23 research outputs found
Crossover Scaling of Wavelength Selection in Directional Solidification of Binary Alloys
We simulate dendritic growth in directional solidification in dilute binary
alloys using a phase-field model solved with an adaptive-mesh refinement. The
spacing of primary branches is examined for a range of thermal gradients and
alloy compositions and is found to undergo a maximum as a function of pulling
velocity, in agreement with experimental observations. We demonstrate that
wavelength selection is unambiguously described by a non-trivial crossover
scaling function from the emergence of cellular growth to the onset of
dendritic fingers, a result validated using published experimental data.Comment: 4 pages, four figures, submitted to Physical Review Letter
Points, Walls and Loops in Resonant Oscillatory Media
In an experiment of oscillatory media, domains and walls are formed under the
parametric resonance with a frequency double the natural one. In this bi-stable
system, %phase jumps by crossing walls. a nonequilibrium transition from
Ising wall to Bloch wall consistent with prediction is confirmed
experimentally. The Bloch wall moves in the direction determined by its
chirality with a constant speed. As a new type of moving structure in
two-dimension, a traveling loop consisting of two walls and Neel points is
observed.Comment: 9 pages (revtex format) and 6 figures (PostScript
Interruption of torus doubling bifurcation and genesis of strange nonchaotic attractors in a quasiperiodically forced map : Mechanisms and their characterizations
A simple quasiperiodically forced one-dimensional cubic map is shown to
exhibit very many types of routes to chaos via strange nonchaotic attractors
(SNAs) with reference to a two-parameter space. The routes include
transitions to chaos via SNAs from both one frequency torus and period doubled
torus. In the former case, we identify the fractalization and type I
intermittency routes. In the latter case, we point out that atleast four
distinct routes through which the truncation of torus doubling bifurcation and
the birth of SNAs take place in this model. In particular, the formation of
SNAs through Heagy-Hammel, fractalization and type--III intermittent mechanisms
are described. In addition, it has been found that in this system there are
some regions in the parameter space where a novel dynamics involving a sudden
expansion of the attractor which tames the growth of period-doubling
bifurcation takes place, giving birth to SNA. The SNAs created through
different mechanisms are characterized by the behaviour of the Lyapunov
exponents and their variance, by the estimation of phase sensitivity exponent
as well as through the distribution of finite-time Lyapunov exponents.Comment: 27 pages, RevTeX 4, 16 EPS figures. Phys. Rev. E (2001) to appea
Pattern formation in directional solidification under shear flow. I: Linear stability analysis and basic patterns
An asymptotic interface equation for directional solidification near the
absolute stabiliy limit is extended by a nonlocal term describing a shear flow
parallel to the interface. In the long-wave limit considered, the flow acts
destabilizing on a planar interface. Moreover, linear stability analysis
suggests that the morphology diagram is modified by the flow near the onset of
the Mullins-Sekerka instability. Via numerical analysis, the bifurcation
structure of the system is shown to change. Besides the known hexagonal cells,
structures consisting of stripes arise. Due to its symmetry-breaking
properties, the flow term induces a lateral drift of the whole pattern, once
the instability has become active. The drift velocity is measured numerically
and described analytically in the framework of a linear analysis. At large flow
strength, the linear description breaks down, which is accompanied by a
transition to flow-dominated morphologies, described in a companion paper.
Small and intermediate flows lead to increased order in the lattice structure
of the pattern, facilitating the elimination of defects. Locally oscillating
structures appear closer to the instability threshold with flow than without.Comment: 20 pages, Latex, accepted for Physical Review
Identification of a Novel Gene Product That Promotes Survival of Mycobacterium smegmatis in Macrophages
BACKGROUND: Bacteria of the suborder Corynebacterineae include significant human pathogens such as Mycobacterium tuberculosis and M. leprae. Drug resistance in mycobacteria is increasingly common making identification of new antimicrobials a priority. Mycobacteria replicate intracellularly, most commonly within the phagosomes of macrophages, and bacterial proteins essential for intracellular survival and persistence are particularly attractive targets for intervention with new generations of anti-mycobacterial drugs. METHODOLOGY/PRINCIPAL FINDINGS: We have identified a novel gene that, when inactivated, leads to accelerated death of M. smegmatis within a macrophage cell line in the first eight hours following infection. Complementation of the mutant with an intact copy of the gene restored survival to near wild type levels. Gene disruption did not affect growth compared to wild type M. smegmatis in axenic culture or in the presence of low pH or reactive oxygen intermediates, suggesting the growth defect is not related to increased susceptibility to these stresses. The disrupted gene, MSMEG_5817, is conserved in all mycobacteria for which genome sequence information is available, and designated Rv0807 in M. tuberculosis. Although homology searches suggest that MSMEG_5817 is similar to the serine:pyruvate aminotransferase of Brevibacterium linens suggesting a possible role in glyoxylate metabolism, enzymatic assays comparing activity in wild type and mutant strains demonstrated no differences in the capacity to metabolize glyoxylate. CONCLUSIONS/SIGNIFICANCE: MSMEG_5817 is a previously uncharacterized gene that facilitates intracellular survival of mycobacteria. Interference with the function of MSMEG_5817 may provide a novel therapeutic approach for control of mycobacterial pathogens by assisting the host immune system in clearance of persistent intracellular bacteria
Spiral instability to line sources in forced chemical pattern turbulence
A stably rotating spiral wave in the Belousov-Zhabotinsky reaction studied in
an open spatial reactor becomes unstable when forced by a periodic chemical
oscillation. The instability starts from the spiral center and occurs when the
ratio of the spiral-rotation period to that of the forcing is close to 3/2. The
resulting disordered state involves drifting line sources that emit waves in
both directions. These unusual objects are reminiscent of the travelling hole
solutions to the one-dimensional complex Ginzburg-Landau equation
Elastic properties of a cellular dissipative structure
Transition towards spatio-temporal chaos in one-dimensional interfacial patterns often involves two degrees of freedom: drift and out-of-phase oscillations of cells, respectively associated to parity breaking and vacillating-breathing secondary bifurcations. In this paper, the interaction between these two modes is investigated in the case of a single domain propagating along a circular array of liquid jets. As observed by Michalland and Rabaud for the printer’s instability [1], the velocity V g of a constant width domain is linked to the angular frequency of oscillations and to the spacing between columns by the relationship . We show by a simple geometrical argument that should be close to instead of the initial value deduced from their analogy with phonons. This fact is in quantitative agreement with our data, with a slight deviation increasing with flow rate. Copyright Springer-Verlag Berlin/Heidelberg 2003
Parity breaking in a one-dimensional pattern: A quantitative study with controlled wavelength
We have quantitatively investigated the parity-breaking bifurcation in
a liquid column pattern with periodic boundary conditions formed below
an overflowing dish. For high enough viscosity homogeneous global
drifting states can be stabilized within a large range of
wavelengths. Data are analyzed in terms of coupled phase and amplitude
equations and show that the bifurcation to a global drifting state is
supercritical, the flow rate and phase gradient behaving as effective control
parameters. The wavelength dependence shows that new non-linearities
must be added to the usual model. We also identify at
large flow rate a regime of spatio-temporal chaos that mixes
oscillations, drifts, coalescences and nucleations of columns