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At least 433 records · Page 24Linked to original sources
PubMed2003

Self-organized pattern formation of a bacteria colony modeled by a reaction diffusion system and nucleation theory.

Self-organized pattern formation is observed in bacterial colony growth. The recently reported knotted-branching pattern of the Bacillus circulans colony consists of the trajectories of aggregates which grow, move, and reproduce simultaneously. We modeled these processes by combining a reaction diffusion system of nutrient dynamics, nucleation theory for aggregate generation, and individual based dynamics of motion and growth of aggregates. The branching pattern produced by computer simulation shows great similarity with experiments. Response to the initial nutrient concentration is also consistent with the experiments.

Bacillus↗
PubMed1999

Theoretical and Experimental Study of Spectral Reflectance and Color of Concentrated Oil-in-Water Emulsions.

The influence of droplet and dye concentration on the optical properties of oil-in-water emulsions has been investigated. The spectral reflectance and tristimulus color coordinates (L, a, b) of a series of n-hexadecane oil-in-water emulsions with the same median droplet diameters ( approximately 0.3 µm), but different droplet concentrations (0.25 to 38.3 wt%) and red dye concentrations (0 to 0.2 wt%), were measured. Spectral reflectances decreased with increasing dye concentration and decreasing droplet concentration and had troughs at the same wavelengths as the peaks in the absorption spectra of the dyes. Emulsion L-values ("lightness") decreased and a- and b-values ("chromacity measures") increased as the dye concentration increased or the droplet concentration decreased. There was good qualitative agreement between the measured spectral reflectance of the emulsions and that predicted by the Kubelka-Munk theory of diffuse reflectance. Excellent agreement between theory and experiment could be obtained using an empirically determined correction-factor that accounts for cuvette effects. Copyright 1999 Academic Press.

Journal Article↗
PubMed1980

Elementary transition state theory of the Soret and Dufour effects.

The elementary transition state approach has been used to obtain a simple model theory for the Soret effect (thermal diffusion) and the Dufour effect. The flow of heat in the Dufour effect is identified as the transport of the enthalpy change of activation as molecules diffuse. The theory as now formulated applies only to thermodynamically ideal mixtures of substances with molecules of nearly equal size. The results of the theory conform to the Onsager reciprocal relationship. When the results were fit to data on thermal diffusion for two liquid systems, a close fit was obtained and yielded reasonable values of between 2 and 3 kcal mol(-1) for enthalpy changes of activation and differences between the entropy changes of activation for the two components of between 0 and 1 cal K(-1) mol(-1).

Journal Article↗
PubMed2001

Self-consistent mode-coupling theory for self-diffusion in quantum liquids.

A closed, self-consistent equation for the velocity autocorrelation function of a quantum liquid within the framework of a quantum mode-coupling theory is derived. The solution of the quantum generalized Langevin equation requires static input which is generated by an appropriate path-integral Monte Carlo scheme. In order to assess the accuracy of our approach we have studied the self-diffusion process of liquid para-hydrogen at two thermodynamic state points. Quantitative agreement for the self-diffusion constant is obtained in comparison to experimental measurements and other theoretical predictions.

Journal Article↗
PubMed2001

Self-consistent theory of rupture by progressive diffuse damage.

We analyze a self-consistent theory of crack growth controlled by a cumulative damage variable d(t) dependent on stress history, in the quasistatic regime where the sound wave velocity is taken as infinite. Depending upon the damage exponent m, which controls the rate of damage dd/dt~sigma(m) as a function of local stress sigma, we find two regimes. For 0 /=2, the rupture dynamics is not defined without the introduction of a regularizing scheme. We investigate three regularization schemes involving, respectively, a saturation of damage, a minimum distance of approach to the crack tip, and a fixed stress maximum. In the first and third schemes, the finite-time singularity is replaced by a crack dynamics defined for all times but which is controlled by either the existence of a microscopic scale at which the stress is regularized or by the maximum sustainable stress. In the second scheme, a finite-time singularity is again found. In the first two schemes within this regime m>/=2, the theory has no continuous limit.

Journal Article↗
PubMed2003

Time-resolved pulse propagation in a strongly scattering material.

Light transport in macroporous gallium phosphide, perhaps the strongest nonabsorbing scatterer of visible light, is studied using phase-sensitive femtosecond pulse interferometry. Phase statistics are measured at optical wavelengths in both reflection and transmission and compared with theory. The diffusion constant of light is measured in both reflection and transmission as a function of thickness and compared with theories for diffusive transport and localization. An unusually high energy velocity due to the bicontinuous structure of the porous network is reported. For such strongly scattering samples, we show that surface properties and the effective index of refraction need to be treated carefully.

Journal Article↗
PubMed1984

Homotopic callosal inhibition.

Previous theories concerning the function of the corpus callosum have included a "topographic" column-to-column excitatory theory and a diffuse "regional" inhibitory theory. Here it is shown that a topographic inhibitory model, in conjunction with a postulate concerning surround inhibition among cortical columns, produces complementary patterns of cortical column firing which, although qualitatively different, do not imply quantitative metabolic asymmetries. The cognitive implications of such homotopic inhibition are discussed.

Cerebral Cortex↗
PubMed1979

The binding of a neutral aromatic molecule to a negatively-charged lipid membrane. II. Kinetics and mechanism.

The kinetics of the binding of the fluorescence indicator N-phenyl naphthylamine to bilayer vesicles of C12-methyl-phosphatidic acid have been investigated by means of the temperature-jump relaxation technique utilizing fluorescence light detection. Single-exponential relaxation curves were observed, with time constants in the range 0.2-3 ms. The concentration dependence of the relaxation time yielded an apparent association rate constant (expressed in terms of monomeric phospholipid) of k(on) = 5 x 10(6) M(-1) s(-1) in aqueous solution at 25 degrees . The activation energy and viscosity dependence associated with the binding rate show that this process is actually diffusion-controlled. The theory of diffusion-controlled reactions then allows a determination of the average size of the bilayer vesicles and of the true rate constant for the association of the indicator molecules with the vesicles. Assuming spherical geometry for the vesicles, the values are: r(ves) = 190 A, which corresponds to 20000 lipid molecules per vesicle and k'(on) = 1 x 10(11) M(-1) s(-1) (25 degrees). The correctness of this size-determination was confirmed semi-quantitatively by electron microscopy. Since in fact a distribution of vesicle sizes must be present, a discussion is included of the relaxation function which the system is expected to take in the general case. Biological implications of diffusion control for the transport of non-polar substances and for lipid mixing are indicated.

Journal Article↗
PubMed2003

Effective methods to change sex-risk among drug users: a review of psychosocial interventions.

This review examines the current state of knowledge on the effectiveness of HIV prevention interventions with respect to sexual behavior among the population of drug users. The review focuses specifically on the relation between intervention methods found to be effective and the underlying theory. Electronic searches were conducted and supplemented by publications gathered through other channels. The studies were reviewed for (1) study design, (2) evident use of theory in intervention development, (3) clear targeting of determinants, (4) description of the study or studies, and (5) evaluation of the behavioral goals and targeted determinants. For each study, a description is given of: (1) the size and nature of the sample; (2) the retention rate; (3) the study design; (4) the nature of the intervention programs, including theoretical methods, practical strategies and theoretical background; (4) the measures of variability. The results show that a limited number of interventions were effective in changing sexual risk behavior among drug users. More successful programs featured several of the following elements: use of multiple theories and methods, inclusion of peers and rehearsal of skills. Moreover, the community-level interventions showed the importance of sustainability. The most successful intervention methods were modeling, skill building and social support enhancement. These methods are generally derived from the Social-Cognitive Theory or the Diffusion of Innovations Theory. Future HIV/AIDS interventions should build on the strengths discussed. Evaluations of interventions should be designed to facilitate comparison, using standardized and specific behavioral outcomes as well as standardized and preferably long-term follow-up levels, and should also evaluate the impact of programs at a psychosocial level to examine whether or not the theoretical methods on which a program was based were actually effective in changing the psychosocial factors targeted and why.

Community Health Services↗
PubMed2005

Structure and stability of Ne+He(n): experiment and diffusion quantum Monte Carlo theory with "on the fly" electronic structure.

New data are reported for the mass-spectrometry fragmentation patterns of helium clusters, either pure or containing a Ne or an Ar atom. The patterns for He(n)+ and Ar+He(n) show clear evidence of structure, while those of Ne+He(n) do not. To better understand the surprising result for the Ne+He(n) fragments, diffusion quantum Monte Carlo (DMC) calculations of the energies and structural properties of these ions were performed using a diatomics-in-molecule (DIM) parametrization of the potential energy. Using DIM for electronic energy evaluation allows us to sample 10(9) configurations even for a cluster as large as Ne+He14. The results of the DMC calculation are very surprising. For n > 7, the DMC random walkers rarely venture within 100 cm(-1) of the minimum potential energy. Analysis of the resulting particle density distributions shows that the zero-point energy does more than spread the wave function around the potential-energy minima, resulting in very diffuse wave functions. For some of the clusters the quantum effects nearly exclude the region of the potential minimum from the overall wave function. An important result of this effect is that the incremental bonding energy of the nth helium atom varies quite smoothly with n, for n > 5. This eliminates the expected shell structure and explains the lack of magic-number-type features in the data.

Journal Article↗
PubMed2006

Application of the Poisson-Nernst-Planck theory with space-dependent diffusion coefficients to KcsA.

The Poisson-Nernst-Planck electrodiffusion theory serves to compute charge fluxes and is here applied to the ion current through a protein channel. KcsA was selected as an example because of the abundance of experimental and theoretical data. The potassium channels MthK and KvAP were used as templates to define two open channel models for KcsA. Channel boundary surfaces and protein charge distributions were defined according to atomic radii and partial atomic charges. To establish the sensitivity of the results to these parameters, two different sets were used. Assigning the potassium diffusion coefficients equal to the value for free-diffusion in water (1.96 x 10(-9) m(2)/s), the computed currents overestimated the experimental data. Ion distributions inside the channel suggest that the overestimate is not due to an excess of charge shielding. A good agreement with the experimental data was achieved by reducing the potassium diffusion coefficient inside the channel to 1.96 x 10(-10) m(2)/s, a value of substantial motility but nonetheless in accord with the intuitive notion that the channel has a high affinity for the ions and therefore slows them down. These results are independent of the open channel model and the parameterization adopted for atomic radii and partial atomic charges. The method offers a reliable estimate of the channel current with low computational effort.

Bacterial Proteins↗
PubMed2000

Evaluation of a data warehouse in an academic health sciences center.

OBJECTIVES: The Clinical data repository (CDR) at the University of Virginia Health System is a data warehouse that provides direct access to data for clinical research and effective decision making. We undertook an evaluation of the CDR to understand factors affecting its adoption. DESIGN: We used a theoretical framework that is based on diffusion of innovation theory. Building on validated survey instruments, we developed a questionnaire and conducted interviews of key executive leaders. Fifty-three individuals with logon ids to the CDR completed our questionnaire. Twelve executive leaders were interviewed. MEASUREMENTS: The outcome variables were the initial and continued use of the CDR. Independent variables included attributes suggested by diffusion theory (i.e. relative advantage, complexity), knowledge and skills expected to correlate with computer usage, and the influence of communication channels. RESULTS: Our overall response rate was 82%. We identified characteristics of users associated with the initial decision to use the CDR. Compatibility with an individual's skills and work style was associated strongly with satisfaction and continued use. Secondly, the importance of organizational culture and the need for data was illuminated by management interviews. CONCLUSIONS: We have shown that diffusion of innovation theory can be used to help understand factors contributing to the success of a data warehouse in a healthcare setting. Our results suggest areas for future research and inquiry as the CDR evolves.

Academic Medical Centers↗
PubMedDate not supplied

Early adoption of BRCA1/2 testing: who and why.

PURPOSE: Relatively little is known about the characteristics of women who chose to undergo BRCA1/2 testing soon after testing became available, including how they became aware of and chose to pursue testing. Diffusion of innovation theory states that acceptance of an innovation is a function of the potential adopter's value for innovation and perceptions of the compatibility, complexity, and relative advantage of the innovation. These factors may contribute to early uptake of BRCA1/2 testing. The purpose of this study was to describe the characteristics of women who were "early adopters" of clinical BRCA1/2 testing and to determine whether diffusion theory explains variation in uptake of testing after participation in genetic counseling for BRCA1/2 testing. METHODS: A retrospective cohort study of participants in genetic counseling for BRCA1/2 testing was conducted at a clinical program in a large academic health system. Measures included components of diffusion of innovation theory (participant's value for innovation, i.e., "innovativeness," and perceptions of the compatibility, complexity, and relative advantage of BRCA1/2 testing), characteristics of how the participant became aware of and sought BRCA1/2 testing, and decisions about testing after counseling. RESULTS: From the 229 respondents, 71 (31%) had undergone testing at the time of the survey. Fifty-seven women (25%) had sought BRCA1/2 testing because a family member had breast or ovarian cancer and 37 (16%) because they had breast or ovarian cancer. Only 15 women (7%) reported seeking testing because of a physician's recommendation. After multivariate adjustment, higher innovativeness and higher ratings of the compatibility of BRCA1/2 testing were associated with undergoing testing after counseling [relative risk (RR) 1.76, 95% confidence interval (CI) 1.2-2.6]. However, ratings of the complexity or relative advantage of testing were not associated with testing decisions. Higher innovativeness was associated with being the first in the family to undergo testing (RR 4.85, 95% CI 1.6-14.9), becoming aware of BRCA1/2 testing through the media (RR 1.50, 95% CI 1.0-2.4), and being aware of BRCA1/2 testing prior to counseling (RR 1.25, 95% CI 1.1-1.4). CONCLUSIONS: The uptake of BRCA1/2 testing among women undergoing genetic counseling was associated with innovative characteristics of the participant and the perceived compatibility of the test with existing values and needs, but not with the complexity or relative advantage of the test. Most "early adopters" had heard of BRCA1/2 testing from a source other than their physician and had sought testing because of a personal or family member's cancer diagnosis. These findings can inform predictions surrounding the introduction of future genetic susceptibility tests and strategies for guiding the further diffusion of BRCA1/2 testing.

Decision Making↗
PubMed1996

Tracer diffusion through F-actin: effect of filament length and cross-linking.

We have determined diffusion coefficients for small (50- to 70-nm diameter) fluorescein-thiocarbamoyl-labeled Ficoll tracers through F-actin as a function of filament length and cross-linking. fx45 was used to regulate filament length and avidin/biotinylated actin or ABP-280 was used to prepare cross-linked actin gels. We found that tracer diffusion was generally independent of filament length in agreement with theoretical predictions for diffusion through solutions of rods. However, in some experiments diffusion was slower through short (< or = 1.0 micron) filaments, although this result was not consistently reproducible. Measured diffusion coefficients through unregulated F-actin and filaments of lengths > 1.0 micron were more rapid than predicted by theory for tracer diffusion through rigid, random networks, which was consistent with some degree of actin bundling. Avidin-induced cross-linking of biotinylated F-actin did not affect diffusion through unregulated F-actin, but in cases where diffusion was slower through short filaments this cross-linking method resulted in enhanced tracer diffusion rates indistinguishable from unregulated F-actin. This finding, in conjunction with increased turbidity of 1.0-micron filaments upon avidin cross-linking, indicated that this cross-linking method induces F-actin bundling. By contrast, ABP-280 cross-linking retarded diffusion through unregulated F-actin and decreased turbidity. Tracer diffusion under these conditions was well approximated by the diffusion theory. Both cross-linking procedures resulted in gel formation as determined by falling ball viscometry. These results demonstrate that network microscopic geometry is dependent on the cross-linking method, although both methods markedly increase F-actin macroscopic viscosity.

Actin Cytoskeleton↗
PubMed2006

Dynamically corrected transition state theory calculations of self-diffusion in anisotropic nanoporous materials.

We apply the dynamically corrected transition state theory to confinements with complex structures. This method is able to compute self-diffusion coefficients for adsorbate-adsorbent systems far beyond the time scales accessible to molecular dynamics. Two example cage/window-type confinements are examined: ethane in ERI- and CHA-type zeolites. In ERI-type zeolites, each hop in the z direction is preceded by a hop in xy direction and diffusion is anisotropic. The lattice for CHA-type zeolite is a rhombohedral Bravais lattice, and diffusion can be considered isotropic in practice. The anisotropic behavior of ERI-type cages reverses with loading, i.e., at low loading the diffusion in the z direction is two times faster than in the xy direction, while for higher loadings this changes to a z diffusivity that is more than two times slower. At low loading the diffusion is impeded by the eight-ring windows, i.e., the exits out of the cage to the next, but at higher loadings the barrier is formed by the center of the cages.

Journal Article↗
PubMed2004

Diffusion of innovations: a world tour.

This contribution to the 40th Anniversary celebration of the Diffusion of Innovations Theory discusses three health communication projects which applied the tenets of Diffusion of Innovation Theory with differing results: Using voodoo practitioners to pave the way for HIV/AIDS education in Haiti. A food-based approach to improving Vitamin A nutrition in Nepal. Diffusion at the horizon of life: The difficulties of communicating reproductive health to youth in Mali. The article illustrates a spectrum of circumstances in which diffusion theory has been applied, in order to show the application of the theory with different populations or target groups, in different sectors, and in different regions of the world.

Diffusion of Innovation↗
PubMed1997

The globalization of behavioral science evidence about battered women: a theory of production and diffusion.

A theoretical framework is proposed for understanding how the innovative use of behavioral science evidence is both produced and diffused among members of the global legal community. Using case law analyses and interviews with key individuals involved in selected cases, we examine how battered woman syndrome (BWS) is produced and diffused between and among Australia, Canada, England, and the United States. The following diffusion mechanisms are proposed: (1) The availability and accessibility of credible dissemination sources; (2) characteristics of the overall practice environment operating in each legal culture; (3) the attitudes and knowledge of attorneys and judges about the use of scientific evidence; (4) political and social support for the use of the evidence in the legal culture; and (5) the level of structural equivalence, communication, and "neighbor effects" between and among legal cultures. Each mechanism is discussed and supplemented with information from interviews with individuals involved in key cases involving BWS evidence.

Australia↗