340 research outputs found
Activated sampling in complex materials at finite temperature: the properly-obeying-probability activation-relaxation technique
While the dynamics of many complex systems is dominated by activated events,
there are very few simulation methods that take advantage of this fact. Most of
these procedures are restricted to relatively simple systems or, as with the
activation-relaxation technique (ART), sample the conformation space
efficiently at the cost of a correct thermodynamical description. We present
here an extension of ART, the properly-obeying-probability ART (POP-ART), that
obeys detailed balance and samples correctly the thermodynamic ensemble.
Testing POP-ART on two model systems, a vacancy and an interstitial in
crystalline silicon, we show that this method recovers the proper
thermodynamical weights associated with the various accessible states and is
significantly faster than MD in the diffusion of a vacancy below 700 K.Comment: 10 pages, 3 figure
Probabilistic Phase Space Trajectory Description for Anomalous Polymer Dynamics
It has been recently shown that the phase space trajectories for the
anomalous dynamics of a tagged monomer of a polymer --- for single polymeric
systems such as phantom Rouse, self-avoiding Rouse, Zimm, reptation, and
translocation through a narrow pore in a membrane; as well as for
many-polymeric system such as polymer melts in the entangled regime --- is
robustly described by the Generalized Langevin Equation (GLE). Here I show that
the probability distribution of phase space trajectories for all these
classical anomalous dynamics for single polymers is that of a fractional
Brownian motion (fBm), while the dynamics for polymer melts between the
entangled regime and the eventual diffusive regime exhibits small, but
systematic deviations from that of a fBm.Comment: 8 pages, two figures, 3 eps figure files, minor changes,
supplementary material included moved to the appendix, references expanded,
to appear in J. Phys.: Condens. Matte
The role of body image disturbance in the onset, maintenance, and relapse of anorexia nervosa:A systematic review
Body image disturbance is an important feature of Anorexia Nervosa (AN). Some researchers have argued that body image disturbance is not just a symptom of AN, but plays a causal role in the development, persistence, and relapse of AN. Our aim was to systematically review the existing empirical evidence concerning the role of the cognitive-affective, perceptual, and behavioral components of body image disturbance in AN. 46 studies fulfilled eligibility criteria reporting about 4928 participants with AN. There is some evidence suggesting that body image disturbance is related to the course of AN. However, experimental studies were missing and operationalizations of body image constructs and AN outcome measures varied greatly across studies. Therefore, on the basis of the available empirical data, it remained unclear whether body image disturbance is indeed a causal risk factor for AN. For future studies, it is crucial to use more consistent terminology and more specific and precise definitions of body image constructs as well as experimental designs, adequately powered samples, and well-validated measures. Altogether, this would set the stage to generate the high-quality data that are necessary to clarify the role of body image disturbance in the onset, maintenance and relapse of AN
Gender Differences in Body Evaluation: Do Men Show More Self-Serving Double Standards Than Women?
Generally speaking, compared to women, men are less dissatisfied with their own body and consider themselves to be better-looking and less overweight. So far, however, it is unclear whether these divergent body ratings arise from the application of double standards. With the present study, we examined whether men apply different standards to their own body than to other men’s bodies and whether they differ from women in this regard. To this aim, we presented n = 104 women and n = 93 men with pictures of thin, average-weight, overweight, athletic and hypermuscular male and female bodies on a computer screen. To manipulate identification, we showed the bodies of the respective participant’s gender once with the participant’s own face and once with the face of another person. Identity cues, such as faces, might activate different body schemata, which influence body ratings and thus lead to the application of double standards. Participants were instructed to rate their emotional reaction to the bodies according to valence and arousal, and to rate the bodies with respect to attractiveness, body fat, and muscle mass. The application of double standards was determined by calculating the difference between the rating of a body presented with the participant’s face and the rating of the same body presented with another person’s face. Both women and men showed self-deprecating double standards in valence, body attractiveness, body fat and muscle mass for the overweight body. Men also revealed self-deprecating double standards for the thin, average-weight and hypermuscular bodies, but evaluated the athletic body as more attractive and with a higher positive feeling when it was presented with their own face. Women did not show any self-serving double standards and showed fewer self-deprecating double standards than men. The results indicate that men devalue non-ideal bodies and upvalue ideal bodies when they are self-related, whereas women more rate in a fair-minded manner. Thus, in contrast to women, an advantage for men may be that they are able to self-enhance in the case of desirable bodies. This ability to self-enhance regarding desirable features might be beneficial for men’s self-worth and body satisfaction
Oblique propagation of arbitrary amplitude electron acoustic solitary waves in magnetized kappa-distributed plasmas
The linear and nonlinear properties of large amplitude electron-acoustic
waves are investigated in a magnetized plasma comprising two distinct electron
populations (hot and cold) and immobile ions. The hot electrons are assumed to
be in a non-Maxwellian state, characterized by an excess of superthermal
particles, here modelled by a kappa-type long-tailed distribution function.
Waves are assumed to propagate obliquely to the ambient magnetic field. Two
types of electrostatic modes are shown to exist in the linear regime, and their
properties are briefly analyzed. A nonlinear pseudopotential type analysis
reveals the existence of large amplitude electrostatic solitary waves and
allows for an investigation of their propagation characteristics and existence
domain, in terms of the soliton speed (Mach number). The effects of the key
plasma configuration parameters, namely, the superthermality index and the cold
electron density, on the soliton characteristics and existence domain, are
studied. The role of obliqueness and magnetic field are discussed.Comment: Submitted to Plasma Physics and Controlled Fusio
Oxidative Activity of Human Polymorphonuclear Leukocytes Stimulated by the Long-Chain Phosphatidic Acids
Summary It has already been suggested that phosphatidic acids (PAs) play an important role in the regulation of signaling pathways involved in the production of reactive oxygen species (ROS) by human polymorphonuclear leukocytes (PMNs). The present study was performed to elucidate the effects of extracellularly added PAs -1,2-distearoyl-(DSPA) and 1-stearoyl-2-arachidonoyl-sn-glycero-phosphate (SAPA) -on the ROS production and on the elastase release by human PMNs. ROS production was monitored by luminol-amplified chemiluminescence and the elastase activity was measured in the supernatant of the PA-stimulated human PMNs by colorimetric assay. Obtained effects were compared with those of cells stimulated by either a chemotactic tripeptide, phorbol ester or calcium ionophore. Our results show that long-chain PAs at concentrations higher than 3 × 10 -5 mol/l stimulate the ROS production by human PMNs, whereas they were ineffective in promoting the elastase release. The chemiluminescence pattern of the SAPA-stimulated cells exhibited a biphasic curve, whereas cell stimulation with DSPA resulted in a monophasic chemiluminescence curve. Stimulation of the ROS production by PAs in dependence of the fatty acid composition required the activity of protein kinases
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Interpretation of Radio Wave Scintillation Observed through LOFAR Radio Telescopes
Radio waves propagating through a medium containing irregularities in the spatial distribution of the electron density develop fluctuations in their intensities and phases. In the case of radio waves emitted from astronomical objects, they propagate through electron density irregularities in the interstellar medium, the interplanetary medium, and Earth’s ionosphere. The LOFAR radio telescope, with stations across Europe, can measure intensity across the VHF radio band and thus intensity scintillation on the signals received from compact astronomical objects. Modeling intensity scintillation allows the estimate of various parameters of the propagation medium, for example, its drift velocity and its turbulent power spectrum. However, these estimates are based on the assumptions of ergodicity of the observed intensity fluctuations and, typically, of weak scattering. A case study of single-station LOFAR observations of the strong astronomical source Cassiopeia A in the VHF range is utilized to illustrate deviations from ergodicity, as well as the presence of both weak and strong scattering. Here it is demonstrated how these aspects can lead to misleading estimates of the propagation medium properties, for example, in the solar wind. This analysis provides a method to model errors in these estimates, which can be used in the characterization of both the interplanetary medium and Earth’s ionosphere. Although the discussion is limited to the case of the interplanetary medium and Earth’s ionosphere, its ideas are also applicable to the case of the interstellar medium
Amplitude and Frequency Spectrum of Thermal Fluctuations of A Translocating RNA Molecule
Using a combination of theory and computer simulations, we study the
translocation of an RNA molecule, pulled through a solid-state nanopore by an
optical tweezer, as a method to determine its secondary structure. The
resolution with which the elements of the secondary structure can be determined
is limited by thermal fluctuations. We present a detailed study of these
thermal fluctuations, including the frequency spectrum, and show that these
rule out single-nucleotide resolution under the experimental conditions which
we simulated. Two possible ways to improve this resolution are strong
stretching of the RNA with a back-pulling voltage across the membrane, and
stiffening of the translocated part of the RNA by biochemical means.Comment: Significantly expanded compared to previous version, 13 pages, 4
figures, to appear in J. Phys.: Condens. Matte
LOFAR observations of the quiet solar corona
The quiet solar corona emits meter-wave thermal bremsstrahlung. Coronal radio
emission can only propagate above that radius, , where the local
plasma frequency eqals the observing frequency. The radio interferometer LOw
Frequency ARray (LOFAR) observes in its low band (10 -- 90 MHz) solar radio
emission originating from the middle and upper corona. We present the first
solar aperture synthesis imaging observations in the low band of LOFAR in 12
frequencies each separated by 5 MHz. From each of these radio maps we infer
, and a scale height temperature, . These results can be combined
into coronal density and temperature profiles. We derived radial intensity
profiles from the radio images. We focus on polar directions with simpler,
radial magnetic field structure. Intensity profiles were modeled by ray-tracing
simulations, following wave paths through the refractive solar corona, and
including free-free emission and absorption. We fitted model profiles to
observations with and as fitting parameters. In the low corona,
solar radii, we find high scale height temperatures up to
2.2e6 K, much more than the brightness temperatures usually found there. But if
all values are combined into a density profile, this profile can be
fitted by a hydrostatic model with the same temperature, thereby confirming
this with two independent methods. The density profile deviates from the
hydrostatic model above 1.5 solar radii, indicating the transition into the
solar wind. These results demonstrate what information can be gleaned from
solar low-frequency radio images. The scale height temperatures we find are not
only higher than brightness temperatures, but also than temperatures derived
from coronograph or EUV data. Future observations will provide continuous
frequency coverage, eliminating the need for local hydrostatic density models
Pore-blockade Times for Field-Driven Polymer Translocation
We study pore blockade times for a translocating polymer of length ,
driven by a field across the pore in three dimensions. The polymer performs
Rouse dynamics, i.e., we consider polymer dynamics in the absence of
hydrodynamical interactions. We find that the typical time the pore remains
blocked during a translocation event scales as ,
where is the Flory exponent for the polymer. In line with our
previous work, we show that this scaling behaviour stems from the polymer
dynamics at the immediate vicinity of the pore -- in particular, the memory
effects in the polymer chain tension imbalance across the pore. This result,
along with the numerical results by several other groups, violates the lower
bound suggested earlier in the literature. We discuss why
this lower bound is incorrect and show, based on conservation of energy, that
the correct lower bound for the pore-blockade time for field-driven
translocation is given by , where is the viscosity of
the medium surrounding the polymer.Comment: 14 pages, 6 figures, slightly shorter than the previous version; to
appear in J. Phys.: Cond. Ma
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