24,021 research outputs found
Phase Separation of Saturated and Mono-unsaturated Lipids as determined from a Microscopic Model
A molecular model is proposed of a bilayer consisting of fully saturated DPPC
and mono unsaturated DOPC. The model not only encompasses the constant density
within the hydrophobic core of the bilayer, but also the tendency of chain
segments to align. It is solved within self-consistent field theory. A model
bilayer of DPPC undergoes a main chain transition to a gel phase, while a
bilayer of DOPC does not do so above zero degrees centigrade because of the
double bond which disrupts order. We examine structural and thermodynamic
properties of these membranes and find our results in reasonable accord with
experiment. In particular, order-parameter profiles are in good agreement with
NMR experiments. A phase diagram is obtained for mixtures of these lipids in a
membrane at zero tension. The system undergoes phase separation below the
main-chain transition temperature of the saturated lipid. Extensions to the
ternary DPPC, DOPC, and cholesterol system are outlined.Comment: 29 pages, 4 figures, 1 table. revised versio
Moderate temperature rechargeable sodium batteries
Cells utilizing the organic electrolyte, NaI in triglyme, operated at approx. 130 C with Na(+) - intercalating cathodes. However, their rate and stability were inadequate. NaAlCl4 was found to be a highly useful electrolyte for cell operation at 165-190 C. Na(+) intercalating chalcogenides reacted with NaAlCl4 during cycling to form stable phases. Thus, VS2 became essentially VS2Cl, with reversible capacity of approx 2.8 e(-)/V, and a mid-discharge voltage of approx 2.5V and 100 deep discharge cycles were readily achieved. A positive electrode consisting of VCl3 and S plus NaAlCl4 was subjected to deep-discharge cycles 300 times and it demonstrated identity with the in-situ-formed BSxCly cathode. NiS2 and NiS which are not Na(+)-intercalating structures formed highly reversible electrodes in NaAlCl4. The indicated discharge mechanism implies a theoretical capacity 4e(-)/Ni for NiS2 and 2e(-)/Ni for NiS. The mid-discharge potentials are, respectively, 2.4V and 2.1V. A Na/NiS2 cell cycling at a C/5 rate has exceeded 500 deep discharge cycles with 2.5e(-)/Ni average utilization. A 4 A-hr nominal capacity prototype Na/NiS2 cell was tested at 190 C. It was voluntarily terminated after 80 cycles. Further development, particularly of cathode structure and hardware should produce a battery capable of at least 50-W-hr/lb and more than 1000 cycles
Sex Differences in Mechanisms of Recovery after Isometric and Dynamic Fatiguing Tasks
Purpose The purpose of this study was to determine whether supraspinal mechanisms contribute to the sex difference in fatigability during and recovery from a dynamic and isometric fatiguing task with the knee extensors.
Methods: Transcranial magnetic stimulation and electrical stimulation were used to determine voluntary activation and contractile properties of the knee extensors in 14 men and 17 women (20.8 ± 1.9 yr) after a 1) 60-s sustained, maximal voluntary isometric contraction (MVIC), and 2) dynamic fatiguing task involving 120 maximal voluntary concentric contractions with a 20% MVIC load.
Results: There were no differences between men and women in the reduction of maximal torque during the sustained MVIC (54.4% ± 18.9% vs 55.9% ± 11.2%, P = 0.49) or in the decrease in power during the dynamic fatiguing task (14.7% ± 20.1% vs 14.2% ± 18.5%, P = 0.92). However, MVIC torque recovered more quickly for women than men after the sustained MVIC and the dynamic task (P \u3c 0.05). The transcranial magnetic stimulation–elicited superimposed twitch was larger for men than for women during the sustained MVIC and in recovery (immediately post, R0.1: 4.7% ± 3.3% vs 2.4% ± 1.9% MVIC; P = 0.02), with no sex difference after the dynamic task (P = 0.35). The reduction in resting twitch amplitude was larger for men than for women immediately after the dynamic task (37% ± 22% vs 23% ± 18%; P = 0.016) with no sex difference after the sustained MVIC (64% ± 16% vs 67% ± 11%; P = 0.46).
Conclusions: Supraspinal fatigue contributed to fatigability of the knee extensors more for men than for women after a maximal isometric task, whereas contractile mechanisms explained the sex difference in torque recovery after the fast-velocity dynamic task. The mechanisms for the sex difference in fatigability are task dependent
Surface Partition of Large Fragments
The surface partition of large fragments is derived analytically within a
simple statistical model by the Laplace-Fourier transformation method. In the
limit of small amplitude deformations, a suggested Hills and Dales Model
reproduces the leading term of the famous Fisher result for the surface entropy
with an accuracy of a few percent. The surface partition of finite fragments is
discussed as well.Comment: 4 pages, 1 figur
Structural characteristics of positionally-disordered lattices: relation to the first sharp diffraction peak in glasses
Positional disorder has been introduced into the atomic structure of certain
crystalline lattices, and the orientationally-averaged structure factor S(k)
and pair-correlation function g(r) of these disordered lattices have been
studied. Analytical expressions for S(k) and g(r) for Gaussian positional
disorder in 2D and 3D are confirmed with precise numerical simulations. These
analytic results also have a bearing on the unsolved Gauss circle problem in
mathematics. As the positional disorder increases, high-k peaks in S(k) are
destroyed first, eventually leaving a single peak, that with the lowest-k
value. The pair-correlation function for lattices with such high levels of
positional disorder exhibits damped oscillations, with a period equal to the
separation between the furthest-separated (lowest-k) lattice planes. The last
surviving peak in S(k) is, for example for silicon and silica, at a wavevector
nearly identical to that of the experimentally-observed first sharp diffraction
peak (FSDP) in the amorphous phases of those materials. Thus, for these
amorphous materials at least, the FSDP can be regarded as arising from
scattering from atomic configurations equivalent to the single family of
positionally-disordered local Bragg planes having the furthest separation.Comment: v2: changes in response to referees' comments: Figure 2 made more
readable, improved discussion of height of peaks in S(k), other minor changes
4 pages, 3 figures, submitted to Physical Review
Compound nuclear decay and the liquid to vapor phase transition: a physical picture
Analyses of multifragmentation in terms of the Fisher droplet model (FDM) and
the associated construction of a nuclear phase diagram bring forth the problem
of the actual existence of the nuclear vapor phase and the meaning of its
associated pressure. We present here a physical picture of fragment production
from excited nuclei that solves this problem and establishes the relationship
between the FDM and the standard compound nucleus decay rate for rare particles
emitted in first-chance decay. The compound thermal emission picture is
formally equivalent to a FDM-like equilibrium description and avoids the
problem of the vapor while also explaining the observation of Boltzmann-like
distribution of emission times. In this picture a simple Fermi gas thermometric
relation is naturally justified and verified in the fragment yields and time
scales. Low energy compound nucleus fragment yields scale according to the FDM
and lead to an estimate of the infinite symmetric nuclear matter critical
temperature between 18 and 27 MeV depending on the choice of the surface energy
coefficient of nuclear matter.Comment: Five page two column pages, four figures, submitted to Phys. Rev.
Pulse shape analysis in segmented detectors as a technique for background reduction in Ge double-beta decay experiments
The need to understand and reject backgrounds in Ge-diode detector
double-beta decay experiments has given rise to the development of pulse shape
analysis in such detectors to discern single-site energy deposits from
multiple-site deposits. Here, we extend this analysis to segmented Ge detectors
to study the effectiveness of combining segmentation with pulse shape analysis
to identify the multiplicity of the energy deposits.Comment: 12 pages, 13 figures, will be submitted to NI
A major role for intestinal epithelial nucleotide oligomerization domain 1 (NOD1) in mediating host bactericidal activity against Campylobacter jejuni
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