Electron localization in metallic quasicrystals.

Bulk icosahedral-quasicrystalline aluminum-palladium-rhenium alloys of high structural quality and thermal stability are found to exhibit low-temperature electrical resistivities that are four orders of magnitude larger than those found in disordered metals and metallic glasses. Experiments suggest that these quasiperiodic alloys, which have a semimetallic electron density, are insulators at low temperature. The findings are discussed in light of theories on electron localization and band-gap formation in ordered metallic systems.

The threat of soil erosion to long-term crop production.

National increases in row crops at the expense of hay and pasture crops, particularly on steeper slopes, have made the control of erosion a difficult prospect. Management practices that fit the various field conditions are needed to accomplish effective erosion control. These measures should be selected on the basis of soil characteristics, landscape type, and the amount of ongoing erosion. The maintenance of a cropland base adequate to our needs must be a primary national goal.

Self-assembly of ligated gold nanoparticles: phenomenological modeling and computer simulations.

We study the assembly of ligated gold nanoparticles by both phenomenological modeling and computer simulations for various ligand chain lengths. First, we develop an effective nanoparticle-nanoparticle pair potential by treating the ligands as flexible polymer chains. Besides van der Waals interactions, we incorporate both the free energy of mixing and elastic contributions from compression of the ligands in our effective pair potentials. The separation of the nanoparticles at the potential minimum compares well with experimental results of gold nanoparticle superlattice constants for various ligand lengths. Next, we use the calculated pair potentials as input to Brownian dynamics simulations for studying the formation of nanoparticle assembly in three dimensions. For dodecanethiol ligated nanoparticles in toluene, our model gives a relatively shallower well depth and the clusters formed after a temperature quench are compact in morphology. Simulation results for the kinetics of cluster growth in this case are compared with phase separations in binary mixtures. For decanethiol ligated nanoparticles, the model well depth is found to be deeper, and simulations show hybrid, fractal-like morphology for the clusters. Cluster morphology in this case shows a compact structure at short length scales and a fractal structure at large length scales. Growth kinetics for this deeper potential depth is compared with the diffusion-limited cluster-cluster aggregation (DLCA) model.

Computer simulation of diffusion-limited cluster-cluster aggregation with an Epstein drag force.

The motion of particles, dispersed in a medium, between collisions with each other can, in limiting situations, be either ballistic (straight line) or diffusive (random walker). The diffusive regime can be divided into two distinct subregimes. The "continuum regime" exhibits Stokes-Einstein-type diffusion (no-slip surface boundary condition) with a frictional coefficient proportional to the particle size (linear dimension). The "Epstein regime," as we shall refer to it, is characterized by a frictional coefficient proportional to the particle cross-sectional area, hence an Epstein-type diffusion (slip surface). The purpose of the current study is to illuminate the dynamics of dilute-limit aggregation in the Epstein regime. We present results from low volume fraction Monte Carlo simulations of cluster-cluster aggregation in the Epstein regime with the particle motion based on each particle's cross-sectional area. Our findings indicate that aggregates grown under Epstein conditions have a fractal dimension of approximately 1.8, similar to that of diffusion-limited cluster-cluster aggregates (DLCA) in the continuum regime. The kinetic exponent z in the Epstein regime is found to be z approximately 0.8, lower than its value for both the continuum regime DLCA (z = 1) and for the ballistic cluster aggregation regime (z approximately 2). Cluster size distribution data for Epstein systems are found to scale at large cluster sizes with exponents consistent with the kinetic data. A scaling argument for predicting the kinetic exponent and kernel homogeneity based on the mass or size dependence of the particle velocity and collision cross section is presented and is seen to give accurate results for dilute and intermediate values of particle volume fractions not only for the current study, but also for work done by other researchers with various choices for the aggregation kernel.

Hybrid superaggregate morphology as a result of aggregation in a cluster-dense aerosol.

From the digitized pictures of soot clusters formed after the explosion of a hydrocarbon gas mixed with oxygen, the cluster morphology was determined by two different methods: structure factor and perimeter analysis. We find a hybrid, superaggregate morphology characterized by a fractal dimension of D approximately equal to 1.8 between the monomer size, ca. 50 nm, and 1 microm and D approximately equal to 2.6 at larger length scales up to approximately 10 microm. The superaggregate morphology is a consequence of late-stage aggregation in a cluster-dense regime near a gel point.

Designing clinical trials on energy healing: ancient art encounters medical science.

Demand for energy healing is growing rapidly in the United States. Until recently, however, few clinical trials have been conducted to investigate its clinical efficacy, risks, and cost-effectiveness. This article discusses principles underlying the research design of clinical trials on energy healing, based on the experience of an interdisciplinary team conducting a large-sample clinical study on qigong funded by the National Institutes of Health. The first part overviews the background and contemporary practice of qigong therapy. The second addresses some difficulties and unique issues to be considered in designing a clinical trial on energy healing. These issues include research emphasis on outcome versus mechanism, randomization, control, expectations/placebo effects, staff and practitioner bias/conflict of interest, patients' belief, selection bias, intent-to-treat analysis, ethics, informed consent, sample size, and outcome report. The ultimate goal is to promote more scholarly and clinical discussion on the evaluation of energy healing.

Aggregation-fragmentation in a model of DNA-mediated colloidal assembly.

We present results from an off-lattice Monte Carlo simulation of DNA-mediated colloidal assembly. In this simulation, the aggregation-fragmentation of a binary mixture of DNA-coated colloidal particles is studied through a simplified model of base-pair hybridization. Bonding between monomers is modeled as a simple temperature-sensitive A/B-type interaction, where type A and B monomers can bond to only the opposite type (no A/A or B/B attachments are allowed). The actual chemistry of base-pair hybridization is not included in the model. The morphological structures of the clusters formed as well as the kinetics of growth are analyzed in our 2D simulations. The fractal dimension and kinetic growth exponents for clusters formed near the DNA "melting" temperature agree with those seen previously for 2D diffusion-limited cluster aggregation (DLCA) models. The clusters appear more compact, exhibiting signs of local order at intermediate temperature values. At higher temperatures, the formation of large clusters is not favorable under the action of temperature-dependent fragmentation, and the system eventually reaches a steady state as a collection of small aggregates. The temperature profile for this dissolution of the colloidal assembly is sharp, indicating that the selective hybridization process provides a highly sensitive measurement tool. At high temperatures, we analyze the steady-state behavior of the average cluster size in terms of an aggregation-fragmentation model.

A field evaluation of noise measuring instruments.

Five sound level measuring instrument responses were compared in four plant noise situations. A sound level meter (SLM), an integrating sound level meter (LOSHA), a 25 dB crest-80 dB threshold dosimeter, a 15 dB crest-80 dB threshold dosimeter, and a 15 dB crest-90 dB threshold dosimeter were used in the study. All instruments were calibrated and maintained as recommended by the manufacturers and were tested in constant noise environments at moderate and high levels, at moderate background levels with moderate impact noise, and at low background levels with high impact noise. Experimentation was conducted in a plant environment designed to encompass various types of noise exposure actually found in industry. The results showed varying differences depending on noise levels between dosimeters with matching crest factors but with differing thresholds. The results showed no appreciable differences among instrument precision, dosimeter response at 25 versus 15 crest in impact noise areas, LOSHA meter response with dosimeter response at matching thresholds, SLM readings with 80 threshold dosimeter response, and LOSHA meter response at 80 threshold with SLM data.

Comparison of heat stress measuring techniques in a steel mill.

Three heat stress measurement devices, currently used to assess heat exposure in the workplace, were compared in indoor and outdoor environments at a steel mill in Orem, Utah, during the month of July, 1982. Sixty sets of environmental data from a total of fourteen different test locations were collected and analyzed. Significance tests, linear regression equations and correlation coefficients were calculated to determine comparability and relationships between standard WBGT Index and Botsball (WGT) and also between standard WBGT Index and an electronic WBGT Index Meter. The results of the significance testing between the standard WBGT, Botsball, and electronically generated WBGT showed that the electronically generated WBGT means were statistically much closer to the standard WBGT values than were the Botsball (WGT) results. The statistical analysis performed shows that a high correlation of variation exists between the standard WBGT and both the Botsball and the electronically generated WBGT.

Computer simulation of selective aggregation in binary colloids.

The morphology of clusters formed by selective aggregation of binary colloids is studied in a two-dimensional Monte Carlo simulation for a large range of number fractions (200:1, 100:1, 10:1, 2:1). We find remarkable similarity in morphology to those observed in experiments, from the formation of closed "micelles" to large branched clusters. Quantitative studies of the fractal dimension, kinetics, and cluster size distribution are also carried out and compared with diffusion-limited cluster aggregation and reaction-limited cluster aggregation models.

The pattern of secondary root formation in curving roots of Arabidopsis thaliana (L.) Heynh.

A gravitational stimulus was used to induce the curvature of the main root of Arabidopsis thaliana. The number of secondary roots increased on the convex side and decreased on the concave side of any curved main root axes in comparison with straight roots used as the control. The same phenomenon was observed with the curved main roots of plants grown on a clinostat and of mutant plants exhibiting random root orientation. The data suggest that the pattern of lateral root formation is associated with curvature but is independent of the environmental stimuli used to induce curvature.