18,720 research outputs found
Effect of dipolar moments in domain sizes of lipid bilayers and monolayers
Lipid domains are found in systems such as multi-component bilayer membranes
and single component monolayers at the air-water interface. It was shown by
Andelman et al. (Comptes Rendus 301, 675 (1985)) and McConnell et al. (Phys.
Chem. {\bf 91}, 6417 (1987)) that in monolayers, the size of the domains
results from balancing the line tension, which favors the formation of a large
single circular domain, against the electrostatic cost of assembling the
dipolar moments of the lipids. In this paper, we present an exact analytical
expression for the electric potential, ion distribution and electrostatic free
energy for different problems consisting of three different slabs with
different dielectric constants and Debye lengths, with a circular homogeneous
dipolar density in the middle slab. From these solutions, we extend the
calculation of domain sizes for monolayers to include the effects of finite
ionic strength, dielectric discontinuities (or image charges) and the
polarizability of the dipoles and further generalize the calculations to
account for domains in lipid bilayers. In monolayers, the size of the domains
is dependent on the different dielectric constants but independent of ionic
strength. In asymmetric bilayers, where the inner and outer leaflets have
different dipolar densities, domains show a strong size dependence with ionic
strength, with molecular-sized domains that grow to macroscopic phase
separation with increasing ionic strength. We discuss the implications of the
results for experiments and briefly consider their relation to other two
dimensional systems such as Wigner crystals or heteroepitaxial growth.Comment: 13 pages, 5 figues in eps Replaced with new version, one citation
added and a few statements corrected. The results of the paper are unchange
Design guidelines for assessing and controlling spacecraft charging effects
The need for uniform criteria, or guidelines, to be used in all phases of spacecraft design is discussed. Guidelines were developed for the control of absolute and differential charging of spacecraft surfaces by the lower energy space charged particle environment. Interior charging due to higher energy particles is not considered. A guide to good design practices for assessing and controlling charging effects is presented. Uniform design practices for all space vehicles are outlined
Multiwavelength observations of the Be/X-ray binary 4U1145-619
We report optical and infrared observations of the massive X-ray binary
system 4U1145-619 (V801 Cen) which show that the circumstellar disc of the Be
star component is in decline. Infrared J,H,K,L magnitudes of V801Cen have been
monitored from 1993 March to 1996 April. H alpha spectra have been obtained
throughout the same period. We find that both the infrared excess and the
Balmer emission have been in decline throughout the period of observations. A
13 year optical and X-ray history of the source has been collated, revealing a
possible correlation between the optical and X-ray activity. In addition, we
have used u,v,b,y,beta indices, corrected for both circumstellar and
interstellar effects, to calculate the physical parameters of the underlying B
star.Comment: 8 pages postscript. Accepted by MNRA
Investigation of latent heat effects at the stratocumulus top using direct numerical simulations
Variations of tropical lapse rates in climate models and their implications for upper tropospheric warming
The vertical temperature structure in the tropics is primarily set by convection and therefore follows a moist adiabat to first order. However, tropical upper tropospheric temperatures differ among climate models and observations, as atmospheric convection remains poorly understood. Here, we quantify the variations in tropical lapse rates in CMIP6 models and explore reasons for these variations. We find that differences in surface temperatures weighted by the regions of strongest convection cannot explain these variations and therefore we hypothesise that the representation of convection itself and associated small scale processes are responsible. We reproduce these variations in perturbed physics experiments with the global atmospheric model ICON-A, in which we vary autoconversion and entrainment parameters. For smaller autoconversion values, additional freezing enthalpy from the cloud water that is not precipitated warms the upper troposphere. Smaller entrainment rates also lead to a warmer upper troposphere, as convection and thus latent heating reaches higher. Furthermore, we show that according to most radiosonde datasets all CMIP6 AMIP simulations overestimate recent upper tropospheric warming. Additionally, all radiosonde datasets agree that climate models on average overestimate the amount of upper tropospheric warming for a given lower tropospheric warming. We demonstrate that increased entrainment rates reduce this overestimation, likely because of the reduction of latent heat release in the upper troposphere. Our results suggest that imperfect convection parameterisations are responsible for a considerable part of the variations in tropical lapse rates and also part of the overestimation of warming compared to the observation
Multi-objective evolutionary–fuzzy augmented flight control for an F16 aircraft
In this article, the multi-objective design of a fuzzy logic augmented flight controller for a high performance fighter jet (the Lockheed-Martin F16) is described. A fuzzy logic controller is designed and its membership functions tuned by genetic algorithms in order to design a roll, pitch, and yaw flight controller with enhanced manoeuverability which still retains safety critical operation when combined with a standard inner-loop stabilizing controller. The controller is assessed in terms of pilot effort and thus reduction of pilot fatigue. The controller is incorporated into a six degree of freedom motion base real-time flight simulator, and flight tested by a qualified pilot instructor
Phase Coexistence of a Stockmayer Fluid in an Applied Field
We examine two aspects of Stockmayer fluids which consists of point dipoles
that additionally interact via an attractive Lennard-Jones potential. We
perform Monte Carlo simulations to examine the effect of an applied field on
the liquid-gas phase coexistence and show that a magnetic fluid phase does
exist in the absence of an applied field. As part of the search for the
magnetic fluid phase, we perform Gibbs ensemble simulations to determine phase
coexistence curves at large dipole moments, . The critical temperature is
found to depend linearly on for intermediate values of beyond the
initial nonlinear behavior near and less than the where no
liquid-gas phase coexistence has been found. For phase coexistence in an
applied field, the critical temperatures as a function of the applied field for
two different are mapped onto a single curve. The critical densities
hardly change as a function of applied field. We also verify that in an applied
field the liquid droplets within the two phase coexistence region become
elongated in the direction of the field.Comment: 23 pages, ReVTeX, 7 figure
Borehole climatology: a discussion based on contributions from climate modeling
Progress in understanding climate variability through the last millennium leans on simulation and reconstruction efforts. Exercises blending both approaches present a great potential for answering questions relevant both for the simulation and reconstruction of past climate, and depend on the specific peculiarities of proxies and methods involved in climate reconstructions, as well as on the realism and limitations of model simulations. This paper explores research specifically related to paleoclimate modeling and borehole climatology as a branch of climate reconstruction that has contributed significantly to our knowledge of the low frequency climate evolution during the last five centuries. The text flows around three main issues that group most of the interaction between model and geothermal efforts: the use of models as a validation tool for borehole climate reconstructions; comparison of geothermal information and model simulations as a means of either model validation or inference about past climate; and implications of the degree of realism on simulating subsurface climate on estimations of future climate change. The use of multi-centennial simulations as a surrogate reality for past climate suggests that within the simplified reality of climate models, methods and assumptions in borehole reconstructions deliver a consistent picture of past climate evolution at long time scales. Comparison of model simulations and borehole profiles indicate that borehole temperatures are responding to past external forcing and that more realism in the development of the soil model components in climate models is desirable. Such an improved degree of realism is important for the simulation of subsurface climate and air-ground interaction; results indicate it could also be crucial for simulating the adequate energy balance within climate change scenario experiments
Magnetization of ferrofluids with dipolar interactions - a Born--Mayer expansion
For ferrofluids that are described by a system of hard spheres interacting
via dipolar forces we evaluate the magnetization as a function of the internal
magnetic field with a Born--Mayer technique and an expansion in the dipolar
coupling strength. Two different approximations are presented for the
magnetization considering different contributions to a series expansion in
terms of the volume fraction of the particles and the dipolar coupling
strength.Comment: 19 pages, 11 figures submitted to PR
Ferromagnetic Liquid Thin Films Under Applied Field
Theoretical calculations, computer simulations and experiments indicate the
possible existence of a ferromagnetic liquid state, although definitive
experimental evidence is lacking. Should such a state exist, demagnetization
effects would force a nontrivial magnetization texture. Since liquid droplets
are deformable, the droplet shape is coupled with the magnetization texture. In
a thin-film geometry in zero applied field, the droplet has a circular shape
and a rotating magnetization texture with a point vortex at the center. We
calculate the elongation and magnetization texture of such ferromagnetic thin
film liquid droplet confined between two parallel plates under a weak applied
magnetic field. The vortex stretches into a domain wall and exchange forces
break the reflection symmetry. This behavior contrasts qualitatively and
quantitatively with the elongation of paramagnetic thin films.Comment: 10 pages, 4 figures, Submitted to Phys. Rev.
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