Saffman-Taylor instability in a non-Brownian suspension: finger selection and destabilization

We study the Saffman-Taylor instability in a non-Brownian suspension by injection of air. We find that flow structuration in the Hele-Shaw cell can be described by an effective viscosity depending on the volume fraction. When this viscosity is used to define the control parameter of the instability, the classical finger selection for Newtonian fluids is recovered. However, this picture breaks down when the cell thickness is decreased below approximatively 10 grain sizes. The discrete nature of the grains plays also a determinant role in the the early destabilization of the fingers observed. The grains produce a perturbation at the interface proportional to the grain size and can thus be considered as a "controlled noise". The finite amplitude instability mechanism proposed earlier by Bensimon et al. allows to link this perturbation to the actual values of the destabilization threshold.Comment: 4 pages, 4 figures, submitted to PR

Infrared properties of SiC particles

We present basic laboratory infrared data on a large number of SiC particulate samples, which should be of great value for the interpretation of the 11.3 micron feature observed in the spectra of carbon-rich stars. The laboratory spectra show a wide variety of the SiC phonon features in the 10-13 micron wavelength range, both in peak wavelength and band shape. The main parameters determining the band profile are morphological factors as grain size and shape and, in many cases, impurities in the material. We discovered the interesting fact that free charge carriers, generated e.g. by nitrogen doping, are a very common characteristics of many SiC particle samples. These free charge carriers produce very strong plasmon absorption in the near and middle infrared, which may also heavily influence the 10-13 micron feature profile via plasmon-phonon coupling. We also found that there is no systematic dependence of the band profile on the crystal type (alpha- vs. beta-SiC). This is proven both experimentally and by theoretical calculations based on a study of the SiC phonon frequencies. Further, we give optical constants of amorphous SiC. We discuss the implications of the new laboratory results for the interpretation of the spectra of carbon stars.Comment: 17 pages, 12 figures. To appear in A&

Infrared Spectra of Meteoritic SiC Grains

We present here the first infrared spectra of meteoritic SiC grains. The mid-infrared transmission spectra of meteoritic SiC grains isolated from the Murchison meteorite were measured in the wavelength range 2.5--16.5 micron, in order to make available the optical properties of presolar SiC grains. These grains are most likely stellar condensates with an origin predominately in carbon stars. Measurements were performed on two different extractions of presolar SiC from the Murchison meteorite. The two samples show very different spectral appearance due to different grain size distributions. The spectral feature of the smaller meteoritic SiC grains is a relatively broad absorption band found between the longitudinal and transverse lattice vibration modes around 11.3 micron, supporting the current interpretation about the presence of SiC grains in carbon stars. In contrast to this, the spectral feature of the large (> 5 micron) grains has an extinction minimum around 10 micron. The obtained spectra are compared with commercially available SiC grains and the differences are discussed. This comparison shows that the crystal structure (e.g., beta-SiC versus alpha-SiC) of SiC grains plays a minor role on the optical signature of SiC grains compared to e.g. grain size.Comment: 7 pages, 6 figures. To appear in A&

Australian environmental issues : an overview.

Australia is experiencing significant environmental problems both on land and in its marine areas and several of these problems are interconnected. Clearing of trees and woodland is still substantial and apart from adding to CO2 emissions, has a number of other unfavourable environmental consequences e.g. on water systems. Australia’s river systems, estuary areas, coastal lakes and lagoons are suffering from serious environmental problems of which eutrophication is one of the more serious. Marine environments are also under stress. For example, seagrass beds have declined substantially, toxic phytoplankton blooms are of growing concern, adverse environmental impacts are experienced by some species of marine reptiles and mammals and some fisheries are adversely affected by environmental changes and excessive harvesting. Wastes from human settlements add to these environmental problems; partly because settlement occurs in dense pockets in Australia. Per capita, Australians use a high level of natural resources. Neither markets nor political mechanisms have been able to solve Australia’s environmental problems in as effective manner as many would hope for

Brunei's quest for sustainable development : diversification and other strategies.

Like many Middle East economies, Brunei is an oil-rich rentier economy with a high degree of dependence on guest workers, concentration of employment of Bruneians in the public sector and with a high degree of specialisation in extractive production. Because its hydrocarbon reserves are diminishing, it faces the problem of how to sustain its income. The potential for it to do this by investing its rental income abroad and by diversifying its economy are discussed, and comparisons are made with oil-rich Middle East countries and MIRAB economies. Doubts are raised about industrial diversification as a suitable sustainable development strategy for Brunei. Expansion of service industries may offer better prospects. Brunei’s entry into APEC could limit its scope for adopting strategic policies to restructure its economy. Keywords: Brunei, development strategies, economic diversification, Malay Muslim Monarchy, oil-rich economies, rentier economies

Coevolution, agricultural practices and sustainability: some major social and ecological issues.

Outlines major social and ecological issues involved in the coevolution of social and ecological systems by initially reviewing relevant aspects of the recent literature relating to economic development and their implications for agricultural development. Coevolutionary qualitative- type models are presented. There has been a failure amongst advocates of structural adjustment policies (involving the extension of markets and economic globalisation) to take account of coevolutionary principles and allow for historical differences in the evolution of communities and their varied circumstances. This lack of sensitivity has had unfortunate social and ecological consequences for some communities eg The Russian Federation and subsistence agriculturalists in some less developed countries. The evolution of globalized market systems involving industrial/commercial agriculture (largely dependent on inputs external to the farm) under the 'patronage' of oligopolistic suppliers is seen to increasingly threaten the balance between social and ecological systems and as undermining the sustainabiltiy of both. Capitalistic processes of technological change eg advances in biotechnology, play a major role in this evolution

The Distance of the First Overtone RR Lyrae Variables in the MACHO LMC Database: A New Method to Correct for the Effects of Crowding

Previous studies have indicated that many of the RR Lyrae variables in the LMC have properties similar to the ones in the Galactic globular cluster M3. Assuming that the M3 RR Lyrae variables follow the same relationships among period, temperature, amplitude and Fourier phase parameter phi31 as their LMC counterparts, we have used the M3 phi31-logP relation to identify the M3-like unevolved first overtone RR Lyrae variables in 16 fields near the LMC bar. The temperatures of these variables were calculated from the M3 logP-logTe relation so that the extinction could be derived for each star separately. Since blended stars have lower amplitudes for a given period, the period amplitude relation should be a useful tool for identifying which stars are affected by crowding. We find that the low amplitude stars are brighter. We remove them from the sample and derive an LMC distance modulus 18.49+/-0.11.Comment: 30 pages, 7 figures, accepted for publication in the Astronomical Journa