Lower Resistance (lower + resistance)

Distribution by Scientific Domains


Selected Abstracts


Species pool size and invasibility of island communities: a null model of sampling effects

ECOLOGY LETTERS, Issue 9 2005
Herben
Abstract The success of alien species on oceanic islands is considered to be one of the classic observed patterns in ecology. Explanations for this pattern are based on lower species richness on islands and the lower resistance of species-poor communities to invaders, but this argument needs re-examination. The important difference between islands and mainland is in the size of species pools, not in local species richness; invasibility of islands should therefore be addressed in terms of differences in species pools. Here I examine whether differences in species pools can affect invasibility in a lottery model with pools of identical native and exotic species. While in a neutral model with all species identical, invasibility does not depend on the species pool, a model with non-zero variation in population growth rates predicts higher invasibility of communities of smaller pools. This is because of species sampling; drawing species from larger pools increases the probability that an assemblage will include fast growing species. Such assemblages are more likely to exclude random invaders. This constitutes a mechanism through which smaller species pools (such as those of isolated islands) can directly underlie differences in invasibility. [source]


Macroporous monolithic chiral stationary phases for capillary electrochromatography: New chiral monomer derived from cinchona alkaloid with enhanced enantioselectivity

ELECTROPHORESIS, Issue 17 2003
Michael Lämmerhofer
Abstract A new chiral monomer derived from cinchona alkaloid, namely O -9-(tert -butylcarbamoyl)-11-[2-(methacryloyloxy)ethylthio]-10,11-dihydroquinine 1, was employed for the preparation of enantioselective monolithic capillary columns by an in situ copolymerization with 2-hydroxyethyl methacrylate 2 (HEMA), ethylene dimethacrylate 3 (EDMA) in the presence of cyclohexanol and 1-dodecanol as porogens (UV or thermal initiation of azobisisobutyronitrile (AIBN) as radical initiator). The porous properties and the electrochromatographic behavior of the new chiral monoliths were comparatively evaluated with previously described analogs obtained from O -9-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine 4 as chiral monomer. Despite close structural and physicochemical similarities of the both chiral monomers, the pore distribution profiles of the resulting monoliths were shifted typically towards larger pore diameters with the new monomer 1. Once more, it was confirmed that a low cross-linking (10 wt% related to total monomers) and a pore diameter of about 1 ,m in the dry state provides the best electrochromatographic efficiency as a result of lower resistance to mass transfer (smaller C-term contribution to peak broadening) and more homogeneous flow profile (smaller A-term). Most importantly, as expected the new poly(1 - co -HEMA- co -EDMA) monoliths showed enhanced enantioselectivities and in addition faster separations as compared to poly(4 - co -HEMA- co -EDMA) analogs, which represents a significant improvement. Further, the elution order was reversed owing to the pseudoenantiomeric behavior of quinine- and quinidine-derived monomers. Fluorescence-labeled 9-fluorenylmethoxycarbonyl (FMOC), dansyl (DNS), 7-dimethylaminosulfonyl-1,3,2-benzoxadiazol-4-yl (DBD), carbazole-9-carbonyl (CC) amino acids could be separated with resolution values between 2 and 4 (with efficiencies typically between 100,000 and 200,000 plates/m) and fluorescence detection (variable wavelength fluorescence detector in-line with UV) yielding routinely a gain in detection sensitivities up to two orders of magnitude without specific optimization of the conditions with regards to fluorescence efficiency. [source]


Planarization of Polymeric Field-Effect Transistors: Improvement of Nanomorphology and Enhancement of Electrical Performance

ADVANCED FUNCTIONAL MATERIALS, Issue 14 2010
Kumar A. Singh
Abstract The planarization of bottom-contact organic field-effect transistors (OFETs) resulting in dramatic improvement in the nanomorphology and an associated enhancement in charge injection and transport is reported. Planar OFETs based on regioregular poly(3-hexylthiophene) (rr-P3HT) are fabricated wherein the Au bottom-contacts are recessed completely in the gate-dielectric. Normal OFETs having a conventional bottom-contact configuration with 50-nm-high contacts are used for comparison purpose. A modified solvent-assisted drop-casting process is utilized to form extremely thin rr-P3HT films. This process is critical for direct visualization of the effect of planarization on the polymer morphology. Atomic force micrographs (AFM) show that in a normal OFET the step between the surface of the contacts and the gate dielectric disrupts the self-assembly of the rr-P3HT film, resulting in poor morphology at the contact edges. The planarization of contacts results in notable improvement of the nanomorphology of rr-P3HT, resulting in lower resistance to charge injection. However, an improvement in field-effect mobility is observed only at short channel lengths. AFM shows the presence of well-ordered nanofibrils extending over short channel lengths. At longer channel lengths the presence of grain boundaries significantly minimizes the effect of improvement in contact geometry as the charge transport becomes channel-limited. [source]


Sex differences in long bone fatigue using a rat model

JOURNAL OF ORTHOPAEDIC RESEARCH, Issue 10 2006
Luisa D. Moreno
Abstract Stress fractures can occur because of prolonged exercise and are associated with cyclic loading. Fatigue is the accumulated damage that results from cyclic loading and bone fatigue damage is of special concern for athletes and army recruits. Existing literature shows that the rates of stress fracture for female athletes and female army recruits are higher than their male counterparts. In this study, we used an ex vivo rat model to investigate the fatigue response of female and male bones. We determined the strain versus number of cycles to failure (S/N) for each sex and found that for a certain initial strain (5,000,7,000 µ,) female bones have shorter fatigue life. To further characterize the bone response to fatigue, we also determined the creep that occurred during the fatigue test. From the creep data, for a certain strain range, female bones accumulated greater residual strains and reached the critical strain at a faster rate. In summary, this study demonstrates that female rat bones have a lower resistance to fatigue in the absence of a physiological response such as muscle fatigue or osteogenic adaptation. From these results, we hypothesized that creep was the underlying mechanism that accounted for the fast deterioration of female bones during fatigue. © 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:1926,1932, 2006 [source]


Comparison of Fixed and Fluidized Activated Carbon Beds for Removal of Organic Vapors

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 7 2004
I.E. Agranovski
Abstract Activated carbons are commonly used for removal of organic vapors from exhaust air streams. Two configurations, including fixed and fluidized carbon beds are usually employed in industry to meet various requirements of the industrial process which is being used. This paper investigates the performance of such configurations and provides a comparative analysis. It was found that for thin carbon layers, the fixed bed performs more efficiently with the difference exceeding 15,% for the layers with the thickness less than 15,mm. This difference is decreasing with increase of the layer thickness and becomes less than 5,% for the beds thicker than 100,mm. Considering various advantages of the fluidized beds over fixed beds, including lower resistance to the gas flow, excellent gas distribution and minimal possibility of clogging in case of existence of alien particles in the gas, they are recommended for use with the minimal thickness of the layer to be larger than 100,mm. [source]