361 research outputs found
Symmetric Diblock Copolymers in Thin Films (I): Phase stability in Self-Consistent Field Calculations and Monte Carlo Simulations
We investigate the phase behavior of symmetric AB diblock copolymers confined
into a thin film. The film boundaries are parallel, impenetrable and attract
the A component of the diblock copolymer. Using a self-consistent field
technique [M.W. Matsen, J.Chem.Phys. {\bf 106}, 7781 (1997)], we study the
ordered phases as a function of incompatibility and film thickness in
the framework of the Gaussian chain model. For large film thickness and small
incompatibility, we find first order transitions between phases with different
number of lamellae which are parallel oriented to the film boundaries. At high
incompatibility or small film thickness, transitions between parallel oriented
and perpendicular oriented lamellae occur. We compare the self-consistent field
calculations to Monte Carlo simulations of the bond fluctuation model for chain
length N=32. In the simulations we quench several systems from to
and monitor the morphology into which the diblock copolymers
assemble. Three film thicknesses are investigated, corresponding to parallel
oriented lamellae with 2 and 4 interfaces and a perpendicular oriented
morphology. Good agreement between self-consistent field calculations and Monte
Carlo simulations is found.Comment: to appear in J.Chem.Phy
First principles calculation of vibrational Raman spectra in large systems: signature of small rings in crystalline SiO2
We present an approach for the efficient calculation of vibrational Raman
intensities in periodic systems within density functional theory. The Raman
intensities are computed from the second order derivative of the electronic
density matrix with respect to a uniform electric field. In contrast to
previous approaches, the computational effort required by our method for the
evaluation of the intensities is negligible compared to that required for the
calculation of vibrational frequencies. As a first application, we study the
signature of 3- and 4-membered rings in the the Raman spectra of several
polymorphs of SiO2, including a zeolite having 102 atoms per unit cell.Comment: 4 pages, 2 figures, revtex4 Minor corrections; accepted in Phys. Rev.
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Phase diagram for diblock copolymer melts under cylindrical confinement
We extensively study the phase diagram of a diblock copolymer melt confined
in a cylindrical nanopore using real-space self-consistent mean-field theory.
We discover a rich variety of new two-dimensional equilibrium structures that
have no analog in the unconfined system. These include non-hexagonally
coordinated cylinder phases and structures intermediate between lamellae and
cylinders. We map the stability regions and phase boundaries for all the
structures we find. As the pore radius is decreased, the pore accommodates
fewer cylindrical domains and structural transitions occur as cylinders are
eliminated. Our results are consistent with experiments, but we also predict
phases yet to be observed.Comment: 12 pages, 3 figures. submitted to Physical Review Letter
Advanced development for space robotics with emphasis on fault tolerance
This paper describes the ongoing work in fault tolerance at the University of Texas at Austin. The paper describes the technical goals the group is striving to achieve and includes a brief description of the individual projects focusing on fault tolerance. The ultimate goal is to develop and test technology applicable to all future missions of NASA (lunar base, Mars exploration, planetary surveillance, space station, etc.)
Stable Carbon Isotope Signature of Methane Released From Phytoplankton
Unidad de excelencia MarÃa de Maeztu CEX2019-000940-MAquatic ecosystems play an important role in global methane cycling and many field studies have reported methane supersaturation in the oxic surface mixed layer (SML) of the ocean and in the epilimnion of lakes. The origin of methane formed under oxic condition is hotly debated and several pathways have recently been offered to explain the "methane paradox." In this context, stable isotope measurements have been applied to constrain methane sources in supersaturated oxygenated waters. Here we present stable carbon isotope signatures for six widespread marine phytoplankton species, three haptophyte algae and three cyanobacteria, incubated under laboratory conditions. The observed isotopic patterns implicate that methane formed by phytoplankton might be clearly distinguished from methane produced by methanogenic archaea. Comparing results from phytoplankton experiments with isotopic data from field measurements, suggests that algal and cyanobacterial populations may contribute substantially to methane formationobserved in the SML of oceans and lakes
Large well-relaxed models of vitreous silica, coordination numbers and entropy
A Monte Carlo method is presented for the simulation of vitreous silica.
Well-relaxed networks of vitreous silica are generated containing up to 300,000
atoms. The resulting networks, quenched under the BKS potential, display
smaller bond-angle variations and lower defect concentrations, as compared to
networks generated with molecular dynamics. The total correlation functions
T(r) of our networks are in excellent agreement with neutron scattering data,
provided that thermal effects and the maximum inverse wavelength used in the
experiment are included in the comparison. A procedure commonly used in
experiments to obtain coordination numbers from scattering data is to fit peaks
in rT(r) with a gaussian. We show that this procedure can easily produce
incorrect results. Finally, we estimate the configurational entropy of vitreous
silica.Comment: 7 pages, 4 figures (two column version to save paper
A symmetric polymer blend confined into a film with antisymmetric surfaces: interplay between wetting behavior and phase diagram
We study the phase behavior of a symmetric binary polymer blend which is
confined into a thin film. The film surfaces interact with the monomers via
short range potentials. We calculate the phase behavior within the
self-consistent field theory of Gaussian chains. Over a wide range of
parameters we find strong first order wetting transitions for the semi-infinite
system, and the interplay between the wetting/prewetting behavior and the phase
diagram in confined geometry is investigated. Antisymmetric boundaries, where
one surface attracts the A component with the same strength than the opposite
surface attracts the B component, are applied. The phase transition does not
occur close to the bulk critical temperature but in the vicinity of the wetting
transition. For very thin films or weak surface fields one finds a single
critical point at . For thicker films or stronger surface fields
the phase diagram exhibits two critical points and two concomitant coexistence
regions. Only below a triple point there is a single two phase coexistence
region. When we increase the film thickness the two coexistence regions become
the prewetting lines of the semi-infinite system, while the triple temperature
converges towards the wetting transition temperature from above. The behavior
close to the tricritical point, which separates phase diagrams with one and two
critical points, is studied in the framework of a Ginzburg-Landau ansatz.
Two-dimensional profiles of the interface between the laterally coexisting
phases are calculated, and the interfacial and line tensions analyzed. The
effect of fluctuations and corrections to the self-consistent field theory are
discussed.Comment: Phys.Rev.E in prin
Ultra-Fast and Optimized Method for the Preparation of Rodent Testicular Cells for Flow Cytometric Analysis
Homogeneity of cell populations is a prerequisite for the analysis of biochemical and molecular events during male gamete differentiation. Given the complex organization of the mammalian testicular tissue, various methods have been used to obtain enriched or purified cell populations, including flow cell sorting. Current protocols are usually time-consuming and may imply loss of short-lived RNAs, which is undesirable for expression profiling. We describe an optimized method to speed up the preparation of suitable testicular cell suspensions for cytometric analysis of different spermatogenic stages from rodents. The procedure takes only 15 min including testis dissection, tissue cutting, and processing through the Medimachine System (Becton Dickinson). This method could be a substitute for the more tedious and time-consuming cell preparation techniques currently in use
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