25,694 research outputs found
Registered and antiregistered phase separation of mixed amphiphilic bilayers
We derive a mean-field free energy for the phase behaviour of coupled bilayer
leaflets, which is implicated in cellular processes and important to the design
of artificial membranes. Our model accounts for amphiphile-level structural
features, particularly hydrophobic mismatch, which promotes antiregistration
(AR), in competition with the `direct' trans-midplane coupling usually studied,
promoting registration (R). We show that the phase diagram of coupled leaflets
allows multiple \textit{metastable} coexistences, then illustrate the kinetic
implications with a detailed study of a bilayer of equimolar overall
composition. For approximate parameters estimated to apply to phospholipids,
equilibrium coexistence is typically registered, but metastable antiregistered
phases can be kinetically favoured by hydrophobic mismatch. Thus a bilayer in
the spinodal region can require nucleation to equilibrate, in a novel
manifestation of Ostwald's `rule of stages'. Our results provide a framework
for understanding disparate existing observations, elucidating a subtle
competition of couplings, and a key role for phase transition kinetics in
bilayer phase behaviour.Comment: Final authors' version. Important typo in Eq. A24 corrected. To
appear in Biophysical Journa
Ion molecule reactions in vinyl fluoride by photoionization. Effects of vibrational excitation on major reaction pathways
Photoionization methods have been used to study the reactions of the molecular ion in vinyl fluoride to yield the ionic products C_3H_3F_2^+, C_3H_4F^+, and C_3H_5^+. Quantitative measurements are reported of the effect of the vibrational state of the reactant ion on the product distribution and overall reaction cross section. Reaction cross sections for all three channels decrease with reactant internal energy. The effect on the reaction pathway producing C_3H_3F_2^+ is especially pronounced, with 0.18 eV of vibrational excitation being sufficient to reduce the reaction probability by 75%. Deactivation of vibrationally excited reactant ions competes with the reaction and is shown to be an efficient process
A Sunyaev-Zel'Dovich-Selected Sample of the Most Massive Galaxy Clusters in the 2500 deg^2 South Pole Telescope Survey
The South Pole Telescope (SPT) is currently surveying 2500 deg^2 of the southern sky to detect massive galaxy clusters out to the epoch of their formation using the Sunyaev-Zel'dovich (SZ) effect. This paper presents a catalog of the 26 most significant SZ cluster detections in the full survey region. The catalog includes 14 clusters which have been previously identified and 12 that are new discoveries. These clusters were identified in fields observed to two differing noise depths: 1500 deg^2 at the final SPT survey depth of 18 ÎźK arcmin at 150 GHz and 1000 deg^2 at a depth of 54 ÎźK arcmin. Clusters were selected on the basis of their SZ signal-to-noise ratio (S/N) in SPT maps, a quantity which has been demonstrated to correlate tightly with cluster mass. The S/N thresholds were chosen to achieve a comparable mass selection across survey fields of both depths. Cluster redshifts were obtained with optical and infrared imaging and spectroscopy from a variety of ground- and space-based facilities. The redshifts range from 0.098 ⤠z ⤠1.132 with a median of z_(med) = 0.40. The measured SZ S/N and redshifts lead to unbiased mass estimates ranging from 9.8 Ă 10^(14) M_â h^(â1)_(70) ⤠M _(200(Ďmean)) ⤠3.1 Ă 10^(15) M_â h^(â1)_(70). Based on the SZ mass estimates, we find that none of the clusters are individually in significant tension with the ÎCDM cosmological model. We also test for evidence of non-Gaussianity based on the cluster sample and find the data show no preference for non-Gaussian perturbations
The water-cryogen heat exchanger
Heat exchanger, using water as heat medium, converts liquid hydrogen to gaseous hydrogen at a very high rate. Possible applications include treatment of liquified natural gas in cities to bring the gas on-line quickly, conversion of liquid oxygen and liquid nitrogen for steel mills, and high volume inert purging
Shifts in hexapod diversification and what Haldane could have said
Data on species richness and taxon age are assembled for the extant hexapod orders (insects and their six-legged relatives). Coupled with estimates of phylogenetic relatedness, and simple statistical null models, these data are used to locate where, on the hexapod tree, significant changes in the rate of cladogenesis (speciation-minus-extinction rate) have occurred. Significant differences are found between many successive pairs of sister taxa near the base of the hexapod tree, all of which are attributable to a shift in diversification rate after the origin of the Neoptera (insects with wing flexion) and before the origin of the Holometabola (insects with complete metamorphosis). No other shifts are identifiable amongst supraordinal taxa. Whilst the Coleoptera have probably diversified faster than either of their putative sister lineages, they do not stand out relative to other closely related clades. These results suggest that any Creator had a fondness for a much more inclusive clade than the Coleoptera, definitely as large as the Eumetabola (Holometabola plus bugs and their relatives), and possibly as large as the entire Neoptera. Simultaneous, hence probable causative events are discussed, of which the origin of wing flexion has been the focus of much attention
Bored pile design in stiff clay II:Mechanisms and uncertainty
The soil mechanics related to pile design in clay has been the subject of substantial engineering research. In a companion paper, various codes of practice were reviewed showing the effect on pile capacity of the different global factors of safety that emerge from the various partial factor combinations for the ultimate limit state. Factors of safety are generally specified based on the opinions of experts. In this paper an assessment will be made of various objective procedures that can be used to reduce uncertainty in the design process, especially regarding the adoption of a pile resistance model and the selection of a soil strength profile as part of a ultimate limit state check, and the estimation of pile head settlement in the context of a serviceability limit state check. It is shown that both total stress and effective stress calculation methods are applicable in London Clay. Estimates of settlement using a non-linear soil stressâstrain relationship are made and compared with published data. It is shown that the compression of the concrete dominates the settlement of long piles. Given the low settlements observed, recommendations are made for a reduction in standard factors of safety for bored pile design in stiff clays. </jats:p
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