilibrium Phase-Ordering with a Global Conservation Law
In all dimensions, infinite-range Kawasaki spin exchange in a quenched Ising
model leads to an asymptotic length-scale
at because the kinetic coefficient is renormalized by the broken-bond
density, . For , activated kinetics recovers the
standard asymptotic growth-law, . However, at all temperatures,
infinite-range energy-transport is allowed by the spin-exchange dynamics. A
better implementation of global conservation, the microcanonical Creutz
algorithm, is well behaved and exhibits the standard non-conserved growth law,
, at all temperatures.Comment: 2 pages and 2 figures, uses epsf.st
Breakdown of Scaling in the Nonequilibrium Critical Dynamics of the Two-Dimensional XY Model
The approach to equilibrium, from a nonequilibrium initial state, in a system
at its critical point is usually described by a scaling theory with a single
growing length scale, , where z is the dynamic exponent
that governs the equilibrium dynamics. We show that, for the 2D XY model, the
rate of approach to equilibrium depends on the initial condition. In
particular, if no free vortices are present in the
initial state, while if free vortices are
present.Comment: 4 pages, 3 figure
Comment on ``Theory of Spinodal Decomposition''
I comment on a paper by S. B. Goryachev [PRL vol 72, p.1850 (1994)] that
presents a theory of non-equilibrium dynamics for scalar systems quenched into
an ordered phase. Goryachev incorrectly applies only a global conservation
constraint to systems with local conservation laws.Comment: 2 pages LATeX (REVTeX macros), no figures. REVISIONS --- more to the
point. microscopic example added, presentation streamlined, long-range
interactions mentioned, to be published in Phys. Rev. Let
Stress-free Spatial Anisotropy in Phase-Ordering
We find spatial anisotropy in the asymptotic correlations of two-dimensional
Ising models under non-equilibrium phase-ordering. Anisotropy is seen for
critical and off-critical quenches and both conserved and non-conserved
dynamics. We argue that spatial anisotropy is generic for scalar systems
(including Potts models) with an anisotropic surface tension. Correlation
functions will not be universal in these systems since anisotropy will depend
on, e.g., temperature, microscopic interactions and dynamics, disorder, and
frustration.Comment: 4 pages, 4 figures include
Persistence in systems with algebraic interaction
Persistence in coarsening 1D spin systems with a power law interaction
is considered. Numerical studies indicate that for sufficiently
large values of the interaction exponent ( in our
simulations), persistence decays as an algebraic function of the length scale
, . The Persistence exponent is found to be
independent on the force exponent and close to its value for the
extremal () model, . For smaller
values of the force exponent (), finite size effects prevent the
system from reaching the asymptotic regime. Scaling arguments suggest that in
order to avoid significant boundary effects for small , the system size
should grow as .Comment: 4 pages 4 figure
Steady-state MreB helices inside bacteria: dynamics without motors
Within individual bacteria, we combine force-dependent polymerization
dynamics of individual MreB protofilaments with an elastic model of
protofilament bundles buckled into helical configurations. We use variational
techniques and stochastic simulations to relate the pitch of the MreB helix,
the total abundance of MreB, and the number of protofilaments. By comparing our
simulations with mean-field calculations, we find that stochastic fluctuations
are significant. We examine the quasi-static evolution of the helical pitch
with cell growth, as well as timescales of helix turnover and denovo
establishment. We find that while the body of a polarized MreB helix treadmills
towards its slow-growing end, the fast-growing tips of laterally associated
protofilaments move towards the opposite fast-growing end of the MreB helix.
This offers a possible mechanism for targeted polar localization without
cytoplasmic motor proteins.Comment: 7 figures, 1 tabl
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