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PubMed2003

From the Cover: Segmented spiral waves in a reaction-diffusion system.

Pattern formation in reaction-diffusion systems is often invoked as a mechanism for biological morphogenesis. Patterns in chemical systems typically occur either as propagating waves or as stationary, spatially periodic, Turing structures. The spiral and concentric (target) waves found to date in spatially extended chemical or physical systems are smooth and continuous; only living systems, such as seashells, lichens, pine cones, or flowers, have been shown to demonstrate segmentation of these patterns. Here, we report observations of segmented spiral and target waves in the Belousov-Zhabotinsky reaction dispersed in water nanodroplets of a water-in-oil microemulsion. These highly ordered chemical patterns, consisting of short wave segments regularly separated by gaps, form a link between Turing and trigger wave patterns and narrow the disparity between chemistry and biology. They exhibit aspects of such fundamental biological behavior as self-replication of structural elements and preservation of morphology during evolutionary development from a simpler precursor to a more complex structure.

Biophysical Phenomena↗
PubMed1973

Temperature dependence of nonelectrolyte permeation across red cell membranes.

The temperature dependence of permeation across human red cell membranes has been determined for a series of hydrophilic and lipophilic solutes, including urea and two methyl substituted derivatives, all the straight-chain amides from formamide through valeramide and the two isomers, isobutyramide and isovaleramide. The temperature coefficient for permeation by all the hydrophilic solutes is 12 kcal mol(-1) or less, whereas that for all the lipophilic solutes is 19 kcal mol(-1) or greater. This difference is consonant with the view that hydrophilic molecules cross the membrane by a path different from that taken by the lipophilic ones. The thermodynamic parameters associated with lipophile permeation have been studied in detail. DeltaG is negative for adsorption of lipophilic amides onto an oil-water interface, whereas it is positive for transfer of the polar head from the aqueous medium to bulk lipid solvent. Application of absolute reaction rate theory makes it possible to make a clear distinction between diffusion across the water-red cell membrane interface and diffusion within the membrane. Diffusion coefficients and apparent activation enthalpies and entropies have been computed for each process. Transfer of the polar head from the solvent into the interface is characterized by DeltaG(double dagger) = 0 kcal mol(-1) and DeltaS(double dagger) negative, whereas both of these parameters have large positive values for diffusion within the membrane. Diffusion within the membrane is similar to what is expected for diffusion through a highly associated viscous fluid.

Amides↗
PubMed2006

Demographic effects of the introduction of steroid contraception in developed countries.

The use of the contraceptive pill increased very rapidly in the 1970s in many developed countries, and fertility almost simultaneously started to decline. We discuss here the possibility of a causal link between these two major changes. We first provide evidence for a relationship between the spread of oral contraceptive use and the change in fertility in many European countries over the last three or four decades. The situation of specific countries is examined more in depth on the basis of available literature. We then review the various theories attempting to explain these trends and see how the family planning variables are treated in these approaches. At the country level, the conclusion is unambiguous: within individual countries, there is no systematic negative correlation between fertility and contraceptive pill use. The development of hormonal contraception cannot be considered as responsible for either starting or the size of the fertility decline. A more subtle chain of causality must be considered, but there is no agreement on a general theory of fertility changes. Most authors, however, agree that the diffusion of modern contraception has certainly contributed to the reduction in the number of unwanted pregnancies and has also facilitated and favoured the adoption of new (more restrictive) norms for the ideal family size.

Adolescent↗
PubMed2002

Effects of disorder on the wave front depinning transition in spatially discrete systems.

Pinning and depinning of wave fronts are ubiquitous features of spatially discrete systems describing a host of phenomena in physics, biology, etc. A large class of discrete systems is described by overdamped chains of nonlinear oscillators with nearest-neighbor coupling and subject to random external forces. The presence of weak randomness shrinks the pinning interval and it changes the critical exponent of the wave front depinning transition from 1/2 to 3/2. This effect is derived by means of a recent asymptotic theory of the depinning transition, extended to discrete drift-diffusion models of transport in semiconductor superlattices and is confirmed by numerical calculations.

Journal Article↗
PubMed2002

Cavitation of Langmuir monolayers.

Cavitation in liquid expanded and liquid condensed Langmuir monolayers induced by laser heating or microbubble coalescence is studied experimentally using fluorescence and Brewster angle microscopy. The kinetics of hole closure of two-dimensional (2D) gaseous cavitation bubbles exhibits a decelerated dynamics for cavities surrounded by a liquid expanded phase and an accelerated dynamics for cavities in a liquid condensed phase. Most of the cavities in liquid condensed phases possess a nonconvex shape and do not close. The results are compared with theoretical predictions derived for 2D cavitation of liquid monolayers of different surface shear viscosities, and for solid monolayers with diffusive flux of vacancies and interstitials. While part of the theory is in qualitative agreement with the experiment, the experimentally observed hole persistence within the liquid condensed phases and the hole closure within liquid expanded phases remains to be explained. The technique of microbubble coalescence might be particularly useful for the study of the rheological properties of hexatic phases.

Journal Article↗
PubMed2003

Anisotropic thermally activated diffusion in percolation systems.

We present a study of static and frequency-dependent diffusion with anisotropic thermally activated transition rates in a two-dimensional bond percolation system. The approach accounts for temperature effects on diffusion coefficients in disordered anisotropic systems. Static diffusion shows an Arrhenius behavior for low temperatures with an activation energy given by the highest energy barrier of the system. From the frequency-dependent diffusion coefficients, we calculate a characteristic frequency omega(c) approximately 1/t(c), related to the time t(c) needed to overcome a characteristic barrier. We find that omega(c) follows an Arrhenius behavior with different activation energies in each direction.

Anisotropy↗
PubMed2003

Transport properties of dense fluid argon.

We calculate using molecular dynamics simulations the transport properties of realistically modeled fluid argon at pressures up to approximately 50 GPa and temperatures up to 3000 K. In this context, we provide a critique of some newer theoretical predictions for the diffusion coefficients of liquids and a discussion of the Enskog theory relevance under two different adaptations: modified Enskog theory and effective diameter Enskog theory. We also analyze a number of experimental data for the thermal conductivity of monoatomic and small diatomic dense fluids.

Journal Article↗
PubMed2004

Stationary dynamics approach to analytical approximations for polymer coexistence curves.

Phase separation in polymer blends is an important process. However, the compositions of the coexisting phases can only be predicted by numerical methods. We provide simple analytical expressions which serve as good approximations for the compositions after phase separation of binary homopolymer blends. These approximations are obtained by a stationary dynamics approach: we calculate the compositions of two polymer mixtures such that the stationary diffusion between these distinguishable mixtures vanishes. For the diffusion equations we employ composition-dependent diffusion coefficients, as derived according to the slow- and fast-mode theory from the Flory-Huggins free energy. The analytical results are in good agreement with exact (numerically calculated) binodal compositions. Our coexistence curves are more accurate than some conventional approximations. Another advantage of the stationary dynamics approach is that it is not only applicable to binary polymer blends or polymer solutions, but also to symmetrical multicomponent blends. The same diffusion coefficients may be used to obtain the exact spinodal compositions in multicomponent systems.

Journal Article↗
PubMed2005

Conformational properties of an adsorbed charged polymer.

The behavior of a strongly charged polymer adsorbed on an oppositely charged surface of a low-dielectric constant is formulated by the functional integral method. By separating the translational, conformational, and fluctuational degrees of freedom, the scaling behaviors for both the height of the polymer and the thickness of the diffusion layer are determined. Unlike the results predicted by scaling theory, we identified the continuous crossover from the weak compression to the compression regime. All the analytical results are found to be consistent with Monte Carlo simulations. Finally, an alternative (operational) definition of a charged polymer adsorption is proposed.

Journal Article↗
PubMed2006

Diffusive atomistic dynamics of edge dislocations in two dimensions.

The fundamental dislocation processes of glide, climb, and annihilation are studied on diffusive time scales within the framework of a continuum field theory, the phase field crystal model. Glide and climb are examined for single edge dislocations subjected to shear and compressive strain, respectively, in a two-dimensional hexagonal lattice. It is shown that the natural features of these processes are reproduced without any explicit consideration of elasticity theory or ad hoc construction of microscopic Peierls potentials. Particular attention is paid to the Peierls barrier for dislocation glide or climb and the ensuing dynamic behavior as functions of strain rate, temperature, and dislocation density. It is shown that the dynamics are accurately described by simple viscous motion equations for an overdamped point mass, where the dislocation mobility is the only adjustable parameter. The critical distance for the annihilation of two edge dislocations as a function of separation angle is also presented.

Journal Article↗
PubMed2004

Helium diffusion through H2O and D2O amorphous ice: observation of a lattice inverse isotope effect.

The diffusion of He through both H2O and D2O amorphous solid water (ASW) has been measured between 55 and 110 K. We find the diffusion rate is dependent on the isotopic composition of the ASW lattice. This lattice isotope effect is the "inverse" of a normal isotope effect in that diffusion is faster in the heavier (D2O) isotope. Transition state theory calculations show that the inverse isotope effect is due to a tight transition state and predominantly arises from the zero-point vibrational energy associated with the frustrated rotational modes of water in the lattice.

Journal Article↗
PubMed2004

Eikonal formulation of the minimal principle for scroll wave filaments.

Recently, Wellner et al. [Proc. Natl. Acad. Sci. U.S.A. 99, 8015 (2002)]] proposed a principle for predicting a stable scroll wave filament shape as a geodesic in a 3D space with a metric determined by the inverse diffusivity tensor of the medium. Using the Hamilton-Jacobi theory we show that this geodesic is the shortest path for a wave propagating through the medium. This allows the use of shortest path algorithms to predict filament shapes, which we confirm numerically for a medium with orthotropic anisotropy. Our method can be used in cardiac tissue experiments since it does not require knowledge of the tissue anisotropy.

Algorithms↗
PubMed2004

Synergy of electron-cyclotron and lower-hybrid current drive in steady-state plasmas.

Improvement (up to a factor of approximately 4) of the electron-cyclotron (EC) current drive efficiency in plasmas sustained by lower-hybrid (LH) current drive has been demonstrated in stationary conditions on the Tore Supra tokamak. This was made possible by feedback controlled discharges at zero loop voltage, constant plasma current, and constant density. This effect, predicted by kinetic theory, results from a favorable interplay of the velocity space diffusions induced by the two waves: the EC wave pulling low-energy electrons out of the Maxwellian bulk, and the LH wave driving them to high parallel velocities.

Journal Article↗
PubMed2006

Observation of strong quantum depletion in a gaseous Bose-Einstein condensate.

We studied quantum depletion in a gaseous Bose-Einstein condensate. An optical lattice enhanced the atomic interactions and modified the dispersion relation resulting in strong quantum depletion. The depleted fraction was directly observed as a diffuse background in the time-of-flight images. Bogoliubov theory provides a semiquantitative description for our observations of depleted fractions in excess of 50%.

Journal Article↗
PubMed1980

Juvenile periodontitis.

Our knowledge of juvenile periodontitis is still fragmentary. In 50 years we have advanced from the concept of diffuse atrophy of the alveolar bone (Gottlieb 1923) through the theory of non-inflammatory, degenerative disease of the periodontium (Orban & Weinmann 1942) to the present conception of juvenile periodontitis (Manson & Lehner 1974, Waerhaug 1977a) as a periodontal disease appearing in young individuals with inflammation always present. Only the clinical picture of the disease is quite clear as Baer described it (1971): rapid destruction of the alveolar bone, not commensurate with the local irritants, around more than one permanent tooth in otherwise healthy adolescents. The etiology and etiopathogenesis of juvenile periodontitis have remained unknown. The bacteriological findings of Scransky et al. (1970) and Newman et al. (1974), suggesting some Gram-negative rods as an etiological factor, are still controversial. Neither is the theory of Lehner and his coworkers (1974), that juvenile periodontitis is a selective, cell-mediated immunodeficiency condition, fully accepted. Heredity is an etiologic factor for which there is more evidence. Several authors have found a familial pattern of the disease and it might be either an autosomal, recessive trait (Fourel 1972, Jorgenson et al. 1975) or an X-linked dominant disease (Melnick et al. 1976). These two statements are, however, also controversial. The prevalence rates vary from 0.1% to 17.6%. Although there could be racial variations, the estimated prevalence rates also vary within racial groups, suggesting that there must be great variations in methods and diagnostics. Juvenile periodontitis seems to exist in all racial groups throughout the world and although comparable prevalence figures are difficult to obtain, it seems that the disease is less common in Caucasoid populations and more frequent in India and Africa. The claim of female preponderence requires further investigation.

Adolescent↗
PubMed2004

Transient temperature rise due to ultrasound absorption at a bone/soft-tissue interface.

Thermal effects due to high ultrasound absorption in bone pose an ongoing safety issue. Of considerable concern is the heating of the soft tissue adjacent to the bone surface. Mathematical models can be useful in predicting the transient temperature near the interface during insonation. This paper develops a model that provides the temperature field in terms of simple expressions that convey the functional dependence of the material properties, and are easily incorporated into standards and ultrasound machine software, yet are able to incorporate the material properties of both bone and soft tissue. The model contains an asymptotic theory based upon a "high-attenuation" assumption: the distance diffused by heat over the time of interest is large compared to the ultrasound attenuation length. Model predictions of temperature rise and location of maximum temperature were in close agreement with finite-element calculations, using parameters appropriate for radiation-force imaging and focused-ultrasound surgery.

Animals↗
PubMed2003

Gas diffusion in zeolite beds: PFG NMR evidence for different tortuosity factors in the Knudsen and bulk regimes.

Self-diffusion of ethane in beds of zeolite NaX is studied using Pulsed Field Gradient (PFG) NMR. The ethane diffusivities were measured for displacements, which are orders of magnitude larger than the size of individual crystals. These diffusivities were compared with those, calculated using simple gas kinetic theory. The results of the comparison indicate that for the same bed of NaX crystals the apparent tortuosity factor in the Knudsen regime ( i.e. when molecule-solid collisions dominate) is significantly larger than that in the bulk regime ( i.e. when molecule-molecule collisions dominate). This finding is attributed to the more pronounced geometrical trapping by the pore structure of the zeolite bed in the Knudsen than in the bulk regime.

Journal Article↗
PubMed1993

On the structure of electrical double layer of IgG immobilized on polystyrene microspheres.

Latex particles used in the development of immunoassays usually present stability problems when the antibody is attached to the surface. This work is an attempt to know the potential distribution parameters around these complexes by electrokinetic measurements. The conversion of mobility data into zeta-potential was carried out by different theoretical approaches developed by Smoluchowski (classical equation), O'Brien and White, and Dukhin and Semenikhin. The zeta-potential calculated with allowance for electrical double layer (e.d.l.) polarization was substantially greater than zeta calculated according to the classical electrokinetic theory. The greater values of zeta D-S in comparison with zeta O-W were readily explained on the basis that in the first theory, the contribution to polarization from all ions of the diffuse layer was taken into account, whereas O'Brien and White accounted for only the ions of the hydrodynamically mobile part of the e.d.l. This is an indication that the anomalous surface conductance of latex particles coated by IgG molecules is much higher than that of bare latex particles.

Adsorption↗