532 research outputs found
Enhanced dielectrophoresis of nanocolloids by dimer formation
We investigate the dielectrophoretic motion of charge-neutral, polarizable
nanocolloids through molecular dynamics simulations. Comparison to analytical
results derived for continuum systems shows that the discrete charge
distributions on the nanocolloids have a significant impact on their coupling
to the external field. Aggregation of nanocolloids leads to enhanced
dielectrophoretic transport, provided that increase in the dipole moment upon
aggregation can overcome the related increase in friction. The dimer
orientation and the exact structure of the nanocolloid charge distribution are
shown to be important in the enhanced transport
Electrokinetic behavior of two touching inhomogeneous biological cells and colloidal particles: Effects of multipolar interactions
We present a theory to investigate electro-kinetic behavior, namely,
electrorotation and dielectrophoresis under alternating current (AC) applied
fields for a pair of touching inhomogeneous colloidal particles and biological
cells. These inhomogeneous particles are treated as graded ones with physically
motivated model dielectric and conductivity profiles. The mutual polarization
interaction between the particles yields a change in their respective dipole
moments, and hence in the AC electrokinetic spectra. The multipolar
interactions between polarized particles are accurately captured by the
multiple images method. In the point-dipole limit, our theory reproduces the
known results. We find that the multipolar interactions as well as the spatial
fluctuations inside the particles can affect the AC electrokinetic spectra
significantly.Comment: Revised version with minor changes: References added and discussion
extende
Dynamics of driven interfaces near isotropic percolation transition
We consider the dynamics and kinetic roughening of interfaces embedded in
uniformly random media near percolation treshold. In particular, we study
simple discrete ``forest fire'' lattice models through Monte Carlo simulations
in two and three spatial dimensions. An interface generated in the models is
found to display complex behavior. Away from the percolation transition, the
interface is self-affine with asymptotic dynamics consistent with the
Kardar-Parisi-Zhang universality class. However, in the vicinity of the
percolation transition, there is a different behavior at earlier times. By
scaling arguments we show that the global scaling exponents associated with the
kinetic roughening of the interface can be obtained from the properties of the
underlying percolation cluster. Our numerical results are in good agreement
with theory. However, we demonstrate that at the depinning transition, the
interface as defined in the models is no longer self-affine. Finally, we
compare these results to those obtained from a more realistic
reaction-diffusion model of slow combustion.Comment: 7 pages, 9 figures, to appear in Phys. Rev. E (1998
Collapses and explosions in self-gravitating systems
Collapse and reverse to collapse explosion transition in self-gravitating
systems are studied by molecular dynamics simulations. A microcanonical
ensemble of point particles confined to a spherical box is considered; the
particles interact via an attractive soft Coulomb potential. It is observed
that the collapse in the particle system indeed takes place when the energy of
the uniform state is put near or below the metastability-instability threshold
(collapse energy), predicted by the mean-field theory. Similarly, the explosion
in the particle system occurs when the energy of the core-halo state is
increased above the explosion energy, where according to the mean field
predictions the core-halo state becomes unstable. For a system consisting of
125 -- 500 particles, the collapse takes about single particle crossing
times to complete, while a typical explosion is by an order of magnitude
faster. A finite lifetime of metastable states is observed. It is also found
that the mean-field description of the uniform and the core-halo states is
exact within the statistical uncertainty of the molecular dynamics data.Comment: 9 pages, 14 figure
Towards Better Integrators for Dissipative Particle Dynamics Simulations
Coarse-grained models that preserve hydrodynamics provide a natural approach
to study collective properties of soft-matter systems. Here, we demonstrate
that commonly used integration schemes in dissipative particle dynamics give
rise to pronounced artifacts in physical quantities such as the compressibility
and the diffusion coefficient. We assess the quality of these integration
schemes, including variants based on a recently suggested self-consistent
approach, and examine their relative performance. Implications of
integrator-induced effects are discussed.Comment: 4 pages, 3 figures, 2 tables, accepted for publication in Phys. Rev.
E (Rapid Communication), tentative publication issue: 01 Dec 200
An autoregressive model based on the generalized hyperbolic distribution
We define a nonlinear autoregressive time series model based on the generalized hyperbolic distribution in an attempt to model time series with non-Gaussian features such as skewness and heavy tails. We show that the resulting process has a simple condition for stationarity and it is also ergodic. An empirical example with a forecasting experiment is presented to illustrate the features of the proposed model.Peer reviewe
The source ambiguity problem: Distinguishing the effects of grammar and processing on acceptability judgments
Judgments of linguistic unacceptability may theoretically arise from either grammatical deviance or significant processing difficulty. Acceptability data are thus naturally ambiguous in theories that explicitly distinguish formal and functional constraints. Here, we consider this source ambiguity problem in the context of Superiority effects: the dispreference for ordering a wh-phrase in front of a syntactically “superior” wh-phrase in multiple wh-questions, e.g., What did who buy? More specifically, we consider the acceptability contrast between such examples and so-called D-linked examples, e.g., Which toys did which parents buy? Evidence from acceptability and self-paced reading experiments demonstrates that (i) judgments and processing times for Superiority violations vary in parallel, as determined by the kind of wh-phrases they contain, (ii) judgments increase with exposure, while processing times decrease, (iii) reading times are highly predictive of acceptability judgments for the same items, and (iv) the effects of the complexity of the wh-phrases combine in both acceptability judgments and reading times. This evidence supports the conclusion that D-linking effects are likely reducible to independently motivated cognitive mechanisms whose effects emerge in a wide range of sentence contexts. This in turn suggests that Superiority effects, in general, may owe their character to differential processing difficulty
Nonlinear Driven Response of a Phase-Field Crystal in a Periodic Pinning Potential
We study numerically the phase diagram and the response under a driving force
of the phase field crystal model for pinned lattice systems introduced recently
for both one and two dimensional systems. The model describes the lattice
system as a continuous density field in the presence of a periodic pinning
potential, allowing for both elastic and plastic deformations of the lattice.
We first present results for phase diagrams of the model in the absence of a
driving force. The nonlinear response to a driving force on an initially pinned
commensurate phase is then studied via overdamped dynamic equations of motion
for different values of mismatch and pinning strengths. For large pinning
strength the driven depinning transitions are continuous, and the sliding
velocity varies with the force from the threshold with power-law exponents in
agreement with analytical predictions. Transverse depinning transitions in the
moving state are also found in two dimensions. Surprisingly, for sufficiently
weak pinning potential we find a discontinuous depinning transition with
hysteresis even in one dimension under overdamped dynamics. We also
characterize structural changes of the system in some detail close to the
depinning transition
Graded and Binary Responses in Stochastic Gene Expression
Recently, several theoretical and experimental studies have been undertaken
to probe the effect of stochasticity on gene expression (GE). In experiments,
the GE response to an inducing signal in a cell, measured by the amount of
mRNAs/proteins synthesized, is found to be either graded or binary. The latter
type of response gives rise to a bimodal distribution in protein levels in an
ensemble of cells. One possible origin of binary response is cellular
bistability achieved through positive feedback or autoregulation. In this
paper, we study a simple, stochastic model of GE and show that the origin of
binary response lies exclusively in stochasticity. The transitions between the
active and inactive states of the gene are random in nature. Graded and binary
responses occur in the model depending on the relative stability of the
activated and deactivated gene states with respect to that of
mRNAs/proteins.The theoretical results on binary response provide a good
description of the ``all-or-none'' phenomenon observed in an eukaryotic system.Comment: to be published in Physical Biolog
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