Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “diffusion theory”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 955 records · Page 53Linked to original sources
PubMed2000

Communication through a diffusive medium: coherence and capacity

Coherent wave propagation in disordered media gives rise to many fascinating phenomena as diverse as universal conductance fluctuations in mesoscopic metals and speckle patterns in light scattering. Here, the theory of electromagnetic wave propagation in diffusive media is combined with information theory to show how interference affects the information transmission rate between antenna arrays. Nontrivial dependencies of the information capacity on the nature of the antenna arrays are found, such as the dimensionality of the arrays and their direction with respect to the local scattering medium. This approach provides a physical picture for understanding the importance of scattering in the transfer of information through wireless communications.

Journal Article↗
PubMedDate not supplied

Convergence of behaviour change models for AIDS risk reduction in sub-Saharan Africa.

There is increasing programme focus on the impact of HIV and AIDS and the advances in biomedical research that are taking central stage even in sub-Saharan Africa. Perhaps out of frustration, communication and behaviour change efforts are receiving lesser attention. But as long as heterosexual intercourse remains the predominant means of transmission of HIV in sub-Saharan Africa, there is the inherent danger of more people getting infected thus mopping up the gains from other approaches. This paper therefore argues for evidence based behaviour change interventions in sub-Saharan Africa, revisits the behaviour change theories and their relevance to the African situation. It proposes an integration of models based on the convergence of three existing theories of social learning, diffusion of innovation and social networks and the author's understanding of some African culture. The beauty of the model is based on the fact that 'social norms are best understood and influenced at the level of social networks'; existing chains of communication and the naturalness of exchange of information. A case is made for application of the 'model' to precipitate sexual behaviour change and HIV reduction in sub-Saharan Africa.

Acquired Immunodeficiency Syndrome↗
PubMed2002

On T(2)-shortening by strongly magnetized spheres: a partial refocusing model.

Computer simulations of water transverse relaxation induced by superparamagnetic particles are shown to disagree with the available theories, covering the slow diffusion domain. Understanding these new simulations, not in the slow diffusion domain, thus requires a new theoretical approach. A "partial refocusing model" is introduced for this purpose; it is based on a spatial division between an inner region where the gradients are too strong for the refocusing pulses to be efficient and an outer region where they are efficient. This model agrees with published simulations of relaxation induced by magnetic dipoles approximated as points. The validity domains of the various models are also compared.

Computer Simulation↗
PubMed1997

Surface Charging Mechanism and Disjoining Pressure of Electrolytic Soap Films

We present a theory developed to describe the diffuse double layer of electrolytic soap films. The hydrophobic nature of the soap molecules is modeled, in an effective field theory for the system, by the introduction of a free energetic potential favoring the presence of the soap molecules at the film surface. This potential can take various forms, depending on how accurately we wish to model the geometry of the system. The resulting theory is analyzed and leads to the estimation of the electrostatic interactions at the mean field level. A whole range of physical observables is predicted including the electrostatic component of the disjoining pressure isotherm and the Gibbs adsorption isotherm of the system. We compare the theory with accurate disjoining pressure measurements (using X-ray specular reflectivity) carried out on systems composed of sodium dodecyl sulphate (SDS) with two different monovalent electrolytes on a wide range of salinity. The theory appears to account particularly well for the experimental data. A comparison with the DLVO theory is given. Finally, we discuss the limitations of the theory and its modification due to the inclusion of dispersion forces. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1985

Dynamic properties of the backbone of an integral membrane polypeptide measured by 2H-NMR.

The 2H-NMR spectrum of the exchangeable hydrogens of the synthetic amphiphilic polypeptide, lys2-gly-leu24-lys2-ala-amide, was measured for the solid peptide at room temperature and, as a function of temperature, for the peptide incorporated into hydrated dipalmitoylphosphatidylcholine (DPPC) bilayers. This study is a prototype of a similar class of experiments which can be carried out on integral membrane proteins to characterize, quantitatively, the dynamic properties of integral membrane proteins. At temperatures below the DPPC gel-liquid crystalline phase transition, the 2H NMR spectrum was very similar to that of the solid peptide indicating that the peptide was immobilized in the lipid bilayer on the time scale (approximately equal to 10(-5) s) of the 2H-NMR measurements. The 2H-NMR spectrum above the phase transition corresponded to that expected from a peptide in the alpha-helical conformation reorienting rapidly about the symmetry axis of the alpha-helix. Measurements of the quadrupolar echo relaxation time, T2e, gave a quantitative measure of the correlation time, tau c, for this motion. The value of tau c decreased rapidly with increasing temperature as the fraction of DPPC molecules in the liquid crystalline phase increased, reaching a value of 2 X 10(-7) s above the phase transition. The observation of a characteristic minimum in T2e as the temperature was raised provided a definitive, quantitative interpretation of the T2e measurements. Using the known geometry of the peptide and the theory of uniaxial rotational diffusion, a value of eta = 1.1 poise was obtained for the effective viscosity of the membrane in close agreement with values obtained previously from transient linear dichroism measurements.

Calorimetry, Differential Scanning↗
PubMed1996

The enzymatic basis of information processing in the living cell.

Analogies are drawn between the computational aspects of information processing and the enzymatic events of cellular metabolism. The origin thereof is linked to the logic-elements involved in the reaction-diffusion processes of the metabolic operation. It is shown how such function entails a 'molecular machine' quality when associated with organized non-equilibrium energy sources in the cell.

Cells↗
PubMed2005

A note on the coating of an inclined plane in the presence of soluble surfactant.

We consider the flow of a thin liquid film coating an inclined plane in the presence of a soluble surfactant. A two-dimensional three-equation model is derived using lubrication theory in the rapid diffusion limit and then used to investigate the stability of the fluid height and the surfactant surface and bulk concentrations. We present solutions for an insoluble surfactant system, which are then contrasted with those obtained for a system containing a soluble surfactant; both transient growth and fully nonlinear two-dimensional simulation results are discussed. Our results indicate that the characteristics of the fingering phenomena which accompany the flow are altered by the effects of solubility. In particular, we find that these effects de-stabilise the system further over an intermediate range of surfactant solubility.

Journal Article↗
PubMed1993

Lateral diffusion in model membranes is independent of the size of the hydrophobic region of molecules.

We have systematically investigated the probe size and shape dependence of lateral diffusion in model dimyristoyl phosphatidylcholine membranes. Linear hydrophobic polymers, which differ in length by an order of magnitude, were used to explore the effect on the lateral diffusion coefficient of hydrodynamic restrictions in the bilayer interior. The polymers employed are isoprenoid alcohols--citronellol, solanesol, and dolichol. Tracer lateral diffusion coefficients were measured by fluorescence photobleaching recovery. Despite the large difference in lengths, the nitrobenzoxadiazole labelled alcohols all diffuse at the rate of lipid self-diffusion (5.0 x 10(-12) m2 s-1, 29 degrees C) in the liquid crystal phase. Companion measurements in isotropic polymer solution, in gel phase lipid membranes and with nonpolar fluorescent polyaromatic hydrocarbons, show a marked dependence of the lateral diffusion coefficient on the probe molecule size. Our results in the liquid crystal phase are in accord with free area theory which asserts that lateral diffusion in the membrane is restricted by the surface-free area. Probe molecules which are significantly longer than the host phospholipid, seven times longer in the case of dolichol, are still restricted in their lateral motion by the surface properties of the bilayer in the liquid crystal phase. Fluorescence quenching experiments indicate that the nitrobenzoxadiazole label does not reside at the aqueous interface, although it must reside in close proximity according to the diffusion measurements.

Diffusion↗
PubMed2001

The diluted aqueous solvation of carbohydrates as inferred from molecular dynamics simulations and NMR spectroscopy.

The purpose of this paper is to review our understanding of the dilute hydration (aqueous solvation) behaviour of disaccharide compounds. To this end we discuss and scrutinize the results that have been obtained for the three model disaccharides: maltose, sucrose and trehalose from experimental NMR studies and from theoretical molecular dynamics studies in explicit aqueous solutions. The focus is on the description of molecular hydration features that will influence macroscopic entities such as diffusion and relaxation: residence times of hydration waters, hydration numbers and hydration densities. The principles of molecular dynamics simulation are briefly outlined while a detailed presentation is given of the key features that characterise hydration: the solvation of the glycosidic linkage, the radial hydration of the solute, the water density anisotropy around the solute, the residential behaviour of water molecules in the periphery of the solute, and rotational and translational diffusion coefficients. With respect to the use of NMR in characterising the structure and dynamics of the hydration, the hydrodynamic theory of rotational and translational diffusion of biomolecules as well as the use of pulse field gradient spin echo experiments are briefly presented. The NMR-defined rotational diffusion coefficients (D(r)) and the experimentally determined translational diffusion (D(t)) coefficients are reported for 4% (w/w) solutions of sucrose, trehalose and maltose. These results are compared with theoretical data obtained from molecular dynamics simulations of sucrose, trehalose and maltose under identical conditions (concentration, temperature, etc.). With our present level of knowledge we can propose that although carbohydrates share a number of hydration characteristics, evidence is accumulating in support of the notion that it is not the amount or overall hydration but rather the detailed individual carbohydrate-water interaction that is likely to determine carbohydrate structure and functionality.

Carbohydrate Conformation↗
PubMed1992

Combined effect of coulombic and van der Waals interactions in the chromatography of proteins.

We have recently proposed a theoretical framework for the effect of the eluting salt ionic strength of the eluent on the retention factor of proteins in ion-exchange chromatography of proteins. It is based on the solution of the linearized Poisson-Boltzmann equation for two oppositely charged planar surfaces in contact with a salt solution and describes the coulombic interaction between the protein and the oppositely charged stationary-phase surface. At sufficiently high salt concentrations in the mobile phase van der Waals interactions between the protein and the stationary phase become important. In this work we consider the effect of salt on the combined coulombic and van der Waals interactions by combining the electrostatic theory with the theory for van der Waals interactions. The combined theory describes the retention of proteins as a function of eluting salt concentration over a wide salt concentration range. The protein molecules are, according to the proposed theory, held in a diffuse layer close to the stationary phase and are not in a distinct layer, which is assumed in the traditional thermodynamic interpretation of the capacity factor. For this reason, we also examine the thermodynamic interpretation of the capacity factor when it is due to distant dependent interactions.

Chromatography, Ion Exchange↗
PubMed2005

Generalized Langevin theory on the dynamics of simple fluids under external fields.

A theory on the time development of the density and current fields of simple fluids under an external field is formulated through the generalized Langevin formalism. The theory is applied to the linear solvation dynamics of a fixed solute regarding the solute as the external field on the solvent. The solute-solvent-solvent three-body correlation function is taken into account through the hypernetted-chain integral equation theory, and the time correlation function of the random force is approximated by that in the absence of the solute. The theoretical results are compared with those of molecular-dynamics (MD) simulation and the surrogate theory. As for the transient response of the density field, our theory is shown to be free from the artifact of the surrogate theory that the solvent can penetrate into the repulsive core of the solute during the relaxation. We have also found a large quantitative improvement of the solvation correlation function compared with the surrogate theory. In particular, the short-time part of the solvation correlation function is in almost perfect agreement with that from the MD simulation, reflecting that the short-time expansion of the theoretical solvation correlation function is exact up to t(2) with the exact three-body correlation function. A quantitative improvement is found in the long-time region, too. Our theory is also applied to the force-force time correlation function of a fixed solute, and similar improvement is obtained, which suggests that our present theory can be a basis to improve the mode-coupling theory on the solute diffusion.

Journal Article↗
PubMed1970

Surface charge and the conductance of phospholipid membranes.

Bilayer membranes, formed from various phospholipids, were studied to assess the influence of the charge of the polar head groups on the membrane conductance mediated by neutral "carriers" of cations and anions. The surface charge of an amphoteric lipid, phosphatidyl ethanolamine, was altered by varying the pH, and the surface charge of several lipids was screened by increasing the ionic strength of the solution with impermeant monovalent and divalent electrolytes. The surface charge should be a key parameter in defining the membrane conductance for a variety of permeation mechanisms; conductance measurements in the presence of carriers may be used to estimate the potential difference, due to surface charge, between the interior of the bilayer and the bulk aqueous phase. The large changes in conductance observed upon varying the surface charge density and the ionic strength agree with those predicted by the Gouy-Chapman theory for an aqueous diffuse double layer. Explicit expressions for the dependence of the membrane conductance on the concentrations of the carrier, the permeant ion, the surface charge density, and the ionic strength are presented.

Hydrogen-Ion Concentration↗
PubMed2000

Properties of hot-melt extruded theophylline tablets containing poly(vinyl acetate).

The objectives of this study were to investigate the properties of poly(vinyl acetate) (PVAc) as a retardant polymer and to study the drug release mechanism of theophylline from matrix tablets prepared by hot-melt extrusion. A physical mixture of drug, polymer, and drug release modifiers was fed into the equipment and heated inside the barrel of the extruder. The cylindrical extrudates were either cut into tablets or ground into granules and compressed with other excipients into tablets. Due to the low glass transition temperature of the PVAc, the melt extrusion process was conducted at approximately 70 degrees C. Theophylline was used as the model drug in this study. Theophylline was present in the extrudate in its crystalline form and was released from the tablets by diffusion. The Higuchi diffusion model and percolation theories were applied to the dissolution data to explain the drug release properties of the matrix systems. The release rate was shown to be dependent on the granule size, drug particle size, and drug loading in the tablets. Water-soluble polymers were demonstrated to be efficient release rate modifiers for this system.

Bronchodilator Agents↗
PubMed2003

The Ca-activated Cl channel and its control in rat olfactory receptor neurons.

Odorants activate sensory transduction in olfactory receptor neurons (ORNs) via a cAMP-signaling cascade, which results in the opening of nonselective, cyclic nucleotide-gated (CNG) channels. The consequent Ca2+ influx through CNG channels activates Cl channels, which serve to amplify the transduction signal. We investigate here some general properties of this Ca-activated Cl channel in rat, as well as its functional interplay with the CNG channel, by using inside-out membrane patches excised from ORN dendritic knobs/cilia. At physiological concentrations of external divalent cations, the maximally activated Cl current was approximately 30 times as large as the CNG current. The Cl channels on an excised patch could be activated by Ca2+ flux through the CNG channels opened by cAMP. The magnitude of the Cl current depended on the strength of Ca buffering in the bath solution, suggesting that the CNG and Cl channels were probably not organized as constituents of a local transducisome complex. Likewise, Cl channels and the Na/Ca exchanger, which extrudes Ca2+, appear to be spatially segregated. Based on the theory of buffered Ca2+ diffusion, we determined the Ca2+ diffusion coefficient and calculated that the CNG and Cl channel densities on the membrane were approximately 8 and 62 micro m-2, respectively. These densities, together with the Ca2+ diffusion coefficient, demonstrate that a given Cl channel is activated by Ca2+ originating from multiple CNG channels, thus allowing low-noise amplification of the olfactory receptor current.

Animals↗
PubMed2000

Transitions in pharmacy practice, part 5: walking the tightrope of change.

A systematic process for reformulating the practice-related attitudes and values of pharmacists to help them adapt to a new practice model is described. The key to motivating pharmacists to commit to practice change lies in fostering a change in intrinsically held professional attitudes and values, not in emphasizing a structured extrinsic reward system. As a systematic process by which managers can motivate their staff to change attitudes and values, the authors offer a customization of the taxonomy of learning in the affective domain proposed by Krathwohl, supplemented by contributions from the literature on the diffusion of innovation, dissonance theory, the transtheoretical model, and instructional psychology. Krathwohl's taxonomy shows affective learning as a five-level hierarchy with stages within each level. Managers can guide practitioners through the first four levels. Practitioners who have been socialized for the distributive model and who then adopt a new practice model such as pharmaceutical care will start by simply receiving information about the new model. Next, they will actively respond to learning about the model and begin to value it as desirable. As their regard for the new model grows, they will reorganize practice priorities. Managers can help pharmacists adopt a new practice model by guiding them through stages of attitude and value formation.

Models, Organizational↗
PubMed1998

Sudden infant death syndrome rates subsequent to the American Academy of Pediatrics supine sleep position.

OBJECTIVES: In April 1992, the American Academy of Pediatrics (AAP) recommended that healthy infants be positioned for sleep on their side or back to reduce the risk of Sudden Infant Death Syndrome (SIDS). The authors hypothesized three different forms of the intervention to examine the impact of the recommendation according to theory such as technology diffusion. Seasonality was included in the models to control its effect when testing. METHODS: Box and Tiao time-series intervention methodology was used to examine the effect of the AAP recommendation on SIDS rates. Sudden Infant Death Syndrome mortality data from Philadelphia and Chicago were examined separately for white and nonwhite populations over 32 quarters. RESULTS: Overall SIDS rates dropped significantly according to an abrupt effect from the intervention. However, the effect appeared to be gradually declining in Philadelphia but permanent in Chicago. In Philadelphia, a decline of 62.3% was estimated in whites in the first quarter after the intervention but decreased to only 5% in the last quarter of 1994. A decline of 35.8% was estimated in nonwhites in the first quarter after the intervention but decreased to only 9.4% in the last quarter of 1994. An abrupt and permanent decrease of 26.7% and 16.5% was found in Chicago for whites and nonwhites, respectively. CONCLUSIONS: Evidence of an abrupt adoption of the recommendation can be explained by the authority innovation decision made by the AAP. Some evidence was found that the effect is temporary, perhaps because physicians are reversing earlier decisions. The demonstrated methodology provides a powerful way to test naturally occurring interventions from quasiexperimental designs to test the impact of policy guidelines.

Chicago↗
PubMed2001

Fast Chebyshev-polynomial method for simulating the time evolution of linear dynamical systems.

We present a fast method for simulating the time evolution of any linear dynamical system possessing eigenmodes. This method does not require an explicit calculation of the eigenvectors and eigenfrequencies, and is based on a Chebyshev polynomial expansion of the formal operator matrix solution in the eigenfrequency domain. It does not suffer from the limitations of ordinary time-integration methods, and can be made accurate to almost machine precision. Among its possible applications are harmonic classical mechanical systems, quantum diffusion, and stochastic transport theory. An example of its use is given for the problem of vibrational wave-packet propagation in a disordered lattice.

Journal Article↗