Geometric diagnostics of complex patterns: Spiral defect chaos

Motivated by the observation of spiral patterns in a wide range of physical, chemical, and biological systems, we present an automated approach that aims at characterizing quantitatively spiral-like elements in complex stripelike patterns. The approach provides the location of the spiral tip and the size of the spiral arms in terms of their arc length and their winding number. In addition, it yields the number of pattern components (Betti number of order 1), as well as their size and certain aspects of their shape. We apply the method to spiral defect chaos in thermally driven Rayleigh- Bénard convection and find that the arc length of spirals decreases monotonically with decreasing Prandtl number of the fluid and increasing heating. By contrast, the winding number of the spirals is nonmonotonic in the heating. The distribution function for the number of spirals is significantly narrower than a Poisson distribution. The distribution function for the winding number shows approximately an exponential decay. It depends only weakly on the heating, but strongly on the Prandtl number. Large spirals arise only for larger Prandtl numbers. In this regime the joint distribution for the spiral length and the winding number exhibits a three-peak structure, indicating the dominance of Archimedean spirals of opposite sign and relatively straight sections. For small Prandtl numbers the distribution function reveals a large number of small compact pattern components.

Desynchronization of cells on the developmental path triggers the formation of spiral waves of cAMP during Dictyostelium aggregation

Whereas it is relatively easy to account for the formation of concentric (target) waves of cAMP in the course of Dictyostelium discoideum aggregation after starvation, the origin of spiral waves remains obscure. We investigate a physiologically plausible mechanism for the spontaneous formation of spiral waves of cAMP in D. discoideum. The scenario relies on the developmental path associated with the continuous changes in the activity of enzymes such as adenylate cyclase and phosphodiesterase observed during the hours that follow starvation. These changes bring the cells successively from a nonexcitable state to an excitable state in which they relay suprathreshold cAMP pulses, and then to autonomous oscillations of cAMP, before the system returns to an excitable state. By analyzing a model for cAMP signaling based on receptor desensitization, we show that the desynchronization of cells on this developmental path triggers the formation of fully developed spirals of cAMP. Developmental paths that do not correspond to the sequence of dynamic transitions no relay-relay-oscillations-relay are less able or fail to give rise to the formation of spirals.

Selection for spiral waves in the social amoebae Dictyostelium

Starving Dictyostelium amoebae emit pulses of the chemoattractant cAMP that are relayed from cell to cell as circular and spiral waves. We have recently modeled spiral wave formation in Dictyostelium. Our model suggests that a secreted protein inhibitor of an extracellular cAMP phosphodiesterase selects for spirals. Herein we test the essential features of this prediction by comparing wave propagation in wild type and inhibitor mutants. We find that mutants rarely form spirals. The territory size of mutant strains is approximately 50 times smaller than wild type, and the mature fruiting bodies are smaller but otherwise normal. These results identify a mechanism for selecting one wave symmetry over another in an excitable system and suggest that the phosphodiesterase inhibitor may be under selection because it helps regulate territory size.

Positive genetic feedback governs cAMP spiral wave formation in Dictyostelium.

The aggregation stage of the life cycle of Dictyostelium discoideum is governed by the chemotactic response of individual amoebae to excitable waves of cAMP. We modeled this process through a recently introduced hybrid automata-continuum scheme and used computer simulation to unravel the role of specific components of this complex developmental process. Our results indicated an essential role for positive feedback between the cAMP signaling and the expression of the genes encoding the signal transduction and response machinery.

The nuclear region of the spiral galaxy M81.

Very-long-baseline radio interferometry images of the nuclear region of the nearby spiral galaxy M81 reveal the most compact galactic core outside the Galaxy of which the size has been determined: 700 x 300 astronomical units (AU). The observations exclude a starburst or supernova interpretation for the core. Instead they favor an active galactic nucleus. There is evidence for a northeastern jet bent by approximately 35 degrees over a length scale from 700 to 4000 AU. The jet is, on average, directed toward an extended emission region, probably a radio lobe, about 1 kiloparsec (kpc) away from the core. A corresponding emission region was found in the southwest at a distance of only 30 pc from the core. The observed jet is extremely stable and likely to be associated with a steady-state channel. There is no detectable motion along the jet beyond the nominal value of -60 +/- 60 km.s-1. The level of activities in the core region of M81 is intermediate between that of SgrA* and that of powerful radio galaxies and quasars.

Stellar populations in bulges of spiral galaxies

We present a few results of a work in progress tackling the radial spectroscopic properties of bulges of spiral galaxies.

Responsivity and resonant properties of dipole, bowtie, and spiral Seebeck nanoantennas

Seebeck nanoantennas, which are based on the thermoelectric effect, have been proposed for electromagnetic energy harvesting and infrared detection. The responsivity and frequency dependence of three types of Seebeck nanoantennas is obtained by electromagnetic simulation for different materials. Results show that the square spiral antenna has the widest bandwidth and the highest induced current of the three analyzed geometries. However, the geometry that presented the highest temperature gradient was the bowtie antenna, which favors the thermoelectric effect in a Seebeck nanoantenna. The results also show that these types of devices can present a voltage responsivity as high as 36  μV/W36  μV/W for titanium–nickel dipoles resonant at far-infrared wavelengths.

Demineralization of carbon-treated secondary effluent by spiral-wound reverse osmosis process /

Mode of access: Internet.

The development and utilization of automated and semi-automated microbial mutagenicity assays : cooperative research and development agreement with Spiral Biotech, Incorporated.

Mode of access: Internet.

Accidents on spiral transition curves in California. Final report.

Federal Highway Administration, Washington, D.C.

The coupling and mutual impedance between balanced wire-arm conical log-spiral antennas /

Includes bibliographical references (76-77)

Polygonal spiral antennas /

"Contract AF33(657)-8460 Project No. 6278, Task No. 40572."

The equiangular spiral antenna /

"Contract AF33(616)-3220 Project No. 6(7-4600), Task No. 40572, Wright Air Development Center."