747 research outputs found
Synthesis and characterisation of end-functionalised poly(N-vinylpyrrolidone) additives by reversible addition–fragmentation transfer polymerisation
We describe herein the synthesis of a series of multi-end functionalized poly(N-vinyl pyrrolidone) (PVP) additives bearing two or three C8F17 fluoroalkyl (CF) groups, designed as additives to modify surface properties. The PVP additives were prepared by reversible addition–fragmentation transfer (RAFT) polymerization, with end functionality imparted via the use of CF functionalized chain transfer agents (CTAs). The resulting PVP additives, when used in modest quantities dispersed in thin films of an unmodified PVP matrix significantly reduce the surface energy, rendering their surfaces more hydrophobic and lipophobic. This is achieved by virtue of the low surface energy of the pendant C8F17 end groups which cause the additive to spontaneously surface segregate during the spin coating process. The resulting thin films have been characterized by static contact angle measurements using dodecane as the contact fluid, and the impact of additive molecular weight, matrix molecular weight, the number of CF groups and additive concentration upon surface properties is reported herein. Significant increases in contact angle were observed with increasing additive concentration, up to a critical aggregation concentration (CAC). Increasing the number of CF groups (from 2 to 3); reducing additive molecular weight or increasing the matrix molecular weight, resulted in increased contact angles and hence surface lipophobicity. Rutherford backscattering (RBS) analysis was performed on films containing varying concentrations of additive, in order to quantitatively measure the near-surface fluorine concentration of these films. The results of these experiments were in excellent agreement with those obtained by contact angle analysis, confirming the surface activity and low surface energy of the additives
The Entropy of a Binary Hidden Markov Process
The entropy of a binary symmetric Hidden Markov Process is calculated as an
expansion in the noise parameter epsilon. We map the problem onto a
one-dimensional Ising model in a large field of random signs and calculate the
expansion coefficients up to second order in epsilon. Using a conjecture we
extend the calculation to 11th order and discuss the convergence of the
resulting series
Single-String Integration Test Measurements of the NEXT Ion Engine Plume
Measurements were made of a 40 cm ion-thruster plume as part of the single-string-integration-test (SSIT) activity of Phase I of the NASA's Evolutionary Xenon Thruster (NEXT) project. The NEXT ion engine incorporates design improvements that extend NSTAR power levels and efficiencies. During SSIT, an engineering model (EM2) 40 cm engine was operated using an advanced xenon propellant system in combination with either a GRC power console or advanced power processing unit. Integral goals of the single-string phase were to characterize engine performance over the full input power range and to detail thruster operation within the specification of the NEXT throttle table. Plume diagnostics measurements of relative Xe(+) and Xe(++) currents were made using near-field and far-field ExB probes. Planar geometry faraday probes were used to obtain beam current density profiles. This paper reports on the characterization of the EM2 plume over a range of SSIT operating conditions, first with the advanced propellant management system teamed with the GRC power console and then with the power-processing unit
Non-linear Response of the trap model in the aging regime : Exact results in the strong disorder limit
We study the dynamics of the one dimensional disordered trap model presenting
a broad distribution of trapping times , when an
external force is applied from the very beginning at , or only after a
waiting time , in the linear as well as in the non-linear response regime.
Using a real-space renormalization procedure that becomes exact in the limit of
strong disorder , we obtain explicit results for many observables,
such as the diffusion front, the mean position, the thermal width, the
localization parameters and the two-particle correlation function. In
particular, the scaling functions for these observables give access to the
complete interpolation between the unbiased case and the directed case.
Finally, we discuss in details the various regimes that exist for the averaged
position in terms of the two times and the external field.Comment: 27 pages, 1 eps figur
Measurement of Fluorescence Phenomena from Yttrium and Gadolinium Oxysulfide Phosphors using a 45-MeV Proton Beam
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Localization properties of the anomalous diffusion phase in the directed trap model and in the Sinai diffusion with bias
We study the anomalous diffusion phase with which
exists both in the Sinai diffusion at small bias, and in the related directed
trap model presenting a large distribution of trapping time . Our starting point is the Real Space Renormalization method in
which the whole thermal packet is considered to be in the same renormalized
valley at large time : this assumption is exact only in the limit
and corresponds to the Golosov localization. For finite , we thus
generalize the usual RSRG method to allow for the spreading of the thermal
packet over many renormalized valleys. Our construction allows to compute exact
series expansions in of all observables : at order , it is
sufficient to consider a spreading of the thermal packet onto at most
traps in each sample, and to average with the appropriate measure over the
samples. For the directed trap model, we show explicitly up to order
how to recover the diffusion front, the thermal width, and the localization
parameter . We moreover compute the localization parameters for
arbitrary
, the correlation function of two particles, and the generating function
of thermal cumulants. We then explain how these results apply to the Sinai
diffusion with bias, by deriving the quantitative mapping between the
large-scale renormalized descriptions of the two models.Comment: 33 pages, 3 eps figure
The fate of assimilated carbon during drought: impacts on respiration in Amazon rainforests
Interannual variations in CO2 exchange across Amazonia, as deduced from atmospheric inversions, correlate with El Niño occurrence. They are thought to result from changes in net ecosystem exchange and fire incidence that are both related to drought intensity. Alterations to net ecosystem production (NEP) are caused by changes in gross primary production (GPP) and ecosystem respiration (Reco). Here, we analyse observations of the components of Reco (leaves, live and dead woody tissue, and soil) to provide first estimates of changes in Reco during short-term (seasonal to interannual) moisture limitation. Although photosynthesis declines if moisture availability is limiting, leaf dark respiration is generally maintained, potentially acclimating upwards in the longer term. If leaf area is lost, then short-term canopy-scale respiratory effluxes from wood and leaves are likely to decline. Using a moderate short-term drying scenario where soil moisture limitation leads to a loss of 0.5 m2 m−2 yr−1 in leaf area index, we estimate a reduction in respiratory CO2 efflux from leaves and live woody tissue of 1.0 (±0.4) t C ha−1 yr−1. Necromass decomposition declines during drought, but mortality increases; the median mortality increase following a strong El Niño is 1.1% (n=46 tropical rainforest plots) and yields an estimated net short-term increase in necromass CO2 efflux of 0.13–0.18 t C ha−1 yr−1. Soil respiration is strongly sensitive to moisture limitation over the short term, but not to associated temperature increases. This effect is underestimated in many models but can lead to estimated reductions in CO2 efflux of 2.0 (±0.5) t C ha−1 yr−1. Thus, the majority of short-term respiratory responses to drought point to a decline in Reco, an outcome that contradicts recent regional-scale modelling of NEP. NEP varies with both GPP and Reco but robust moisture response functions are clearly needed to improve quantification of the role of Reco in influencing regional-scale CO2 emissions from Amazonia
Anomalous diffusion, Localization, Aging and Sub-aging effects in trap models at very low temperature
We study in details the dynamics of the one dimensional symmetric trap model,
via a real-space renormalization procedure which becomes exact in the limit of
zero temperature. In this limit, the diffusion front in each sample consists in
two delta peaks, which are completely out of equilibrium with each other. The
statistics of the positions and weights of these delta peaks over the samples
allows to obtain explicit results for all observables in the limit .
We first compute disorder averages of one-time observables, such as the
diffusion front, the thermal width, the localization parameters, the
two-particle correlation function, and the generating function of thermal
cumulants of the position. We then study aging and sub-aging effects : our
approach reproduces very simply the two different aging exponents and yields
explicit forms for scaling functions of the various two-time correlations. We
also extend the RSRG method to include systematic corrections to the previous
zero temperature procedure via a series expansion in . We then consider the
generalized trap model with parameter and obtain that the
large scale effective model at low temperature does not depend on in
any dimension, so that the only observables sensitive to are those
that measure the `local persistence', such as the probability to remain exactly
in the same trap during a time interval. Finally, we extend our approach at a
scaling level for the trap model in and obtain the two relevant time
scales for aging properties.Comment: 33 pages, 3 eps figure
Random walks and polymers in the presence of quenched disorder
After a general introduction to the field, we describe some recent results
concerning disorder effects on both `random walk models', where the random walk
is a dynamical process generated by local transition rules, and on `polymer
models', where each random walk trajectory representing the configuration of a
polymer chain is associated to a global Boltzmann weight. For random walk
models, we explain, on the specific examples of the Sinai model and of the trap
model, how disorder induces anomalous diffusion, aging behaviours and Golosov
localization, and how these properties can be understood via a strong disorder
renormalization approach. For polymer models, we discuss the critical
properties of various delocalization transitions involving random polymers. We
first summarize some recent progresses in the general theory of random critical
points : thermodynamic observables are not self-averaging at criticality
whenever disorder is relevant, and this lack of self-averaging is directly
related to the probability distribution of pseudo-critical temperatures
over the ensemble of samples of size . We describe the
results of this analysis for the bidimensional wetting and for the
Poland-Scheraga model of DNA denaturation.Comment: 17 pages, Conference Proceedings "Mathematics and Physics", I.H.E.S.,
France, November 200
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