Small Density Changes Research Articles

Top-Down Effects in Soybean Agroecosystems: Spider Density Affects Herbivore Damage

Spiders are abundant predators in many agricultural systems. The goal of this study was to determine if the influence of spiders as predators could cascade through the food web and affect primary producers. We examined the effects of spider augmentation and removal in soybean monocultures to assess their impact on the damage experienced by the plants. In two of three years (1990 and 1992), leaf damage was reduced in areas where spiders were added, and in one of two years (1992), there was increased damage in areas where spiders were removed. The biomass of insects killed by the spiders was positively correlated with spider mass and the leaf damage near augmentation sites was negatively correlated with the mass of insects killed by the spiders. In this study, small changes in spider density had a significant localized effect on the plants, especially in years when damage due to pest insects was high. These data also verified that trophic cascades and top-down control are in operation at measurable levels in soybean food webs

An evolution equation modeling inversion of tulip flames

We attempt to reduce the number of physical ingredients needed to model the phenomenon of tulip-flame inversion to a bare minimum. This is achieved by synthesising the nonlinear, first-order Michelson-Sivashinsky (MS) equation with the second order linear dispersion relation of Landau and Darrieus, which adds only one extra term to the MS equation without changing any of its stationary behaviour and without changing its dynamics in the limit of small density change when the MS equation is asymptotically valid. However, as demonstrated by spectral numerical solutions, the resulting second-order nonlinear evolution equation is found to describe the inversion of tulip flames in good qualitative agreement with classical experiments on the phenomenon. This shows that the combined influences of front curvature, geometric nonlinearity and hydrodynamic instability (including its second-order, or inertial effects, which are an essential result of vorticity production at the flame front) are sufficient to reproduce the inversion process.

Is tubal ligation a risk factor for low bone density and increased risk of fracture?

OBJECTIVE: Osteoporosis is a major women's health problem, because it is responsible for about 1.3 million fractures in the United States each year. Estrogen deficiency is a major risk factor in the pathogenesis of osteoporosis. Recent evidence has indicated that tubal ligation may cause menstrual dysfunction and estrogen deficiency. This study examined the association between tubal ligation and bone mass in a group of elderly postmenopausal women. STUDY DESIGN: Subjects were 2215 white women ≥65 years old participating in the Baltimore center of the Study of Osteoporotic Fractures. Bone mineral density of the proximal and distal radius and the calcaneus was measured by single photon absorptiometry. Multiple regression analysis was performed to determine whether tubal ligation had an independent effect on bone density. The effect of tubal ligation on the risk of hip and osteoporotic fractures was estimated by Cox proportional hazards model. RESULTS: Women who reported a tubal ligation had lower, although not statistically significant, bone density of the radius and calcaneus. The relative risk of hip (1.05, 95% confidence limit 0.84 to 1.32) and osteoporotic fractures (1.01, 0.80 to 1.29) was not significantly increased in women with tubal ligation. CONCLUSION: We conclude that elderly women who had a tubal ligation have small changes in bone density that are not of sufficient magnitude to increase their risk of osteoporotic fractures.

Atomic information entropies beyond the Hartree-Fock limit

Atomic information entropies are computed from configuration interaction wavefunctions for members of the lithium isoelectronic series. For each member of the series, a sequence of wavefunctions built from increasingly larger basis sets is generated which satisfies a density convergence criterion, and exhibits a monotonic behaviour with respect to the density generated from the wavefunctions. Studies of the behaviour of the entropies along a sequence for a particular atom includes trends which have been shown to be present at the Hartree-Fock level. Results suggest that the position and momentum entropies are sensitive to small changes in density and thus can be useful indicators of the quality of the density. The sum of the entropies is also shown to be a useful indicator when density differences are larger. The nature of the information entropy-energy relationship is examined.

A method of anisotropic steel modelling using finite element method with confirmation by experimental results

The modelling methods developed so far for grain-oriented electrical steels can not account for the true properties of materials. When these models were developed, more consideration was given to obtain fast convergence speed for numerical iterations than the accuracy of models themselves. For magnetic materials, the small changes in flux density B can cause dramatic change in permeability, so for the purpose of modelling devices, material modelling has to be as accurate as possible to reflect the true magnetic characteristics. >

A review of local field effects in lasing without inversion

We review the effects of density dependent near dipole-dipole, or local field, interactions on lasing without inversion. In particular, we consider the nonlinear behaviour of the macroscopic electric susceptibility of a dense, coherently prepared, three-level Lambda system. For certain values of the atomic density, we observe enhancement of the absorptionless index of refraction and inversionless gain by more than two orders of magnitude. We also predict a piezophotonic switching effect whereby a small change in density induces the system to flip from a strong absorber into a local field enhanced inversionless amplifier.

Bone remodeling and structural optimization

Bone remodeling has been viewed both as a process which adapts bone tissue to the mechanical environment at each point in the structure, and as a process which optimally adjusts the tissue distribution within bones to bear the loads placed on them. We have developed a connection between these two views of bone remodeling, in a restricted sense. We start with a remodeling rate equation based on strain energy density. We then define an indicator function which is a weighted sum of total strain energy and a measure of bone mass, and we show that finding bone density distributions in which the remodeling rate equation predicts no changes with time is the same as finding density distributions in which the indicator function is insensitive to small changes in density. The set point in the remodeling rate equation corresponds to a parameter in the indicator function whith determines the relative importance of bone mass and strain energy in the optimization indicator function. We have not assessed whether the density distributions which make the density rate of change zero are actually local or global minima for the indicator function in this study, but a related study shows that there is a single unique minimum for the indicator function developed here, implying that a unique solution exists for the bone remodeling rate equations considered in this study.

Self-Propagating High Temperature Synthesis (SHS) of Intermetallic Compounds Titanium and Nickel Aluminides

Abstract The self-propagating high temperature synthesis (SHS) of titanium and nickel aluminides was investigated. Effect of experimental variables on SHS temperature, burning rate, product composition, and microhardness are discussed. The densities of the products are 70.80% of theoretical values and the microhardness of Ni3Al produced is similar to that of low carbon steel. It is also shown that the SHS process is affected by the reactant ratios. Small reactant particle size and moderate preheating result in high SHS rates. A small addition of boron and a small change in the green density do not significantly affect the process.

Effect of different water levels on the properties of HSR class G cement

Changes in water level corresponding to small changes in slurry density have a substantial effect upon the physical cementing properties of HSR Class G oilwell cement under comparable conditions. The implications of this for practical oilwell cement slurry formulations for downhole usage are discussed.

Evaluation of Metal Powders Using Arnold Density Meter and Hall Flowmeter

AbstractA comparative study of the performance of two instruments widely used by the US PM industry to obtain precise, rapid evaluation of metal powder apparent density, the Arnold density meter and the Hall flowmeter, has been conducted. As received water atomised iron powder, screened fractions of this powder, and reblends of the size fractions were tested at various relative humidities. The Arnold meter was found to be more sensitive to small changes in apparent density and to give results more representative of flow in industrial die filling operations, whereas the Hall meter showed greater sensitivity to powder surface smoothness. PM/0582

Sensitivity of 3T3 cells to low serum concentration and the associated problems of serum withdrawal.

The sensitivity of cells to serum deprivation depends upon whether they are transformed. Most supplies of 3T3 cells are of this type and are considerably less sensitive than untransformed cells. In addition, the apparently simple manoeuvre of reducing serum levels has considerable effects on cell fragility, viability, growth rate and metabolism, which were found to be due to small changes in pH, substrate availability, cell density and other parameters, many of which cannot be attributed to the absence of growth factors from the medium. Supplementation of medium with bovine serum albumin (BSA) to compensate for low normal serum protein did not aid growth nor offset the disturbances caused by low serum levels themselves. Problems associated with the altered precursor availability for DNA, RNA and protein synthesis are also discussed.

An in vitro study of the characteristics of a computer-aided radiographic evaluation (CARE) system for longitudinal assessment of density changes.

In recent years, several systems for computerized analysis of radiographs have been introduced, most of which use digital conversion of the image followed by subtraction of consecutive images to assess changes. This paper introduces a computer-based qualitative and quantitative radiographic evaluation system based on the CADIA algorithm. Problems associated with computerized radiographic analysis are discussed and evaluation criteria for this type of system are suggested. These criteria include evaluation of system noise and threshold setting, reproducibility, and establishment of the system working curve (validity). The CARE system noise ranged from a -10 to +10 CADIA value and the threshold was set on 13 for all measurements. The reproducibility was high, both for the radiographic technique and for repeated measurements. The working curve was established and showed the system's ability to detect small density changes of 0.048 O.D. which corresponds to 0.27 mm of aluminium thickness or compact bone equivalent. The linear range of the curve was between 0.7 and 1.8 O.D. By controlling the exposure parameters it was possible to work within the linear range of the curve. Based on these evaluation criteria, the CARE system can be used to quantitatively evaluate small density changes on sequential radiographs for early detection of caries and periodontal disease.

Changes of buoyant density during the S-phase of the cell cycle. Direct evidence demonstrated in acute myeloid leukemia by flowcytometry

Studies with synchronized or exponentially growing bacteria and mammalian cell lines are not able to demonstrate small changes in buoyant density during the cell cycle. Flowcytometric analysis of density separated acute myeloid leukemia cells, a system not dependent on time-related variables, shows that the cellular buoyant density increases slightly with up to 0.008 g/ml during the S-phase, at least in cryo-preserved cells used in this study. This contrasts with the generally accepted belief that S-phase cells have a lower or constant buoyant density. A practical implication is that separation of cell (sub)populations based on differences in buoyant density could be flawed to the extent that these populations contain S-phase cells.

Phantoms for evaluating the performance characteristics of bone densitometers.

Osteoporosis is characterized by a gradual loss of osseous tissue within the framework of the bone, which lowers bone strength and so increases the risk of fracture. A variety of techniques exist for identifying early bone loss, such as single- or dual-photon absorptiometry (SPA, DP A), quantitative computed tomography (QCT), etc. (Speller et al, 1989). Those commonly employed concentrate on the rate of decrease in bone density or bone mineral content within a particular site. The skeletal sites that become most susceptible to fracture are those with high trabecular content, namely the distal radius, the femoral neck and the lumbar vertebrae. For diagnosis of osteoporosis serial density measurements need to be performed at regular intervals over a period of many months. The frequency at which measurements are performed must be determined by the precision of the machine as compared with the decrease in bone density over that period of time. The reported rates of bone loss for normal post-menopausal women are 1.2% per year for trabecular bone (vertebral QCT measurements; Cann et al, 1985) and 0.9% per year for cortical bone (SPA measurements of mid-radius; Mazess, 1982). Therefore, the precision of the method must be sufficient to allow the machine to detect the small changes in bone density that occur in the space of a few months. Thus, knowledge of the precision and accuracy of the machine is essential. All systems when installed are supplied with a calibration phantom.

Experimental measure of dynamic spatial-hole burning in DFB lasers

Spatially resolved measurements of carrier density in two 1.3- mu m distributed feedback laser diodes are reported. By measuring the response of the spontaneous emission intensity to a 50-MHz drive current, small relative changes in the carrier density can be observed. When biased below threshold, the carrier density fluctuates uniformly in the laser cavity with a phase delay corresponding to the carrier lifetime. When biased above threshold, the carrier density response shows clear evidence of spatial hole burning. The location of the minimum and the net change in carrier density, however, depend on the particular laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermodynamic properties of the fluid, fcc, and bcc phases of monodisperse charge-stabilized colloidal suspensions within the Yukawa model.

The thermodynamic properties of the Yukawa model for colloidal suspensions are determined theoretically from the Rogers-Young integral equation for the fluid phase and from a recently introduced van der Waals-like theory for the solid phases. Very good agreement with the Monte Carlo simulations of Meijer and Frenkel [J. Chem. Phys. 94, 2269 (1991)] is found for both the fluid and the (fcc-bcc) solid phases. The location of the two-phase coexistences, however, is shown to involve such small free-energy and density changes that no definite statements about the phase diagram are possible within the present accuracy.

The phase instability of molecular fluid mixtures: dipolar and neutral hard spheres

The many degrees of freedom in fluid mixtures of nonspherical molecules can lead to different kinds of phase transition. We propose to characterize the phase transition uniquely by the eigenvector belonging to the vanishing eigenvalue of the coefficient matrix which multiplies the fluctuation in an expansion of the free energy up to second order. As an example we study the phase transition of a mixture of dipolar hard spheres and neutral hard spheres. We evaluate the coefficient matrix from the calculated correlation functions and find a demixing phase transition combined with small total density changes.

Domains and domain walls in graphite intercalation compounds.

The structure of domains and domain walls in the low-temperature solid phase of stage-2 alkali-metal graphite intercalation compounds is studied using molecular-dynamics (MD) simulation and a model potential obtained from fitting to the liquid structure data. For ${\mathrm{RbC}}_{24}$, we find small domains of (\ensuremath{\surd}7 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}7 ) structure containing mostly seven Rb atoms and with domain walls consisting of (2\ifmmode\times\else\texttimes\fi{}2) and (2\ifmmode\times\else\texttimes\fi{}3) elemental plaquettes. These nanostructures provide a clear picture of the discommensurations and a consistent understanding of the x-ray-diffraction measurements. Based on the simulation results, we propose a periodic domain-wall model that explains very well the dominant features of the x-ray structure factor for both ${\mathrm{RbC}}_{24}$ and ${\mathrm{CsC}}_{24}$ when we allow for small changes in the planar density.

Bulk Density of a Sandy Loam: Traffic, Tillage, and Irrigation‐Method Effects

AbstractModern crop production creates a cycle between soil compaction caused by traffic and alleviation of this condition by tillage or natural processes such as freezing and thawing. The objective of this study was to evaluate important management practices as they relate to changes in bulk density of a tilled sandy loam soil. Practices evaluated were irrigation method, time between tillage and traffic, tire pressure and wheel load of applied traffic, and controlled traffic. Relationships among bulk density, penetration resistance, and infiltration rate were determined. Experiments were conducted in the San Joaquin Valley of California, on a sandy loam soil (Entisol) with an organic‐matter content of &lt;1%. After tillage, settling and trafficking of a soil resulted in rapid changes in its bulk density until a new equilibrium was reached. Tire pressure of 408 kPa and wheel weight of 2724 kg applied at moisture contents near field capacity resulted in a bulk density of 1.92 Mg m−3, compared with a value of 1.67 for no traffic. The time interval between tillage and traffic did not affect final bulk density. Drip irrigation, which did not saturate the soil, resulted in a bulk density of ≈0.1 Mg m−3 lower than flood irrigation, which saturated the soil surface. Wheel traffic in the furrow resulted in only small changes in the bulk density within the row. When tillage did not occur between cropping seasons, traffic caused high bulk densities in the furrow but only small changes in the row. An increase in bulk density from 1.7 to 1.89 Mg m−3 decreased the infiltration rate by four times and increased resistance to penetration at the end of the season by three times. Knowledge of how management practices affect bulk density can aid growers in reducing recompaction following tillage.

Finite thermal diffusivity at onset of convection in autocatalytic systems: Continuous fluid density.

The linear stability of exothermic autocatalytic reaction fronts is considered using the viscous thermohydrodynamic equations for a fluid with finite thermal diffusivity. For upward front propagation and a thin front, the vertical thermal gradient near the front is reminiscent of the Rayleigh-B\'enard problem of a fluid layer heated from below. The problem is also similar to flame propagation, except that here the front propagation speed is limited by catalyst diffusion rather than by activation kinetics. For small density changes in a laterally unbounded system, the curvature dependence of the front propagation speed stabilizes perturbations with short wavelengths \ensuremath{\lambda}${\ensuremath{\lambda}}_{\mathit{c}}$, whereas long wavelengths are unstable to convection. The critical wavelengths ${\ensuremath{\lambda}}_{\mathit{c}}$ are calculated and compared with experiments and with theoretical results for a similar problem that is driven by a Rayleigh-Taylor instability arising from a discontinuous density difference at the reaction front.