12,620 research outputs found
Stiff Polymers, Foams and Fiber Networks
We study the elasticity of fibrous materials composed of generalized stiff
polymers. It is shown that in contrast to cellular foam-like structures affine
strain fields are generically unstable. Instead, a subtle interplay between the
architecture of the network and the elastic properties of its building blocks
leads to intriguing mechanical properties with intermediate asymptotic scaling
regimes. We present exhaustive numerical studies based on a finite element
method complemented by scaling arguments.Comment: 4 pages, 5 figure
Entropic forces generated by grafted semiflexible polymers
The entropic force exerted by the Brownian fluctuations of a grafted
semiflexible polymer upon a rigid smooth wall are calculated both analytically
and by Monte Carlo simulations. Such forces are thought to play an important
role for several cellular phenomena, in particular, the physics of
actin-polymerization-driven cell motility and movement of bacteria like
Listeria. In the stiff limit, where the persistence length of the polymer is
larger than its contour length, we find that the entropic force shows scaling
behavior. We identify the characteristic length scales and the explicit form of
the scaling functions. In certain asymptotic regimes we give simple analytical
expressions which describe the full results to a very high numerical accuracy.
Depending on the constraints imposed on the transverse fluctuations of the
filament there are characteristic differences in the functional form of the
entropic forces; in a two-dimensional geometry the entropic force exhibits a
marked peak.Comment: 21 pages, 18 figures, minor misprints correcte
Manual for 70 mm hand-held photography from Skylab
A manual and atlas used on the Skylab mission for hand-held photography are presented. The manual covers terrain, environmental, meteorological, and dim light photography while the atlas covers sections from the Army Map Service 1:40.000,000 world map, a glossary of geologic terms, geologic maps, and recommended exposure times
Crossover from Isotropic to Directed Percolation
Percolation clusters are probably the simplest example for scale--invariant
structures which either are governed by isotropic scaling--laws
(``self--similarity'') or --- as in the case of directed percolation --- may
display anisotropic scaling behavior (``self--affinity''). Taking advantage of
the fact that both isotropic and directed bond percolation (with one preferred
direction) may be mapped onto corresponding variants of (Reggeon) field theory,
we discuss the crossover between self--similar and self--affine scaling. This
has been a long--standing and yet unsolved problem because it is accompanied by
different upper critical dimensions: for isotropic, and
for directed percolation, respectively. Using a generalized
subtraction scheme we show that this crossover may nevertheless be treated
consistently within the framework of renormalization group theory. We identify
the corresponding crossover exponent, and calculate effective exponents for
different length scales and the pair correlation function to one--loop order.
Thus we are able to predict at which characteristic anisotropy scale the
crossover should occur. The results are subject to direct tests by both
computer simulations and experiment. We emphasize the broad range of
applicability of the proposed method.Comment: 19 pages, written in RevTeX, 12 figures available upon request (from
[email protected] or [email protected]), EF/UCT--94/2, to be
published in Phys. Rev. E (May 1994
Coexistence versus extinction in the stochastic cyclic Lotka-Volterra model
Cyclic dominance of species has been identified as a potential mechanism to
maintain biodiversity, see e.g. B. Kerr, M. A. Riley, M. W. Feldman and B. J.
M. Bohannan [Nature {\bf 418}, 171 (2002)] and B. Kirkup and M. A. Riley
[Nature {\bf 428}, 412 (2004)]. Through analytical methods supported by
numerical simulations, we address this issue by studying the properties of a
paradigmatic non-spatial three-species stochastic system, namely the
`rock-paper-scissors' or cyclic Lotka-Volterra model. While the deterministic
approach (rate equations) predicts the coexistence of the species resulting in
regular (yet neutrally stable) oscillations of the population densities, we
demonstrate that fluctuations arising in the system with a \emph{finite number
of agents} drastically alter this picture and are responsible for extinction:
After long enough time, two of the three species die out. As main findings we
provide analytic estimates and numerical computation of the extinction
probability at a given time. We also discuss the implications of our results
for a broad class of competing population systems.Comment: 12 pages, 9 figures, minor correction
An anionic phosphenium complex as an ambident nucleophile
A unique anionic phosphenium complex was prepared from reaction of an N-heterocyclic chlorophosphine with Collman's reagent or K[HFe(CO)(4)]/NaH and characterized by spectral and XRD data. The complex behaves as an ambident nucleophile. Reactions with acetic acid, ClSnPh3, and a further equivalent of an N-heterocyclic chlorophosphine proceed via electrophilic functionalization at the metal site to yield appropriate mono- or bis-phosphenium complexes. Reaction with MeI at -70 degrees C produces a P-alkylation product as the first spectroscopically detectable intermediate, which decays at a higher temperature to give a mixture of free P-methylated N-heterocyclic phosphine and its Fe(CO)(4) complex. The different reaction products were characterized by spectral and XRD data. Computational studies indicate that the NHP units in all complexes display p-acceptor behaviour but show no disposition to adopt phosphide-like character or formally oxidize the metal centre.Peer reviewe
Machine Translation for Accessible Multi-Language Text Analysis
English is the international standard of social research, but scholars are
increasingly conscious of their responsibility to meet the need for scholarly
insight into communication processes globally. This tension is as true in
computational methods as any other area, with revolutionary advances in the
tools for English language texts leaving most other languages far behind. In
this paper, we aim to leverage those very advances to demonstrate that
multi-language analysis is currently accessible to all computational scholars.
We show that English-trained measures computed after translation to English
have adequate-to-excellent accuracy compared to source-language measures
computed on original texts. We show this for three major analytics -- sentiment
analysis, topic analysis, and word embeddings -- over 16 languages, including
Spanish, Chinese, Hindi, and Arabic. We validate this claim by comparing
predictions on original language tweets and their backtranslations: double
translations from their source language to English and back to the source
language. Overall, our results suggest that Google Translate, a simple and
widely accessible tool, is effective in preserving semantic content across
languages and methods. Modern machine translation can thus help computational
scholars make more inclusive and general claims about human communication.Comment: 5000 words, 6 figure
Noise and Correlations in a Spatial Population Model with Cyclic Competition
Noise and spatial degrees of freedom characterize most ecosystems. Some
aspects of their influence on the coevolution of populations with cyclic
interspecies competition have been demonstrated in recent experiments [e.g. B.
Kerr et al., Nature {\bf 418}, 171 (2002)]. To reach a better theoretical
understanding of these phenomena, we consider a paradigmatic spatial model
where three species exhibit cyclic dominance. Using an individual-based
description, as well as stochastic partial differential and deterministic
reaction-diffusion equations, we account for stochastic fluctuations and
spatial diffusion at different levels, and show how fascinating patterns of
entangled spirals emerge. We rationalize our analysis by computing the
spatio-temporal correlation functions and provide analytical expressions for
the front velocity and the wavelength of the propagating spiral waves.Comment: 4 pages of main text, 3 color figures + 2 pages of supplementary
material (EPAPS Document). Final version for Physical Review Letter
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