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At least 451 records · Page 25Linked to original sources
PubMed1992

The macrodynamics of international migration as a sociocultural diffusion process. Part A: theory.

"This study formulates a model of the macrodynamics of international migration using a differential equation to capture the push-pull forces that propel it. The model's architecture rests on the functioning of information feedback between settled friends and family at the destination and potential emigrants at the origin." The author tests the model using data on Greek emigration to the United States since 1820 and on total emigration from Cyprus since 1946.

Americas↗
PubMedDate not supplied

Extended theory of the early diffusion of multiple indicators: bounds on permeability ratios, with applications to intestinal capillaries.

The determination of ratios of capillary permeabilities to multiple indicators is secured by constructing upper bounds on effects of early back-diffusion following a single arterial injection. The analysis does not exclude highly extracted indicators, and it allows for effects of asymmetric blood-tissue exchange and of tissue concentration gradients. The concept of flow limitation of uptake is delimited operationally. It is shown how to identify effects of heterogeneity of uptake for data sets extended over large ranges of rate of blood flow. These new methods are used to show, for several sets of previously published data from muscle, that back-diffusion cannot change appreciably the permeability ratios determined previously without any quantification of back-diffusion. The main application is to data published by Perry and Granger (1981) on the permeability of cat intestinal capillaries to raffinose and inulin. The permeability ratios deduced from these data imply paradoxically restricted diffusion; it is shown that this cannot be attributed to effects of back-diffusion, heterogeneity of extraction, or of solvent drag. It is shown that the paradox is removed if tissue-to-blood permeability to raffinose is at least double the blood-to-tissue permeability, possibly as part of the active transport of sugars from the intestinal lumen. It is shown that corresponding results on permeability ratios in dog gastric capillaries are not paradoxical (against the standard of unrestricted diffusion) under symmetric permeability to raffinose.

Animals↗
PubMed1991

Theory and application of diffuse infrared biotelemetry.

For biomedical applications the most significant is diffuse indoor infrared telemetry. The advantages and disadvantages of infrared diffuse biotelemetry in comparison to RF biotelemetry are presented. Design of the transmitter-receiver link is described and it is shown how efficiency coefficient of the link can be optimized. The whole room can be covered with infrared radiation because of room wall reflections. The distribution and reflections of infrared radiation are analyzed to calculate their most significant parameters and compare them with measured data. One very efficient infrared telemetry system is described and its characteristics are given. To realize a multichannel system inside one room, an asynchronous and a synchronous mode are feasible. The asynchronous mode is characterized with simplicity but the number of channels is very limited. However, the synchronous mode is more sophisticated and complex but acts as a multichannel system without any interference between channels. The realization of both systems is described. In the case that the infrared radiation in some part of the room is not sufficient, an infrared relay can be applied. At the end, a survey of the infrared telemetry application in medicine is given.

Electronics↗
PubMed1988

On the theory of the disk diffusion test. Evidence for a non-linear relationship between critical concentration and MIC, and its practical implications for susceptibility testing of Haemophilus influenzae.

The procedure for disk diffusion susceptibility testing has been worked out for rapidly growing non-fastidious bacteria. Using the general zone diameter breakpoints for interpretation of susceptibility, it was found that clinical isolates of H. influenzae were assigned to the wrong SIR category in fifty per cent of the strains for erythromycin, and ten per cent for doxycycline. New species- and laboratory-specific interpretive zone diameter breakpoints corresponding to the recommended MIC limits were therefore worked out. The Standard Curve regression Analysis (SCA) method used for this purpose is based on the correlation between zone size and disk content, using two reference strains with different MICs. In its original version (the Single strain Regression Analysis, SRA) only one reference strain was used. This equation was found not to be generally valid since the relationship between MIC and the critical concentration is not constant, as was originally assumed. The slope and intercept of the regression line obtained by SCA is species related, while a general regression line based on results from many different species assumes that there is the same relation between the zone size and MIC for all species. Breakpoints for erythromycin and doxycycline calculated by the SCA equation gave more accurate results in routine susceptibility testing of H. influenzae and reduced the error rate from fifty three and ten per cent to four and three per cent for the two antibiotics.

Doxycycline↗
PubMed1988

A method to account for acoustic microstreaming when predicting bubble growth rates produced by rectified diffusion.

A reinterpretation of existing theory for rectified diffusion, the process by which bubbles in a sound field may grow in radius, is presented in order to quantitate the effect of acoustic microstreaming on bubble growth rates. The 1/t term in the growth rate equation is defined as the "decay term" and t as the "decay time," the time required for the gas concentration in the liquid contacting the bubble to rise (or fall) from its initial to its final value. In the absence of microstreaming, t is the duration of sonification. In the presence of microstreaming, t may be calculated from the streaming velocity and the bubble radius. A comparison between theory and the experimental results of Eller [A. Eller, J. Acoust. Soc. Am. 46, 1246-1250 (1969)] and of Gould [R.K. Gould, J. Acoust. Soc. Am. 56, 1740-1746 (1974)] shows reasonable agreement in the low kHz range. Theoretical results in the frequency range of 1-10 MHz at 1 and 4 bar are also presented.

Acoustics↗
PubMed2003

Scaling exponent of the maximum growth probability in diffusion-limited aggregation.

An early (and influential) scaling relation in the multifractal theory of diffusion limited aggregation (DLA) is the Turkevich-Scher conjecture that relates the exponent alpha(min) that characterizes the "hottest" region of the harmonic measure and the fractal dimension D of the cluster, i.e., D=1+alpha(min). Due to lack of accurate direct measurements of both D and alpha(min), this conjecture could never be put to a serious test. Using the method of iterated conformal maps, D was recently determined as D=1.713+/-0.003. In this paper, we determine alpha(min) accurately with the result alpha(min)=0.665+/-0.004. We thus conclude that the Turkevich-Scher conjecture is incorrect for DLA.

Journal Article↗
PubMed2006

Modeling fluid diffusion using the lattice density functional theory approach: counterdiffusion in an external field.

The dependence of the diffusivity on temperature, pressure, and composition is not understood well; consequently, data is preferred significantly over correlations in most practical situations. Even in dilute gases, the contributions of attractions and repulsions to the diffusivity are difficult to understand on a molecular level without performing simulations. We have developed a Lattice Density Functional Theory (LDFT) approach for modeling diffusion to supplement existing methods that are very rigorous but computationally demanding. The LDFT approach is analogous to the van der Waals equation in how it accounts for molecular interactions in that it has first-order corrections to ideal behavior; it is an extension of the Equilibrium LDFT for adsorption and phase behavior. In this work, the LDFT approach is presented and demonstrated by modeling the problem of color counterdiffusion in an externally-applied potential field. This potential field, in combination with the intermolecular potential function, creates a diffusion regime in which repulsions cause oscillations in the density profile. Using the LDFT approach, the oscillations were described and attributed to nearest-neighbor and next nearest-neighbor interactions. The LDFT approach gives qualitative and quantitative agreement with dual control-volume Grand Canonical Molecular Dynamics simulations.

Journal Article↗