4,779 research outputs found
Constraints on Dimensional Warped Spaces
In order to investigate the phenomenological implications of allowing gauge
fields to propagate in warped spaces of more than five dimensions, we consider
a toy model of a space warped by the presence of a anisotropic bulk
cosmological constant. After solving the Einstein equation, three classes of
solutions are found, those in which the additional () dimensions are
growing, shrinking or remaining constant. It is found that gauge fields
propagating in these spaces have a significantly different Kaluza Klein (KK)
mass spectrum and couplings from that of the Randall and Sundrum model. This
leads to a greatly reduced lower bound on the KK scale, arising from
electroweak constraints, for spaces growing towards the IR brane.Comment: 6 pages, 5 figures PASCOS2010 International Symposium proceedin
Structure, phase behavior and inhomogeneous fluid properties of binary dendrimer mixtures
The effective pair potentials between different kinds of dendrimers in
solution can be well approximated by appropriate Gaussian functions. We find
that in binary dendrimer mixtures the range and strength of the effective
interactions depend strongly upon the specific dendrimer architecture. We
consider two different types of dendrimer mixtures, employing the Gaussian
effective pair potentials, to determine the bulk fluid structure and phase
behavior. Using a simple mean field density functional theory (DFT) we find
good agreement between theory and simulation results for the bulk fluid
structure. Depending on the mixture, we find bulk fluid-fluid phase separation
(macro-phase separation) or micro-phase separation, i.e., a transition to a
state characterized by undamped periodic concentration fluctuations. We also
determine the inhomogeneous fluid structure for confinement in spherical
cavities. Again, we find good agreement between the DFT and simulation results.
For the dendrimer mixture exhibiting micro-phase separation, we observe rather
striking pattern formation under confinement.Comment: 8 pages, 10 figure
Mean-field dynamical density functional theory
We examine the out-of-equilibrium dynamical evolution of density profiles of
ultrasoft particles under time-varying external confining potentials in three
spatial dimensions. The theoretical formalism employed is the dynamical density
functional theory (DDFT) of Marini Bettolo Marconi and Tarazona [J. Chem. Phys.
{\bf 110}, 8032 (1999)], supplied by an equilibrium excess free energy
functional that is essentially exact. We complement our theoretical analysis by
carrying out extensive Brownian Dynamics simulations. We find excellent
agreement between theory and simulations for the whole time evolution of
density profiles, demonstrating thereby the validity of the DDFT when an
accurate equilibrium free energy functional is employed.Comment: 8 pagers, 4 figure
Two-dimensional colloidal fluids exhibiting pattern formation
Fluids with competing short range attraction and long range repulsive
interactions between the particles can exhibit a variety of microphase
separated structures. We develop a lattice-gas (generalised Ising) model and
analyse the phase diagram using Monte Carlo computer simulations and also with
density functional theory (DFT). The DFT predictions for the structures formed
are in good agreement with the results from the simulations, which occur in the
portion of the phase diagram where the theory predicts the uniform fluid to be
linearly unstable. However, the mean-field DFT does not correctly describe the
transitions between the different morphologies, which the simulations show to
be analogous to micelle formation. We determine how the heat capacity varies as
the model parameters are changed. There are peaks in the heat capacity at state
points where the morphology changes occur. We also map the lattice model onto a
continuum DFT that facilitates a simplification of the stability analysis of
the uniform fluid.Comment: 13 pages, 15 figure
Density functional approach for inhomogeneous star polymers
We propose microscopic density functional theory for inhomogeneous star
polymers. Our approach is based on fundamental measure theory for hard spheres,
and on Wertheim's first- and second-order perturbation theory for the
interparticle connectivity. For simplicity we consider a model in which all the
arms are of the same length, but our approach can be easily extended to the
case of stars with arms of arbitrary lengths.Comment: 4 pages, 3 figures, submitte
Thermodynamically consistent description of the hydrodynamics of free surfaces covered by insoluble surfactants of high concentration
In this paper we propose several models that describe the dynamics of liquid
films which are covered by a high concentration layer of insoluble surfactant.
First, we briefly review the 'classical' hydrodynamic form of the coupled
evolution equations for the film height and surfactant concentration that are
well established for small concentrations. Then we re-formulate the basic model
as a gradient dynamics based on an underlying free energy functional that
accounts for wettability and capillarity. Based on this re-formulation in the
framework of nonequilibrium thermodynamics, we propose extensions of the basic
hydrodynamic model that account for (i) nonlinear equations of state, (ii)
surfactant-dependent wettability, (iii) surfactant phase transitions, and (iv)
substrate-mediated condensation. In passing, we discuss important differences
to most of the models found in the literature.Comment: 31 pages, 2 figure
Investigations Using Laboratory Testbeds to Interpret Flight Instrument Datasets from Mars Robotic Missions
The Astromaterials Research and Exploration Science Directorate at the NASA Johnson Space Center (JSC) has laboratory instrumentation that mimic the capabilities of corresponding flight instruments to enable interpretation of datasets returned from Mars robotic missions. The lab instruments have been and continue to be applied to datasets for the Moessbauer Spectrometer (MB) on the Mars Exploration Rovers (MER), the Thermal & Evolved Gas Analyzer (TEGA) on the Mars Phoenix Scout, the CRISM instrument on the Mars Reconnaissance Orbiter Missions and will be applied to datasets for the Sample Analysis at Mars (SAM), Chemistry and Mineralogy (CheMin) and Chemistry & Camera (ChemCam) instruments onboard the Mars Science Laboratory (MSL). The laboratory instruments can analyze analog samples at costs that are substantially lower than engineering models of flight instruments, but their success to enable interpretation of flight data depends on how closely their capabilities mimic those of the flight instrument. The JSC lab MB instruments are equivalent to the MER instruments except without flight qualified components and no reference channel Co-57 source. Data from analog samples were critical for identification of Mg-Fe carbonate at Gusev crater. Fiber-optic VNIR spectrometers are used to obtain CRISM-like spectral data over the range 350-2500 nm, and data for Fephyllosilicates show irreversible behavior in the electronic transition region upon dessication. The MB and VNIR instruments can be operated within chambers where, for example, the absolute H2O concentration can be measured and controlled. Phoenix's TEGA consisted of a calorimeter coupled to a mass spectrometer (MS). The JSC laboratory testbed instrument consisted of a differential scanning calorimeter (DSC) coupled to a MS configured to operate under total pressure (12 mbar), heating rate (20 C/min), and purge gas composition (N2) analogous to the flight TEGA. TEGA detected CO2 release at both low (400-680 C) and high (725-820 C) temperature and an endothermic reaction in concert with the high temperature release. The high-temperature thermal decomposition is consistent with calcite, dolomite, or ankerite, (3-6 wt.%) or any combination of these phase based upon laboratory testbed experiments. Recent laboratory experiments suggest that the low temperature CO2 release was caused by a reaction between calcium carbonate and hydrated magnesium perchlorate; although, CO2 release by the oxidation of organic materials and Fe-/Mg-rich carbonates cannot be ruled out. MSL landed in Gale crater on August 5, 2012. Although numerous analog samples have been analyzed on the JSC laboratory testbeds, no SAM, CheMin, or ChemCam analyses have been acquired by MSL to date. The JSC SAM laboratory testbed consists of a thermal analyzer coupled with a MS configured to operate under total pressure (30 mbar), heating rate (35 C/min), and purge gas composition (He) analogous to the flight SAM. The CheMin and ChemCam laboratory testbeds were developed and built by inXitu, Inc. and Los Alamos National Laboratory, respectively, to acquire datasets relevant to the MSL CheMin and ChemCam flight instruments
Fluid-fluid demixing transitions in colloid--polyelectrolyte star mixtures
We derive effective interaction potentials between hard, spherical colloidal
particles and star-branched polyelectrolytes of various functionalities and
smaller size than the colloids. The effective interactions are based on a
Derjaguin-like approximation, which is based on previously derived potentials
acting between polyelectrolyte stars and planar walls. On the basis of these
interactions we subsequently calculate the demixing binodals of the binary
colloid--polyelectrolyte star mixture, employing standard tools from
liquid-state theory. We find that the mixture is indeed unstable at moderately
high overall concentrations. The system becomes more unstable with respect to
demixing as the star functionality and the size ratio grow.Comment: 24 pages, 9 figures, submitted to Journal of Physics: Condensed
Matte
The Asakura-Oosawa model in the protein limit: the role of many-body interactions
We study the Asakura-Oosawa model in the "protein limit", where the
penetrable sphere radius is much greater than the hard sphere radius
. The phase behaviour and structure calculated with a full many-body
treatment show important qualitative differences when compared to a description
based on pair potentials alone. The overall effect of the many-body
interactions is repulsive.Comment: 9 pages and 11 figures, submitted to J. Phys.: Condensed Matter,
special issue "Effective many-body interactions and correlations in soft
matter
Exotic fluids and crystals of soft polymeric colloids
We discuss recent developments and present new findings in the colloidal
description of soft polymeric macromolecular aggregates. For various
macromolecular architectures, such as linear chains, star polymers, dendrimers
and polyelectrolyte stars, the effective interactions between suitably chosen
coordinates are shown to be ultrasoft, i.e., they either remain finite or
diverge very slowly at zero separation. As a consequence, the fluid phases have
unusual characteristics, including anomalous pair correlations and mean-field
like thermodynamic behaviour. The solid phases can exhibit exotic, strongly
anisotropic as well as open crystal structures. For example, the diamond and
the A15-phase are shown to be stable at sufficiently high concentrations.
Reentrant melting and clustering transitions are additional features displayed
by such systems, resulting in phase diagrams with a very rich topology. We
emphasise that many of these effects are fundamentally different from the usual
archetypal hard sphere paradigm. Instead, we propose that these fluids fall
into the class of mean-field fluids.Comment: 22 pages, uses iopart.cls and iopart10.clo; submitted to Journal of
Physics Condensed Matter, special issue in honour of professor Peter Puse
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