David A. Noever

Fractal geometry of individual river channels and its computer simulation

A new method for analyzing the self-similarity and self-affinity of single-thread channels is proposed. It permits the determination of the fractal scaling exponents, of the characteristic scales, and the evaluation of the degree of anisotropy for self-similar fractal lines. Based upon the application of this method to the Dniester and Pruth rivers we established the self-similarity of the river pattern on small scales and the self-affinity on large scales. For these rivers we obtained the fractal scaling exponents, the characteristic scales, and the anisotropy parameters. A computer model has been developed which simulates river patterns whose fractal properties are close to the properties of natural objects. A generalized model of fractal behavior of natural rivers is proposed. On the basis of self-affinity of natural and simulated rivers on large scales, a hypothesis has been formulated which explains the violation of the dimension principle in the well-known relation between the river length and the catchment area.

WEIGHTLESS BUBBLE LATTICES : A CASE OF FROTH WICKING

In the absence of gravity drainage, froth wicking draws excess fluid onto a bubble lattice. Capillary forces only cause fluid transport; a moving front moves stably and without fluid fingering along a constant velocity bubble–fluid contact line. This percolation of fluid crawling up the lattice shows (1) fluid coverage on lattice borders varies linearly with available surface area (proportional to lattice perimeter); (2) fluid accelerates through regions or nests of high bubble density (number of bubbles cm−2). The development of nearly two‐dimensional bubble lattices in variable gravity (step function between 0.01 and 1.8 times earthly gravity) are examined experimentally and a zeroth‐order model for froth wetting is presented, which captures many of the principal observations. Possible applications for bubble lattices include adhesion casting of metals and separation of biological cells, bacteria, and particles.

Computerized in vitro test for chemical toxicity based on Tetrahymena swimming patterns

An apparatus and a method for rapidly determining chemical toxicity have been evaluated as an alternative to the rabbit eye initancy test (Draize). The toxicity monitor includes an automated scoring of how motile biological cells (Tetrahymena pyriformis) slow down or otherwise change their swimming patterns in a hostile chemical environment. The method, called the Motility Assay (MA), is tested for 30 s to determine the chemical toxicity in 20 aqueous samples containing trace organics and salts. With equal or better detection limits, results compare favorably to in vivo animal tests of eye irritancy.

SELF‐ORGANIZED CRITICALITY IN CLOSED ECOSYSTEMS: CARBON DIOXIDE FLUCTUATIONS IN BIOSPHERE 2

A little understood question in climate and ecological modelling is when a system appropriately can be considered in statistical equilibrium or quasi-steady state. The answer bears on a host of central issues, including the ability of small perturbations to cause large catastrophes, the constant drift of unsettled systems, and the maximum amount of environmental control theoretically possible. Using Biosphere 2 records, the behaviour of carbon dioxide fluctuations was tested for correspondence with theories now known collectively as self-organized criticality. The signature of agreement with other large, composite systems, including forest fires, stock markets, and earthquakes, is a common frequency spectrum or power-law correlations. In this case, the large- and small-scale ends of the spectrum share a common driving force and consequently no single cut-off exists for excluding or ignoring small environmental changes. From the Biosphere 2 carbon dioxide data, the fluctuations in internal atmospheres vary in both small and large steps. The time fluctuations were examined as they varied over 2 years and over three orders of magnitude in fluctuation size, then binned into characteristic size classes. The statistics show a power-law scaling exponent of −1·3, compared with −1 for classical flicker noise (1/f spectrum) and −2·5 for analogous sand-pile experiments developed to test the predictions of a self-organized, critical system. For comparison with open ecosystems, the Byrd climatic record of global CO2 over the last 50 ka has a similar power-law relation but with −2·3 as the scaling exponent. For generalizing self-organized criticality, the design suggests that otherwise unrelated biological and physical models may share a common correlation between the frequency of small and large length-scales or equivalently exhibit temporal similarity laws. The results potentially have wide implications for environmental control in otherwise chaotic or difficult to predict ecological behaviour.

A bioassay for monitoring cadmium based on bioconvective patterns

The effects of cadmium, one of the most lethal bivalent heavy metals, tend to concentrate in protozoa far above natural levels and therein begin transferring through freshwater food chains to animals and humans. A simple assay using the toxic response of the protozoa, Tetrahymena pyriformis, is described. The assay relies on macroscopic bioconvective patterns to measure the toxic response, giving a sensitivity better than 1 μg/1 and a toxicity threshold to 7μg/1 for Cd+2. Unlike previous efforts, this method does not require electronic or chemical analyses to monitor toxicity.

The baroeffect and an appropriate momentum boundary condition

As disparate molecular weight gases isothermally diffuse between two ends of a capillary tube, they can support a pressure gradient. The magnitude of this pressure gradient depends critically on viscous wall stress and becomes a measure of the boundary condition. This baroeffect has been used to test the state of a gas surface layer. Specifically, it allows one to quantify whether a binary gas has a finite wall velocity (diffusive slip). Here, a one‐dimensional analytical model is proposed that allows for specular gas reflection from the wall. It explains anomalous (4/3) correction factors required previously to match experiment to baroeffect models. It predicts a new physical phenomenon, a surface‐driven baroeffect for equal molecular weight gases. Diffusive slip contributions exceed the order of convective diffusion for Peclet number Pe<1 and approximately equal convective diffusion for 1<Pe<4. For binary gases, this model further extends the baroeffect experiment as a means to find momentum accommodati...

Surface plasmon resonance evaluation of colloidal silver aerogel filters

Abstract Aerogels containing silver nanoparticles were fabricated for gas catalysis applications. By applying the concept of an average or effective dielectric constant to the heterogeneous interlayer surrounding each particle, we extend the technique of immersion spectroscopy to heterogeneous or porous media. Specifically, we extend the predominant effective medium theories for the determination of the average fractional composition of each component in this inhomogeneous layer. Hence, the surface area of metal available for catalytic gas reaction is determined. The technique is satisfactory for statistically random metal particle distributions but needs modification for aggregated systems.

Preferred negative geotactic orientation in mobile cells: Tetrahymena results

For the protozoan species Tetrahymena a series of airplane experiments are reported, which varied gravity as an active laboratory parameter and tested for corresponding changes in geotaxic orientation of single cells. The airplane achieved alternating periods of low (0.01 g) and high (1.8 g; g = 980 cm/s) gravity by flying repeated Keplerian parabolas. The experimental design was undertaken to clearly distinguish gravity from competing aerodynamic and chemical gradients. In this way, each culture served as its own control, with gravity level alone determining the orientational changes. On average, 6.3% of the Tetrahymena oriented vertically in low gravity, while 27% oriented vertically in high-gravity phases. Simplified physical models are explored for describing these cell trajectories as a function of gravity, aerodynamic drag, and lift. The notable effect of gravity on turning behavior is emphasized as the biophysical cause of the observed negative geotaxis in Tetrahymena. A fundamental investigation of the biological gravity receptor (if it exists) and improved modeling for vertical migration in important types of ocean plankton motivate the present research.

Diffusive slip and surface transport properties

Abstract The phenomenon of diffusive slip appears when a gas concentration gradient drives a counterdiffusing mass flux parallel to a solid. To arrive here at a new diffusive slip velocity, an Arrhenius equation is written in terms of a surface potential. Slip velocity is solved for not as an extrapolation from the bulk, but directly on the wall. The model incorporates important new features to slip dynamics: surface reflection physics, surface diffusion coefficients, and a result generalizable to different gas-solid combinations. The models principal advantage rests in its ability to account for previous anomalies such as empirical correction factors and finite slip for equal molecular weight gases. The latter result matches experiment and predicts a surface-driven flow for gases having unequal surface mobilities.

A note on the no-slip condition applied to diffusing gases

Abstract The no-slip boundary condition is shown to be inconsistent with a steady-state diffusion assumption when concentration gradients exist parallel to a gas-solid interface.