797 research outputs found
Particle linear theory on a self-gravitating perturbed cubic Bravais lattice
Discreteness effects are a source of uncontrolled systematic errors of N-body
simulations, which are used to compute the evolution of a self-gravitating
fluid. We have already developed the so-called "Particle Linear Theory" (PLT),
which describes the evolution of the position of self-gravitating particles
located on a perturbed simple cubic lattice. It is the discrete analogue of the
well-known (Lagrangian) linear theory of a self-gravitating fluid. Comparing
both theories permits to quantify precisely discreteness effects in the linear
regime. It is useful to develop the PLT also for other perturbed lattices
because they represent different discretizations of the same continuous system.
In this paper we detail how to implement the PLT for perturbed cubic Bravais
lattices (simple, body and face-centered) in a cubic simulation box. As an
application, we will study the discreteness effects -- in the linear regime --
of N-body simulations for which initial conditions have been set-up using these
different lattices.Comment: 9 pages, 4 figures and 4 tables. Minor corrections to match published
versio
A fast high-order method to calculate wakefield forces in an electron beam
In this paper we report on a high-order fast method to numerically calculate
wakefield forces in an electron beam given a wake function model. This method
is based on a Newton-Cotes quadrature rule for integral approximation and an
FFT method for discrete summation that results in an computational
cost, where is the number of grid points. Using the Simpson quadrature rule
with an accuracy of , where is the grid size, we present numerical
calculation of the wakefields from a resonator wake function model and from a
one-dimensional coherent synchrotron radiation (CSR) wake model. Besides the
fast speed and high numerical accuracy, the calculation using the direct line
density instead of the first derivative of the line density avoids numerical
filtering of the electron density function for computing the CSR wakefield
force
Contact area of rough spheres: Large scale simulations and simple scaling laws
We use molecular simulations to study the nonadhesive and adhesive
atomic-scale contact of rough spheres with radii ranging from nanometers to
micrometers over more than ten orders of magnitude in applied normal load. At
the lowest loads, the interfacial mechanics is governed by the contact
mechanics of the first asperity that touches. The dependence of contact area on
normal force becomes linear at intermediate loads and crosses over to Hertzian
at the largest loads. By combining theories for the limiting cases of nominally
flat rough surfaces and smooth spheres, we provide parameter-free analytical
expressions for contact area over the whole range of loads. Our results
establish a range of validity for common approximations that neglect curvature
or roughness in modeling objects on scales from atomic force microscope tips to
ball bearings.Comment: 2 figures + Supporting Materia
Forces between functionalized silica nanoparticles in solution
To prevent the flocculation and phase separation of nanoparticles in
solution, nanoparticles are often functionalized with short chain surfactants.
Here we present fully-atomistic molecular dynamics simulations which
characterize how these functional coatings affect the interactions between
nanoparticles and with the surrounding solvent. For 5 nm diameter silica
nanoparticles coated with poly(ethylene oxide) (PEO) oligomers in water, we
determined the hydrodynamic drag on two approaching nanoparticles moving
through solvent and on a single nanoparticle as it approaches a planar surface.
In most circumstances, acroscale fluid theory accurately predicts the drag on
these nano-scale particles. Good agreement is seen with Brenner's analytical
solutions for wall separations larger than the soft nanoparticle radius. For
two approaching coated nanoparticles, the solvent-mediated
(velocity-independent) and lubrication (velocity-dependent) forces are purely
repulsive and do not exhibit force oscillations that are typical of uncoated
rigid spheres.Comment: 4 pages, 3 fig
Protocol for <em>in vitro</em> co-culture, proliferation, and cell cycle analyses of patient-derived leukemia cells
\ua9 2024 The Authors. Leukemia niche impacts quiescence; however, culturing patient-derived samples ex vivo is technically challenging. Here, we present a protocol for in vitro co-culture of patient-derived xenograft acute lymphoblastic leukemia (PDX-ALL) cells with human mesenchymal stem cells (MSCs). We describe steps for labeling PDX-ALL cells with CellTrace Violet dye to demonstrate MSC-primed PDX-ALL cycling. We then detail procedures to identify MSC-primed G0/quiescent PDX-ALL cells via Hoechst-33342/Pyronin Y live cell cycle analysis. For complete details on the use and execution of this protocol, please refer to Pal et al
Potential flows in a core-dipole-shell system: numerical results
Numerical solutions for: the integral curves of the velocity field
(streamlines), the density contours, and the accretion rate of a steady-state
flow of an ideal fluid with p=K n^(gamma) equation of state orbiting in a
core-dipole-shell system are presented. For 1 < gamma < 2, we found that the
non-linear contribution appearing in the partial differential equation for the
velocity potential has little effect in the form of the streamlines and density
contour lines, but can be noticed in the density values. The study of several
cases indicates that this appears to be the general situation. The accretion
rate was found to increase when the constant gamma decreases.Comment: RevTex, 8 pages, 5 eps figures, CQG to appea
Signaling, Entanglement, and Quantum Evolution Beyond Cauchy Horizons
Consider a bipartite entangled system half of which falls through the event
horizon of an evaporating black hole, while the other half remains coherently
accessible to experiments in the exterior region. Beyond complete evaporation,
the evolution of the quantum state past the Cauchy horizon cannot remain
unitary, raising the questions: How can this evolution be described as a
quantum map, and how is causality preserved? What are the possible effects of
such nonstandard quantum evolution maps on the behavior of the entangled
laboratory partner? More generally, the laws of quantum evolution under extreme
conditions in remote regions (not just in evaporating black-hole interiors, but
possibly near other naked singularities and regions of extreme spacetime
structure) remain untested by observation, and might conceivably be non-unitary
or even nonlinear, raising the same questions about the evolution of entangled
states. The answers to these questions are subtle, and are linked in unexpected
ways to the fundamental laws of quantum mechanics. We show that terrestrial
experiments can be designed to probe and constrain exactly how the laws of
quantum evolution might be altered, either by black-hole evaporation, or by
other extreme processes in remote regions possibly governed by unknown physics.Comment: Combined, revised, and expanded version of quant-ph/0312160 and
hep-th/0402060; 13 pages, RevTeX, 2 eps figure
Towards photostatistics from photon-number discriminating detectors
We study the properties of a photodetector that has a number-resolving
capability. In the absence of dark counts, due to its finite quantum
efficiency, photodetection with such a detector can only eliminate the
possibility that the incident field corresponds to a number of photons less
than the detected photon number. We show that such a {\em non-photon}
number-discriminating detector, however, provides a useful tool in the
reconstruction of the photon number distribution of the incident field even in
the presence of dark counts.Comment: 7 pages, 4 figure
Coulomb Interactions via Local Dynamics: A Molecular--Dynamics Algorithm
We derive and describe in detail a recently proposed method for obtaining
Coulomb interactions as the potential of mean force between charges which are
dynamically coupled to a local electromagnetic field. We focus on the Molecular
Dynamics version of the method and show that it is intimately related to the
Car--Parrinello approach, while being equivalent to solving Maxwell's equations
with freely adjustable speed of light. Unphysical self--energies arise as a
result of the lattice interpolation of charges, and are corrected by a
subtraction scheme based on the exact lattice Green's function. The method can
be straightforwardly parallelized using standard domain decomposition. Some
preliminary benchmark results are presented.Comment: 8 figure
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