4,116 research outputs found
Efficient solutions of self-consistent mean field equations for dewetting and electrostatics in nonuniform liquids
We use a new configuration-based version of linear response theory to
efficiently solve self-consistent mean field equations relating an effective
single particle potential to the induced density. The versatility and accuracy
of the method is illustrated by applications to dewetting of a hard sphere
solute in a Lennard-Jones fluid, the interplay between local hydrogen bond
structure and electrostatics for water confined between two hydrophobic walls,
and to ion pairing in ionic solutions. Simulation time has been reduced by more
than an order of magnitude over previous methods.Comment: Supplementary material included at end of main pape
A new approach for efficient simulation of Coulomb interactions in ionic fluids
We propose a simplified version of local molecular field (LMF) theory to
treat Coulomb interactions in simulations of ionic fluids. LMF theory relies on
splitting the Coulomb potential into a short-ranged part that combines with
other short-ranged core interactions and is simulated explicitly. The averaged
effects of the remaining long-ranged part are taken into account through a
self-consistently determined effective external field. The theory contains an
adjustable length parameter sigma that specifies the cut-off distance for the
short-ranged interaction. This can be chosen to minimize the errors resulting
from the mean-field treatment of the complementary long-ranged part. Here we
suggest that in many cases an accurate approximation to the effective field can
be obtained directly from the equilibrium charge density given by the Debye
theory of screening, thus eliminating the need for a self-consistent treatment.
In the limit sigma -> 0, this assumption reduces to the classical Debye
approximation. We examine the numerical performance of this approximation for a
simple model of a symmetric ionic mixture. Our results for thermodynamic and
structural properties of uniform ionic mixtures agree well with similar results
of Ewald simulations of the full ionic system. In addition we have used the
simplified theory in a grand-canonical simulation of a nonuniform ionic mixture
where an ion has been fixed at the origin. Simulations using short-ranged
truncations of the Coulomb interactions alone do not satisfy the exact
condition of complete screening of the fixed ion, but this condition is
recovered when the effective field is taken into account. We argue that this
simplified approach can also be used in the simulations of more complex
nonuniform systems.Comment: To be published in Journal of Chemical Physic
Density fluctuations and the structure of a nonuniform hard sphere fluid
We derive an exact equation for density changes induced by a general external
field that corrects the hydrostatic approximation where the local value of the
field is adsorbed into a modified chemical potential. Using linear response
theory to relate density changes self-consistently in different regions of
space, we arrive at an integral equation for a hard sphere fluid that is exact
in the limit of a slowly varying field or at low density and reduces to the
accurate Percus-Yevick equation for a hard core field. This and related
equations give accurate results for a wide variety of fields
Local molecular field theory for the treatment of electrostatics
We examine in detail the theoretical underpinnings of previous successful
applications of local molecular field (LMF) theory to charged systems. LMF
theory generally accounts for the averaged effects of long-ranged components of
the intermolecular interactions by using an effective or restructured external
field. The derivation starts from the exact Yvon-Born-Green hierarchy and shows
that the approximation can be very accurate when the interactions averaged over
are slowly varying at characteristic nearest-neighbor distances. Application of
LMF theory to Coulomb interactions alone allows for great simplifications of
the governing equations. LMF theory then reduces to a single equation for a
restructured electrostatic potential that satisfies Poisson's equation defined
with a smoothed charge density. Because of this charge smoothing by a Gaussian
of width sigma, this equation may be solved more simply than the detailed
simulation geometry might suggest. Proper choice of the smoothing length sigma
plays a major role in ensuring the accuracy of this approximation. We examine
the results of a basic confinement of water between corrugated wall and justify
the simple LMF equation used in a previous publication. We further generalize
these results to confinements that include fixed charges in order to
demonstrate the broader impact of charge smoothing by sigma. The slowly-varying
part of the restructured electrostatic potential will be more symmetric than
the local details of confinements.Comment: To be published in J Phys-Cond Matt; small misprint corrected in Eq.
(12) in V
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
Vacuum Energy Density for Massless Scalar Fields in Flat Homogeneous Spacetime Manifolds with Nontrivial Topology
Although the observed universe appears to be geometrically flat, it could
have one of 18 global topologies. A constant-time slice of the spacetime
manifold could be a torus, Mobius strip, Klein bottle, or others. This global
topology of the universe imposes boundary conditions on quantum fields and
affects the vacuum energy density via Casimir effect. In a spacetime with such
a nontrivial topology, the vacuum energy density is shifted from its value in a
simply-connected spacetime. In this paper, the vacuum expectation value of the
stress-energy tensor for a massless scalar field is calculated in all 17
multiply-connected, flat and homogeneous spacetimes with different global
topologies. It is found that the vacuum energy density is lowered relative to
the Minkowski vacuum level in all spacetimes and that the stress-energy tensor
becomes position-dependent in spacetimes that involve reflections and
rotations.Comment: 25 pages, 11 figure
Task 4 supporting technology. Densification requirements definition and test objectives. Propellant densification requirements definition
The primary challenge of the X-33 CAN is to build and test a prototype LO2 and LH2 densification ground support equipment (GSE) unit, and perform tank thermodynamic testing within the 15 month phase 1 period. The LO2 and LH2 propellant densification system will be scaled for the IPTD LO2 and LH2 tank configurations. The IPTD tanks were selected for the propellant technology demonstration because of the potential benefits to the phase 1 plan: tanks will be built in time to support thermodynamic testing; minimum cost; minimum schedule risk; future testing at MSFC will build on phase 1 data base; and densification system will be available to support X-33 and RLV engine test at IPTD. The objective of the task 1 effort is to define the preliminary requirements of the propellant densification GSE and tank recirculation system. The key densification system design parameters to be established in Task 1 are: recirculation flow rate; heat exchanger inlet temperature; heat exchanger outlet temperature; maximum heat rejection rate; vent flow rate (GN2 and GH2); densification time; and tank pressure level
Circles in the Sky: Finding Topology with the Microwave Background Radiation
If the universe is finite and smaller than the distance to the surface of
last scatter, then the signature of the topology of the universe is writ large
on the microwave background sky. We show that the microwave background will be
identified at the intersections of the surface of last scattering as seen by
different ``copies'' of the observer. Since the surface of last scattering is a
two-sphere, these intersections will be circles, regardless of the background
geometry or topology. We therefore propose a statistic that is sensitive to all
small, locally homogeneous topologies. Here, small means that the distance to
the surface of last scatter is smaller than the ``topology scale'' of the
universe.Comment: 14 pages, 10 figures, IOP format. This paper is a direct descendant
of gr-qc/9602039. To appear in a special proceedings issue of Class. Quant.
Grav. covering the Cleveland Topology & Cosmology Worksho
Reconstructing the global topology of the universe from the cosmic microwave background
If the universe is multiply-connected and sufficiently small, then the last
scattering surface wraps around the universe and intersects itself. Each circle
of intersection appears as two distinct circles on the microwave sky. The
present article shows how to use the matched circles to explicitly reconstruct
the global topology of space.Comment: 6 pages, 2 figures, IOP format. To be published in the proceedings of
the Cleveland Cosmology and Topology Workshop 17-19 Oct 1997. Submitted to
Class. Quant. Gra
Segue Between Favorable and Unfavorable Solvation
Solvation of small and large clusters are studied by simulation, considering
a range of solvent-solute attractive energy strengths. Over a wide range of
conditions, both for solvation in the Lennard-Jones liquid and in the SPC model
of water, it is shown that the mean solvent density varies linearly with
changes in solvent-solute adhesion or attractive energy strength. This behavior
is understood from the perspective of Weeks' theory of solvation [Ann. Rev.
Phys. Chem. 2002, 53, 533] and supports theories based upon that perspective.Comment: 8 pages, 7 figure
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