2,954 research outputs found
The influence of short range forces on melting along grain boundaries
We investigate a model which couples diffusional melting and nanoscale
structural forces via a combined nano-mesoscale description. Specifically, we
obtain analytic and numerical solutions for melting processes at grain
boundaries influenced by structural disjoining forces in the experimentally
relevant regime of small deviations from the melting temperature. Though
spatially limited to the close vicinity of the tip of the propagating melt
finger, the influence of the disjoining forces is remarkable and leads to a
strong modification of the penetration velocity. The problem is represented in
terms of a sharp interface model to capture the wide range of relevant length
scales, predicting the growth velocity and the length scale describing the
pattern, depending on temperature, grain boundary energy, strength and length
scale of the exponential decay of the disjoining potential. Close to
equilibrium the short-range effects near the triple junctions can be expressed
through a contact angle renormalisation in a mesoscale formulation. For higher
driving forces strong deviations are found, leading to a significantly higher
melting velocity than predicted from a purely mesoscopic description.Comment: 10 page
Kinetic cross coupling between non-conserved and conserved fields in phase field models
We present a phase field model for isothermal transformations of two
component alloys that includes Onsager kinetic cross coupling between the
non-conserved phase field and the conserved concentration field. We also
provide the reduction of the phase field model to the corresponding macroscopic
description of the free boundary problem. The reduction is given in a general
form. Additionally we use an explicit example of a phase field model and check
that the reduced macroscopic description, in the range of its applicability, is
in excellent agreement with direct phase field simulations. The relevance of
the newly introduced terms to solute trapping is also discussed
Successful invasions of hymenopteran insects into NW Patagonia
We describe the successful invasion of hymenopteran insects into NW Patagonia. We analyse the importance of the invading species and the characteristics of the invaded community, as well as the role of disturbance on the invasion process, by presenting the most conspicuous of the best documented case studies: the wasps Vespula germanica and Sirex noctilio, the bumblebee Bombus ruderatus, and the leaf cutting ant Acromyrmex lobicornis. In their native habitats, these insects are common and have a wide geographical range. In turn, ecological plasticity appears to be the most important demographic trait related to invasion success shared by these species. We believe that climatic matching between the community invaded and the invader’s native range together with the absence of natural enemies are the community characteristics better related to invasion success. The role played by biotic resistance remains unclear. The successful establishment of the studied cases is related to some extent to resource liberation due to exogenous disturbance, or competitive displacement of a native species. This might suggest that the native hymenoptera community of NW Patagonia is species saturated, which in turn, could imply that species interactions are important in the community structure in environments where physical variables have been regarded as key factors
Anomalous Dynamic Scaling in Locally-Conserved Coarsening of Fractal Clusters
We report two-dimensional phase-field simulations of locally-conserved
coarsening dynamics of random fractal clusters with fractal dimension D=1.7 and
1.5. The correlation function, cluster perimeter and solute mass are measured
as functions of time. Analyzing the correlation function dynamics, we identify
two different time-dependent length scales that exhibit power laws in time. The
exponents of these power laws are independent of D, one of them is apparently
the classic exponent 1/3. The solute mass versus time exhibits dynamic scaling
with a D-dependent exponent, in agreement with a simple scaling theory.Comment: 5 pages, 4 figure
Influence of biotic, chemical and mechanical plant defenses on the foraging pattern of the leaf-cutter ant (Acromyrmex striatus) in a subtropical forest
Se estudió la dieta de la hormiga cortadora Acromyrmex striatus como evidencia indirecta de las actividades anthiherbÃvoras de la hormiga nectÃvora Camponotus blandus y de las defensas quÃmicas y mecánicas presentes en las plantas. Se determinó la actividad de ambos tipos de hormigas y la abundancia vegetal de distintas especies de plantas y se cosechó material foliar para análisis de defensas quÃmicas y fÃsicas durante la primavera en un bosque subtropical del Chaco Argentino. Se encontró una relación inversa entre la proporción de visitas de C. blandus y la actividad recolectora de A. striatus para las especies vegetales que ofrecen néctar. Por otro lado, hallamos una correlación positiva entre la abundancia de las especies vegetales del bosque, y su proporción en la dieta de la hormiga cortadora. En general, las defensas fÃsicas y quÃmicas no mostraron correlación significativa cots la dieta de las hormigas cortadoras. Algunas especies mostraron poseer niveles de defensas superiores a las tolerables por A. striatus. Los resultados sugieren dos razones principales para explicar la ausencia o baja representación de las especies vegetales que ofrecen néctar en la dieta de A. striatus: actividad de hormigas nectÃvoras y baja abundancia de las especies vegetales.The diet of the leaf-cutter ant Acromyrmex striatus was used as indirect evidence for potential antiherbivore activity by the nectivorous ant Camponotus blandus and by chemical and mechanical plant defenses. Data on ant activity, plant abundance, and foliar material for analyses of chemical and mechanical plant defenses were collected during spring in a subtropical forest in the Argentinean Chaco. We found a negative relationship between the proportion of visits by C. blandus and leaf harvesting of A. striatus for the plant species that offer nectar. However, the most abundant plant species in the forest comprised the greatest part of the leaf-cutter diet. In general, no relationship was found between chemical or mechanical defenses and leaf-cutter diet for all the plant species of the forest. A few plant species with high levels of plant defenses suffered little or no harvesting by leaf-cutter ants. Our findings suggest two main reasons for the absence or low representation of nectar-offering plant species in the diet of A. striatus: nectivorous ant activity and low plant species abundances
Probing terahertz surface plasmon waves in graphene structures
Epitaxial graphene mesas and ribbons are investigated using terahertz (THz)
nearfield microscopy to probe surface plasmon excitation and THz transmission
properties on the sub-wavelength scale. The THz near-field images show
variation of graphene properties on a scale smaller than the wavelength, and
excitation of THz surface waves occurring at graphene edges, similar to that
observed at metallic edges. The Fresnel reflection at the substrate SiC/air
interface is also found to be altered by the presence of graphene ribbon
arrays, leading to either reduced or enhanced transmission of the THz wave
depending on the wave polarization and the ribbon width.Comment: accepted for publication in Applied Physics Lette
Phase Field Modeling of Fast Crack Propagation
We present a continuum theory which predicts the steady state propagation of
cracks. The theory overcomes the usual problem of a finite time cusp
singularity of the Grinfeld instability by the inclusion of elastodynamic
effects which restore selection of the steady state tip radius and velocity. We
developed a phase field model for elastically induced phase transitions; in the
limit of small or vanishing elastic coefficients in the new phase, fracture can
be studied. The simulations confirm analytical predictions for fast crack
propagation.Comment: 5 pages, 11 figure
Gold substrate-induced single-mode lasing of GaN nanowires
We demonstrate a method for mode-selection by coupling a GaN nanowire laser to an underlying gold substrate. Multimode lasing of GaN nanowires is converted to single-mode behavior following placement onto a gold film. A mode-dependent loss is generated by the absorbing substrate to suppress multiple transverse-mode operation with a concomitant increase in lasing threshold of only ∼13%. This method provides greater flexibility in realizing practical single-mode nanowire lasers and offers insight into the design of metal-contacted nanoscale optoelectronics
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