63,079 research outputs found
Non-equilibrium dynamics of an active colloidal "chucker"
We report Monte Carlo simulations of the dynamics of a "chucker": a colloidal
particle which emits smaller solute particles from its surface, isotropically
and at a constant rate k_c. We find that the diffusion constant of the chucker
increases for small k_c, as recently predicted theoretically. At large k_c the
chucker diffuses more slowly due to crowding effects. We compare our simulation
results to those of a "point particle" Langevin dynamics scheme in which the
solute concentration field is calculated analytically, and in which
hydrodynamic effects can be included albeit in an approximate way. By
simulating the dragging of a chucker, we obtain an estimate of its apparent
mobility coefficient which violates the fluctuation-dissipation theorem. We
also characterise the probability density profile for a chucker which sediments
onto a surface which either repels or absorbs the solute particles, and find
that the steady state distributions are very different in the two cases. Our
simulations are inspired by the biological example of
exopolysaccharide-producing bacteria, as well as by recent experimental,
simulation and theoretical work on phoretic colloidal "swimmers".Comment: re-submission after referee's comment
Coarse-grained Interaction Potentials for Anisotropic Molecules
We have proposed an efficient parameterization method for a recent variant of
the Gay-Berne potential for dissimilar and biaxial particles and demonstrated
it for a set of small organic molecules. Compared to the previously proposed
coarse-grained models, the new potential exhibits a superior performance in
close contact and large distant interactions. The repercussions of thermal
vibrations and elasticity has been studied through a statistical method. The
study justifies that the potential of mean force is representable with the same
functional form, extending the application of this coarse-grained description
to a broader range of molecules. Moreover, the advantage of employing
coarse-grained models over truncated atomistic summations with large distance
cutoffs has been briefly studied.Comment: 8 pages, 4 tables and 6 figures. To appear in J. Chem. Phy
Plasmas generated by ultra-violet light rather than electron impact
We analyze, in both plane and cylindrical geometries, a collisionless plasma
consisting of an inner region where generation occurs by UV illumination, and
an un-illuminated outer region with no generation. Ions generated in the inner
region flow outwards through the outer region and into a wall. We solve for
this system's steady state, first in the quasi-neutral regime (where the Debye
length vanishes and analytic solutions exist) and then in the
general case, which we solve numerically. In the general case a double layer
forms where the illuminated and un-illuminated regions meet, and an
approximately quasi-neutral plasma connects the double layer to the wall
sheath; in plane geometry the ions coast through the quasi-neutral section at
slightly more than the Bohm speed . The system, although simple, therefore
has two novel features: a double layer that does not require counter-streaming
ions and electrons, and a quasi-neutral plasma where ions travel in straight
lines with at least the Bohm speed. We close with a pr\'{e}cis of our
asymptotic solutions of this system, and suggest how our theoretical
conclusions might be extended and tested in the laboratory.Comment: 10 pages, 3 figures, accepted by Physics of Plasma
Ion-water clusters, bulk medium effects, and ion hydration
Thermochemistry of gas-phase ion-water clusters together with estimates of
the hydration free energy of the clusters and the water ligands are used to
calculate the hydration free energy of the ion. Often the hydration
calculations use a continuum model of the solvent. The primitive quasichemical
approximation to the quasichemical theory provides a transparent framework to
anchor such efforts. Here we evaluate the approximations inherent in the
primitive quasichemical approach and elucidate the different roles of the bulk
medium. We find that the bulk medium can stabilize configurations of the
cluster that are usually not observed in the gas phase, while also
simultaneously lowering the excess chemical potential of the ion. This effect
is more pronounced for soft ions. Since the coordination number that minimizes
the excess chemical potential of the ion is identified as the optimal or most
probable coordination number, for such soft ions, the optimum cluster size and
the hydration thermodynamics obtained without account of the bulk medium on the
ion-water clustering reaction can be different from those observed in
simulations of the aqueous ion. The ideas presented in this work are expected
to be relevant to experimental studies that translate thermochemistry of
ion-water clusters to the thermodynamics of the hydrated ion and to evolving
theoretical approaches that combine high-level calculations on clusters with
coarse-grained models of the medium
Elementary Excitations of a Bose-Einstein Condensate in an Effective Magnetic Field
We calculate the low energy elementary excitations of a Bose-Einstein
Condensate in an effective magnetic field. The field is created by the
interplay between light beams carrying orbital angular momentum and the trapped
atoms. We examine the role of the homogeneous magnetic field, familiar from
studies of rotating condensates, and also investigate spectra for vector
potentials with a more general radial dependence. We discuss the instabilities
which arise and how these may be manifested.Comment: 8 pages, 4 figure
Analysis and application of digital spectral warping in analog and mixed-signal testing
Spectral warping is a digital signal processing transform which shifts the frequencies contained within a signal along the frequency axis. The Fourier transform coefficients of a warped signal correspond to frequency-domain 'samples' of the original signal which are unevenly spaced along the frequency axis. This property allows the technique to be efficiently used for DSP-based analog and mixed-signal testing. The analysis and application of spectral warping for test signal generation, response analysis, filter design, frequency response evaluation, etc. are discussed in this paper along with examples of the software and hardware implementation
Depletion effects and loop formation in self-avoiding polymers
Langevin dynamics is employed to study the looping kinetics of self-avoiding
polymers both in ideal and crowded solutions. A rich kinetics results from the
competition of two crowding-induced effects: the depletion attraction and the
enhanced viscous friction. For short chains, the enhanced friction slows down
looping, while, for longer chains, the depletion attraction renders it more
frequent and persistent. We discuss the possible relevance of the findings for
chromatin looping in living cells.Comment: 4 pages, 3 figure
Is a single photon's wave front observable?
The ultimate goal and the theoretical limit of weak signal detection is the
ability to detect a single photon against a noisy background. [...] In this
paper we show, that a combination of a quantum metamaterial (QMM)-based sensor
matrix and quantum non-demolition (QND) readout of its quantum state allows, in
principle, to detect a single photon in several points, i.e., to observe its
wave front.
Actually, there are a few possible ways of doing this, with at least one
within the reach of current experimental techniques for the microwave range.
The ability to resolve the quantum-limited signal from a remote source against
a much stronger local noise would bring significant advantages to such diverse
fields of activity as, e.g., microwave astronomy and missile defence.
The key components of the proposed method are 1) the entangling interaction
of the incoming photon with the QMM sensor array, which produces the spatially
correlated quantum state of the latter, and 2) the QND readout of the
collective observable (e.g., total magnetic moment), which characterizes this
quantum state. The effects of local noise (e.g., fluctuations affecting the
elements of the matrix) will be suppressed relative to the signal from the
spatially coherent field of (even) a single photon.Comment: 13 pages, 4 figure
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