250 research outputs found
Lubricating Bacteria Model for Branching growth of Bacterial Colonies
Various bacterial strains (e.g. strains belonging to the genera Bacillus,
Paenibacillus, Serratia and Salmonella) exhibit colonial branching patterns
during growth on poor semi-solid substrates. These patterns reflect the
bacterial cooperative self-organization. Central part of the cooperation is the
collective formation of lubricant on top of the agar which enables the bacteria
to swim. Hence it provides the colony means to advance towards the food. One
method of modeling the colonial development is via coupled reaction-diffusion
equations which describe the time evolution of the bacterial density and the
concentrations of the relevant chemical fields. This idea has been pursued by a
number of groups. Here we present an additional model which specifically
includes an evolution equation for the lubricant excreted by the bacteria. We
show that when the diffusion of the fluid is governed by nonlinear diffusion
coefficient branching patterns evolves. We study the effect of the rates of
emission and decomposition of the lubricant fluid on the observed patterns. The
results are compared with experimental observations. We also include fields of
chemotactic agents and food chemotaxis and conclude that these features are
needed in order to explain the observations.Comment: 1 latex file, 16 jpeg files, submitted to Phys. Rev.
Global distributed evolution of L-systems fractals
Internet based parallel genetic programming (GP) creates
fractal patterns like Koch’s snow flake.
Pfeiffer, http://www.cs.ucl.ac.uk/staff/W.Langdon/pfeiffer.html,
by analogy with seed/embryo development, uses Lindenmayer grammars
and LOGO style turtle graphics written in Javascript and Perl. 298 novel
pictures were produced. Images are placed in animated snow globes (computerised
snowstorms) by www web browsers anywhere on the planet.
We discuss artificial life (Alife) evolving autonomous agents and virtual
creatures in higher dimensions from a free format representation in the
context of neutral networks, gene duplication and the evolution of higher
order genetic operators
Surface morphology and structural types of natural impact apographitic diamonds
External and internal morphologies of natural impact apographitic diamonds (paramorphoses) have been studied.Вивчено зовнішню і внутрішню морфологію природного імпактного апографітового алмазу – параморфоз.Изучена внешняя и внутреняя морфология природного импактного апографитового алмаза – параморфоз
Planktic foraminifera form their shells via metastable carbonate phases
The calcium carbonate shells of planktic foraminifera provide our most valuable geochemical archive of ocean surface conditions and climate spanning the last 100 million years, and play an important role in the ocean carbon cycle. These shells are preserved in marine sediments as calcite, the stable polymorph of calcium carbonate. Here, we show that shells of living planktic foraminifers Orbulina universa and Neogloboquadrina dutertrei originally form from the unstable calcium carbonate polymorph vaterite, implying a non-classical crystallisation pathway involving metastable phases that transform ultimately to calcite. The current understanding of how planktic foraminifer shells record climate, and how they will fare in a future high-CO 2 world is underpinned by analogy to the precipitation and dissolution of inorganic calcite. Our findings require a re-evaluation of this paradigm to consider the formation and transformation of metastable phases, which could exert an influence on the geochemistry and solubility of the biomineral calcite
Angular Conditions,Relations between Breit and Light-Front Frames, and Subleading Power Corrections
We analyze the current matrix elements in the general collinear (Breit)
frames and find the relation between the ordinary (or canonical) helicity
amplitudes and the light-front helicity amplitudes. Using the conservation of
angular momentum, we derive a general angular condition which should be
satisfied by the light-front helicity amplitudes for any spin system. In
addition, we obtain the light-front parity and time-reversal relations for the
light-front helicity amplitudes. Applying these relations to the spin-1 form
factor analysis, we note that the general angular condition relating the five
helicity amplitudes is reduced to the usual angular condition relating the four
helicity amplitudes due to the light-front time-reversal condition. We make
some comments on the consequences of the angular condition for the analysis of
the high- deuteron electromagnetic form factors, and we further apply the
general angular condition to the electromagnetic transition between spin-1/2
and spin-3/2 systems and find a relation useful for the analysis of the
N- transition form factors. We also discuss the scaling law and the
subleading power corrections in the Breit and light-front frames.Comment: 24 pages,2 figure
The magmatic to hydrothermal evolution of the intrusive Mont Saint-Hilaire Complex: Insights into the late-stage evolution of peralkaline rocks
The Cretaceous Mont Saint-Hilaire complex (Quebec, Canada) comprises
three major rock units that were emplaced in the following sequence: (I)
gabbros; (II) diorites; (III) diverse partly agpaitic foid syenites. The
major element compositions of the rock-forming minerals, age-corrected
Nd and oxygen isotope data for mineral separates and trace element data
of Fe-Mg silicates from the various lithologies imply a common source
for all units. The distribution of the rare earth elements in
clinopyroxene from the gabbros indicates an ocean island basalt type
composition for the parental magma. Gabbros record temperatures of 1200
to 800 degrees C, variable silica activities between 0 center dot 7 and
0 center dot 3, and f(O2) values between -0 center dot 5 and +0 center
dot 7 (log delta FMQ, where FMQ is fayalite-magnetite-quartz). The
diorites crystallized under uniform a(SiO2) (a(SiO2) = 0 center dot 4-0
center dot 5) and more reduced f(O2) conditions (log delta FMQ similar
to-1) between similar to 1100 and similar to 800 degrees C. Phase
equilibria in various foid syenites indicate that silica activities
decrease from 0 center dot 6-0 center dot 3 at similar to 1000 degrees C
to < 0 center dot 3 at similar to 550 degrees C. Release of an aqueous
fluid during the transition to the hydrothermal stage caused a(SiO2) to
drop to very low values, which results from reduced SiO(2) solubilities
in aqueous fluids compared with silicate melts. During the hydrothermal
stage, high water activities stabilized zeolite-group minerals. Fluid
inclusions record a complex post-magmatic history, which includes
trapping of an aqueous fluid that unmixed from the restitic foid
syenitic magma. Cogenetic aqueous and carbonic fluid inclusions reflect
heterogeneous trapping of coexisting immiscible external fluids in the
latest evolutionary stage. The O and C isotope characteristics of
fluid-inclusion hosted CO(2) and late-stage carbonates imply that the
surrounding limestones were the source of the external fluids. The
mineral-rich syenitic rocks at Mont Saint-Hilaire evolved as follows:
first, alkalis, high field strength and large ion lithophile elements
were pre-enriched in the (late) magmatic and subsequent hydrothermal
stages; second, percolation of external fluids in equilibrium with the
carbonate host-rocks and mixing processes with internal fluids as well
as fluid-rock interaction governed dissolution of pre-existing minerals,
element transport and precipitation of mineral assemblages determined by
locally variable parameters. It is this hydrothermal interplay between
internal and external fluids that is responsible for the mineral wealth
found at Mont Saint-Hilaire
Modeling the observed tropospheric BrO background: Importance of multiphase chemistry and implications for ozone, OH, and mercury
Aircraft and satellite observations indicate the presence of ppt (ppt ≡ pmol/mol) levels of BrO in the free troposphere with important implications for the tropospheric budgets of ozone, OH, and mercury. We can reproduce these observations with the GEOS-Chem global tropospheric chemistry model by including a broader consideration of multiphase halogen (Br–Cl) chemistry than has been done in the past. Important reactions for regenerating BrO from its non-radical reservoirs include HOBr+Br−/Cl− in both aerosols and clouds, and oxidation of Br− by ClNO3 and ozone. Most tropospheric BrO in the model is in the free troposphere, consistent with observations, and originates mainly from the photolysis and oxidation of ocean-emitted CHBr3. Stratospheric input is also important in the upper troposphere. Including production of gas phase inorganic bromine from debromination of acidified sea salt aerosol increases free tropospheric Bry by about 30 %. We find HOBr to be the dominant gas-phase reservoir of inorganic bromine. Halogen (Br-Cl) radical chemistry as implemented here in GEOS-Chem drives 14 % and 11 % decreases in the global burdens of tropospheric ozone and OH, respectively, a 16 % increase in the atmospheric lifetime of methane, and an atmospheric lifetime of 6 months for elemental mercury. The dominant mechanism for the Br-Cl driven tropospheric ozone decrease is oxidation of NOx by formation and hydrolysis of BrNO3 and ClNO3
Engineering the Photonic Density of States with metamaterials
The photonic density of states (PDOS), like its' electronic coun- terpart, is
one of the key physical quantities governing a variety of phenom- ena and hence
PDOS manipulation is the route to new photonic devices. The PDOS is
conventionally altered by exploiting the resonance within a device such as a
microcavity or a bandgap structure like a photonic crystal. Here we show that
nanostructured metamaterials with hyperbolic dispersion can dramatically
enhance the photonic density of states paving the way for metamaterial based
PDOS engineering
Vector meson production and nucleon resonance analysis in a coupled-channel approach for energies m_N < sqrt(s) < 2 GeV I: pion-induced results and hadronic parameters
We present a nucleon resonance analysis by simultaneously considering all
pion- and photon-induced experimental data on the final states gamma N, pi N, 2
pi N, eta N, K Lambda, K Sigma, and omega N for energies from the nucleon mass
up to sqrt(s) = 2 GeV. In this analysis we find strong evidence for the
resonances P_{31}(1750), P_{13}(1900), P_{33}(1920), and D_{13}(1950). The
omega N production mechanism is dominated by large P_{11}(1710) and
P_{13}(1900) contributions. In this first part, we present the results of the
pion-induced reactions and the extracted resonance and background properties
with emphasis on the difference between global and purely hadronic fits.Comment: 54 pages, 26 figures, discussion extended, typos corrected,
references updated, to appear in Phys. Rev.
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