349 research outputs found
NUMERICAL ANALYSIS OF STATE-CHANGING PROCESSES IN GRANULAR ASSEMBLIES
A discrete model made for the analysis of state-changing processes of granular assemblies
is introduced here. Our numerical experiments can help in the analysis of some theoretically
suggested variables that could be used later in the macro-level description of the behaviour
of granular materials
Comment on "Mechanical analog of temperature for the description of force distribution in static granular packings"
It has been proposed by Ngan [Phys. Rev. E 68, 011301 (2003)] that the
granular contact force distribution may be analytically derived by minimizing
the analog of a thermodynamic free energy, in this case consisting of the total
potential energy stored in the compressed contacts minus a particular form of
entropy weighted by a parameter. The parameter is identified as a mechanical
temperature. I argue that the particular form of entropy cannot be correct and
as a result the proposed method produces increasingly errant results for
increasing grain rigidity. This trend is evidenced in Ngan's published results
and in other numerical simulations and experiments.Comment: 4 pages, 1 figure, minor editorial correction
Assessing the acoustic behaviour of Anopheles gambiae (s.l.) dsxF mutants: implications for vector control
BACKGROUND: Release of gene-drive mutants to suppress Anopheles mosquito reproduction is a promising method of malaria control. However, many scientific, regulatory and ethical questions remain before transgenic mosquitoes can be utilised in the field. At a behavioural level, gene-drive carrying mutants should be at least as sexually attractive as the wildtype populations they compete against, with a key element of Anopheles copulation being acoustic courtship. We analysed sound emissions and acoustic preference in a doublesex mutant previously used to collapse Anopheles gambiae (s.l.) cages. METHODS: Anopheles rely on flight tones produced by the beating of their wings for acoustic mating communication. We assessed the impact of disrupting a female-specific isoform of the doublesex gene (dsxF) on the wing beat frequency (WBF; measured as flight tone) of males (XY) and females (XX) in homozygous dsxF- mutants (dsxF-/-), heterozygous dsxF- carriers (dsxF+/-) and G3 dsxF+ controls (dsxF+/+). To exclude non-genetic influences, we controlled for temperature and wing length. We used a phonotaxis assay to test the acoustic preferences of mutant and control mosquitoes. RESULTS: A previous study showed an altered phenotype only for dsxF-/- females, who appear intersex, suggesting that the female-specific dsxF allele is haplosufficient. We identified significant, dose-dependent increases in the WBF of both dsxF-/- and dsxF+/- females compared to dsxF+/+ females. All female WBFs remained significantly lower than male equivalents, though. Males showed stronger phonotactic responses to the WBFs of control dsxF+/+ females than to those of dsxF+/- and dsxF-/- females. We found no evidence of phonotaxis in any female genotype. No male genotypes displayed any deviations from controls. CONCLUSIONS: A prerequisite for anopheline copulation is the phonotactic attraction of males towards female flight tones within mating swarms. Reductions in mutant acoustic attractiveness diminish their mating efficiency and thus the efficacy of population control efforts. Caged population assessments may not successfully reproduce natural mating scenarios. We propose to amend existing testing protocols to better reflect competition between mutants and target populations. Our findings confirm that dsxF disruption has no effect on males; for some phenotypic traits, such as female WBFs, the effects of dsxF appear dose-dependent rather than haplosufficient
Influence of Tinospora cordifolia on wound healing in wistar rats
Background: T.cordifolia is widely used in veterinary folk medicine ayurvedic system of medicine. It is known to possess various properties. In a study T.cordifolia promoted wound healing in diabetic patients. However, one study showed that octacosanol content in T.cordifolia possessed anti-angiogenic activity which can hinder wound healing. Therefore, effect of T.cordifolia on wound healing appears to be controversial and there is scarcity of information regarding its effect on wound healing in animal models.Methods: Excision wound, resutured incision wound and dead space wounds were inflicted under light thiopentone anesthesia in male wistar rats (n=6 in each group). Methanol extract of T.cordifolia stem in the dose of 250 mg/kg was administered orally once a day for 10 days in resutured incision (assessed by wound breaking strength), dead space (granuloma dry weight and histopathology of granulation tissue) excision wounds was monitored by planimetry. Data was expressed as mean±SEM and analyzed by student’s t-test. p <0.05 was considered as significant.Results: The results of the present study revealed that T.cordifolia significantly promotes wound healing in all the three models viz. enhanced wound contraction and decreased days for complete epithelization in excision wound; increased breaking strength in resutured incision wound; increased granuloma dry weight and cellular infiltration in granulation tissue.Conclusions: T.cordifolia significantly (p<0.05) promoted wound healing in all the three models of wound in male wistar rats
Stress and Strain in Flat Piling of Disks
We have created a flat piling of disks in a numerical experiment using the
Distinct Element Method (DEM) by depositing them under gravity. In the
resulting pile, we then measured increments in stress and strain that were
associated with a small decrease in gravity. We first describe the stress in
terms of the strain using isotropic elasticity theory. Then, from a
micro-mechanical view point, we calculate the relation between the stress and
strain using the mean strain assumption. We compare the predicted values of
Young's modulus and Poisson's ratio with those that were measured in the
numerical experiment.Comment: 9 pages, 1 table, 8 figures, and 2 pages for captions of figure
Average stresses and force fluctuations in non-cohesive granular materials
A lattice model is presented for investigating the fluctuations in static
granular materials under gravitationally induced stress. The model is similar
in spirit to the scalar q-model of Coppersmith et al., but ensures balance of
all components of forces and torques at each site. The geometric randomness in
real granular materials is modeled by choosing random variables at each site,
consistent with the assumption of cohesionless grains. Configurations of the
model can be generated rapidly, allowing the statistical study of relatively
large systems. For a 2D system with rough walls, the model generates
configurations consistent with continuum theories for the average stresses
(unlike the q-model) without requiring the assumption of a constitutive
relation. For a 2D system with periodic boundary conditions, the model
generates single-grain force distributions similar to those obtained from the
q-model with a singular distribution of q's.Comment: 18 pages, 10 figures. Uses aps,epsfig,graphicx,floats,revte
Reactive oxygen species and cyclooxygenase 2-derived thromboxane A2 reduce angiotensin II type 2 receptor vasorelaxation in diabetic rat resistance arteries
Angiotensin II has a key role in the control of resistance artery tone and local blood flow. Angiotensin II possesses 2 main receptors. Although angiotensin II type 1 receptor is well known and is involved in the vasoconstrictor and growth properties of angiotensin II, the role of the angiotensin II type 2 receptor (AT2R) remains much less understood. Although AT2R stimulation induces vasodilatation in normotensive rats, it induces vasoconstriction in pathological conditions involving oxidative stress and cyclooxygenase 2 expression. Thus, we studied the influence of cyclooxygenase 2 on AT2R-dependent tone in diabetes mellitus. Mesenteric resistance arteries were isolated from Zucker diabetic fatty (ZDF) and lean Zucker rats and studied using in vitro using wire myography. In ZDF rats, AT2R-induced dilation was lower than in lean rats (11% versus 21% dilation). Dilation in ZDF rats returned to the control (lean rats) level after acute superoxide reduction (Tempol and apocynin), cyclooxygenase 2 inhibition (NS398), or thromboxane A(2) synthesis inhibition (furegrelate). Cyclooxygenase 2 expression and superoxide production were significantly increased in ZDF rat arteries compared with arteries of lean rats. After chronic treatment with Tempol, AT2R-dependent dilation was equivalent in ZDF and lean rats. Chronic treatment of ZDF rats with NS398 also restored AT2R-dependent dilation to the control (lean rats) level. Plasma thromboxane B(2) (thromboxane A(2) metabolite), initially high in ZDF rats, was decreased by chronic Tempol and by chronic NS398 to the level found in lean Zucker rats. Thus, in type 2 diabetic rats, superoxide and thromboxane A(2) reduced AT2R-induced dilation. These findings are important to take into consideration when choosing vasoactive drugs for diabetic patients
Internal states of model isotropic granular packings. III. Elastic properties
In this third and final paper of a series, elastic properties of numerically
simulated isotropic packings of spherical beads assembled by different
procedures and subjected to a varying confining pressure P are investigated. In
addition P, which determines the stiffness of contacts by Hertz's law, elastic
moduli are chiefly sensitive to the coordination number, the possible values of
which are not necessarily correlated with the density. Comparisons of numerical
and experimental results for glass beads in the 10kPa-10MPa range reveal
similar differences between dry samples compacted by vibrations and lubricated
packings. The greater stiffness of the latter, in spite of their lower density,
can hence be attributed to a larger coordination number. Voigt and Reuss bounds
bracket bulk modulus B accurately, but simple estimation schemes fail for shear
modulus G, especially in poorly coordinated configurations under low P.
Tenuous, fragile networks respond differently to changes in load direction, as
compared to load intensity. The shear modulus, in poorly coordinated packings,
tends to vary proportionally to the degree of force indeterminacy per unit
volume. The elastic range extends to small strain intervals, in agreement with
experimental observations. The origins of nonelastic response are discussed. We
conclude that elastic moduli provide access to mechanically important
information about coordination numbers, which escape direct measurement
techniques, and indicate further perspectives.Comment: Published in Physical Review E 25 page
Force correlations and arches formation in granular assemblies
In the context of a simple microscopic schematic scalar model we study the
effects of spatial correlations in force transmission in granular assemblies.
We show that the parameters of the normalized weights distribution function,
, strongly depend on the spatial extensions,
, of such correlations. We show, then, the connections between
measurable macroscopic quantities and microscopic mechanisms enhancing
correlations. In particular we evaluate how the exponential cut-off,
, and the small forces power law exponent, , depend
on the correlation length, . If correlations go to infinity, weights are
power law distributed.Comment: 6 page
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