Three-dimensional quantification of the morphology and intragranular void ratio of a shelly carbonate sand

Shelly carbonate sands represent an extreme soil type in terms of their mechanical behavior which derives from the bioclastic nature of the constituent grains. In their uncemented form, these deposits exhibit very high compressibility, which has posed a number of geotechnical engineering problems; in most cases related to the reduction in the bearing capacities of both shallow and deep foundations. Remarkable features of these carbonate sands include the complex shape and the structural weakness of the grains and the high inter and intra granular porosity. Previous studies, have quoted the interlocking of the angular shelly particles to be at the origin of their high friction angles and high initial void ratio, however, up until now, no scientific micro-scale examination has been carried out. This paper presents a non-invasive image based investigation into the grain morphology of a carbonate sand from the Persian Gulf. This sand has a median grain size of 570μm and a high CaCO3 content in the form of aragonite and calcite. Three-dimensional images from x-ray computed tomography (3DXRCT) with a size of 6μm were used. The presence of various skeletal bodies such as shells of small organisms with distinct densities and composition poses real challenges for an accurate segmentation. Image processing algorithms were developed in order to identify the individual sand grains and quantify their properties. Earlier work on silica sands has highlighted the importance of 3D non-invasive techniques in providing an accurate distribution of the grain sizes when compared to more traditional techniques such as sieving analysis and 2D microscopy. The methodology here proposed allows an accurate quantification of grain shape and size and the assessment of grain damage following mechanical deformation. This study, contributes towards improving our understanding of the engineering properties of carbonate sands and thus, predicting their response under loading

A global simulation for laser driven MeV electrons in 50μm50\mu m-diameter fast ignition targets

The results from 2.5-dimensional Particle-in-Cell simulations for the interaction of a picosecond-long ignition laser pulse with a plasma pellet of 50-$\mu m$ diameter and 40 critical density are presented. The high density pellet is surrounded by an underdense corona and is isolated by a vacuum region from the simulation box boundary. The laser pulse is shown to filament and create density channels on the laser-plasma interface. The density channels increase the laser absorption efficiency and help generate an energetic electron distribution with a large angular spread. The combined distribution of the forward-going energetic electrons and the induced return electrons is marginally unstable to the current filament instability. The ions play an important role in neutralizing the space charges induced by the the temperature disparity between different electron groups. No global coalescing of the current filaments resulted from the instability is observed, consistent with the observed large angular spread of the energetic electrons.Comment: 9 pages, 6 figures, to appear in Physics of Plasmas (May 2006

Behavioral suites mediate group-level foraging dynamics in communities of tropical stingless bees

Competition for floral resources is a key force shaping pollinator communities, particularly among social bees. The ability of social bees to recruit nestmates for group foraging is hypothesized to be a major factor in their ability to dominate rich resources such as mass-flowering trees. We tested the role of group foraging in attaining dominance by stingless bees, eusocial tropical pollinators that exhibit high diversity in foraging strategies. We provide the first experimental evidence that meliponine group foraging strategies, large colony sizes and aggressive behavior form a suite of traits that enable colonies to improve dominance of rich resources. Using a diverse assemblage of Brazilian stingless bee species and an array of artificial “flowers” that provided a sucrose reward, we compared species’ dominance and visitation under unrestricted foraging conditions and with experimental removal of group-foraging species. Dominance does not vary with individual body size, but rather with foraging group size. Species that recruit larger numbers of nestmates (Scaptotrigona aff. depilis, Trigona hyalinata, Trigona spinipes) dominated both numerically (high local abundance) and behaviorally (controlling feeders). Removal of group-foraging species increased feeding opportunities for solitary foragers (Frieseomelitta varia, Melipona quadrifasciata and Nannotrigona testaceicornis). Trigona hyalinata always dominated under unrestricted conditions. When this species was removed, T. spinipes or S. aff. depilis controlled feeders and limited visitation by solitary-foraging species. Because bee foraging patterns determine plant pollination success, understanding the forces that shape these patterns is crucial to ensuring pollination of both crops and natural areas in the face of current pollinator declines

New sources of resistance to bacterial wilt identified in dry bean germplasm collection.

Os objetivos do presente trabalho foram a obtenção de informação preliminar sobre a variabilidade de Curtobacterium pv. flaccumfasciens (Cff) e a identificação de novas fontes de resistência à murcha-de-Curtobacterium no feijoeiro comum. Três isolados de Cff foram inoculados nas cultivares Pérola, Xamego, Rosinha G-2, Diamante Negro e Valente. Como a variação não ambiental foi devida principalmente aos isolados e de cultivares, o isolado Cff CNF 4 foi selecionado para o teste de genótipos de feijoeiro comum para resistência à doença devido à sua maior patogenicidade no conjunto das seis cultivares. Dos 333 acessos testados, 12 apresentaram reação de resistência, enquanto 85 deles apresentaram plantas com reações compatíveis e incompatíveis com o patógeno. As plantas resistentes foram selecionadas e as famílias provenientes delas serão testadas para confirmação de resistência. Os acessos resistentes, e os selecionados que confirmarem sua classe de reação serão incorporadas no programa de melhoramento da Embrapa Arroz e Feijão

Fontes de resistência à antracnose, crestamento-bacteriano-comum e murcha-de-curtobacterium em coletas de feijoeiro comum.

o presente trabalho teve por objetivo a identificação de novas fontes de resistência à antracnose, CBC e MCB em acessos de feijão provenientes de coletas efetuadas nos Estados da Bahia, Espírito Santo, Minas Gerais, Goiás, Paraná, Santa Catarina e Rio Grande do Sul