112 research outputs found
Information systems requirements for the microgravity science and applications program
NASA's Microgravity Science and Applications (MSAD) Program is presented. Additionally, the types of information produced within the program and the anticipated growth in information system requirements as the program transitions to Space Station Freedom utilization are discussed. Plans for payload operations support in the Freedom era are addressed, as well as current activities to define research community requirements for data and sample archives
Monte Carlo Simulation of Long Chain Polymer Melts: Crossover from Rouse to Reptation Dynamics
We present data of Monte Carlo simulations for monodisperse linear polymer
chains in dense melts with degrees of polymerization between N=16 and N=512.
The aim of this study is to investigate the crossover from Rouse-like dynamics
for short chains to reptation-like dynamics for long chains. To address this
problem we calculate a variety of different quantities: standard mean-square
displacements of inner monomers and of the chain's center of mass, the recently
proposed cubic invariant, persistence of bond-vector orientation with time, and
the auto-correlation functions of the bond vector, the end-to-end vector and
the Rouse modes. This analysis reveals that the crossover from non- to
entangled dynamics is very protracted. Only the largest chain length N=512,
which is about 13 times larger than the entanglement length, shows evidence for
reptation.Comment: 38 pages of REVTeX, 14 PostScript figure
Comparison of Dissipative Particle Dynamics and Langevin thermostats for out-of-equilibrium simulations of polymeric systems
In this work we compare and characterize the behavior of Langevin and
Dissipative Particle Dynamics (DPD) thermostats in a broad range of
non-equilibrium simulations of polymeric systems. Polymer brushes in relative
sliding motion, polymeric liquids in Poiseuille and Couette flows, and
brush-melt interfaces are used as model systems to analyze the efficiency and
limitations of different Langevin and DPD thermostat implementations. Widely
used coarse-grained bead-spring models under good and poor solvent conditions
are employed to assess the effects of the thermostats. We considered
equilibrium, transient, and steady state examples for testing the ability of
the thermostats to maintain constant temperature and to reproduce the
underlying physical phenomena in non-equilibrium situations. The common
practice of switching-off the Langevin thermostat in the flow direction is also
critically revisited. The efficiency of different weight functions for the DPD
thermostat is quantitatively analyzed as a function of the solvent quality and
the non-equilibrium situation.Comment: 12 pages, introduction improved, references added, to appear in Phys.
Rev.
Static and dynamic properties of the interface between a polymer brush and a melt of identical chains
Molecular dynamics simulations of a short-chain polymer melt between two
brush-covered surfaces under shear have been performed. The end-grafted
polymers which constitute the brush have the same chemical properties as the
free chains in the melt and provide a soft deformable substrate. Polymer chains
are described by a coarse-grained bead-spring model with Lennard-Jones
interactions between the beads and a FENE potential between nearest neighbors
along the backbone of the chains. The grafting density of the brush layer
offers a way of controlling the behavior of the surface without altering the
molecular interactions. We perform equilibrium and non-equilibrium Molecular
Dynamics simulations at constant temperature and volume using the Dissipative
Particle Dynamics thermostat. The equilibrium density profiles and the behavior
under shear are studied as well as the interdigitation of the melt into the
brush, the orientation on different length scales (bond vectors, radius of
gyration, and end-to-end vector) of free and grafted chains, and velocity
profiles. The viscosity and slippage at the interface are calculated as
functions of grafting density and shear velocity.Comment: 12 pages, submitted to J Chem Phy
Nonlinear effects in charge stabilized colloidal suspensions
Molecular Dynamics simulations are used to study the effective interactions
in charged stabilized colloidal suspensions. For not too high macroion charges
and sufficiently large screening, the concept of the potential of mean force is
known to work well. In the present work, we focus on highly charged macroions
in the limit of low salt concentrations. Within this regime, nonlinear
corrections to the celebrated DLVO theory [B. Derjaguin and L. Landau, Acta
Physicochem. USSR {\bf 14}, 633 (1941); E.J.W. Verwey and J.T.G. Overbeck, {\em
Theory of the Stability of Lyotropic Colloids} (Elsevier, Amsterdam, 1948)]
have to be considered. For non--bulklike systems, such as isolated pairs or
triples of macroions, we show, that nonlinear effects can become relevant,
which cannot be described by the charge renormalization concept [S. Alexander
et al., J. Chem. Phys. {\bf 80}, 5776 (1984)]. For an isolated pair of
macroions, we find an almost perfect qualitative agreement between our
simulation data and the primitive model. However, on a quantitative level,
neither Debye-H\"uckel theory nor the charge renormalization concept can be
confirmed in detail. This seems mainly to be related to the fact, that for
small ion concentrations, microionic layers can strongly overlap, whereas,
simultaneously, excluded volume effects are less important. In the case of
isolated triples, where we compare between coaxial and triangular geometries,
we find attractive corrections to pairwise additivity in the limit of small
macroion separations and salt concentrations. These triplet interactions arise
if all three microionic layers around the macroions exhibit a significant
overlap. In contrast to the case of two isolated colloids, the charge
distribution around a macroion in a triple is found to be anisotropic.Comment: 10 pages, 9 figure
Goodness-of-fit testing for left-truncated two-parameter weibull distributions with known truncation point
The left-truncated Weibull distribution is used in life-time analysis, it has many applications ranging from financial market analysis and insurance claims to the earthquake inter-arrival times. We present a comprehensive analysis of the left-truncated Weibull distribution when the shape, scale or both parameters are unknown and they are determined from the data using the maximum likelihood estimator. We demonstrate that if both the Weibull parameters are unknown then there are sets of sample configurations, with measure greater than zero, for which the maximum likelihood equations do not possess non trivial solutions. The modified critical values of the goodness-of-fit test from the Kolmogorov-Smirnov test statistic when the parameters are unknown are obtained from a quantile analysis. We find that the critical values depend on sample size and truncation level, but not on the actual Weibull parameters. Confirming this behavior, we present a complementary analysis using the Brownian bridge approach as an asymptotic limit of the Kolmogorov-Smirnov statistics and find that both approaches are in good agreement. A power testing is performed for our Kolmogorov-Smirnov goodness-of-fit test and the issues related to the left-truncated data are discussed. We conclude the paper by showing the importance of left-truncated Weibull distribution hypothesis testing on the duration times of failed marriages in the US, worldwide terrorist attacks, waiting times between stock market orders, and time intervals of radioactive decay.Ayşe Kızılersü, Markus Kreer, Anthony W. Thoma
Self-similar solutions with fat tails for Smoluchowski's coagulation equation with locally bounded kernels
The existence of self-similar solutions with fat tails for Smoluchowski's
coagulation equation has so far only been established for the solvable and the
diagonal kernel. In this paper we prove the existence of such self-similar
solutions for continuous kernels that are homogeneous of degree and satisfy . More precisely,
for any we establish the existence of a continuous weak
self-similar profile with decay as
AR/VR technologies in management and education
The potential of some technological innovations for a long period of time remains either not fully explored or underestimated. The article gives a brief description of the AR and VR technologies, shows the possibilities of their use in various sectors and industries of the modern world economy. The paper provides an overview of the positive aspects and competitive advantages that AR and VR technologies can bring to companies engaged in the development and production of devices and instruments that allow using these technologies. This is a growth in sales of products and services, a reduction in operating costs, an increase in customer satisfaction, an improvement in the quality of product, services and service. The study also considers the limitations of using AR and VR technologies in educational environments. According to the authors, there are more industries and areas where AR and VR technologies can be applied.The paper analyses the volume of investments in the field of VR / AR, gives the forecasts of possible improvements of VR and AR devices, leading to creating of wider opportunities for their utilization in industrial, financial and educational spheres. The article presents the most successful examples of the implementation of AR and VR technologies in 2012-2021 in foreign and Russian companies. For statistical confirmation of the obtained results the authors performed a correlation analysis of the forecast estimates of the development of AR and VR technologies in 2021–2023. The paper makes the forecast of sales of technologies and devices in some sectors of the economy by 2024. Based on the analysis, the study concludes that there is a significant relationship between the electronics market as a whole and the VR/AR market
Self-Diffusion of a Polymer Chain in a Melt
Self-diffusion of a polymer chain in a melt is studied by Monte Carlo
simulations of the bond fluctuation model, where only the excluded volume
interaction is taken into account. Polymer chains, each of which consists of
segments, are located on an simple cubic lattice
under periodic boundary conditions, where each segment occupies unit cells. The results for
and 512 at the volume fraction are reported, where
for and L=192 for . The -dependence of the
self-diffusion constant is examined. Here, is estimated from the mean
square displacements of the center of mass of a single polymer chain at the
times larger than the longest relaxation time. From the data for , 384
and 512, the apparent exponent , which describes the apparent power
law dependence of on as , is estimated as
. The ratio seems to be a
constant for and 512, where and
denote the longest relaxation time and the mean square end-to-end distance,
respectively.Comment: 4 pages, 3 figures, submitted to J. Phys. Soc. Jp
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