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At least 487 records · Page 27Linked to original sources
PubMed2005

Stability of thin stagnant film on a solid surface with a viscoelastic air-liquid interface.

Linear stability analysis for a film on a solid surface with a viscoelastic air-liquid interface is presented. The interfacial dilatational and shear viscoelastic properties were described by Maxwell models. Dilatational and shear interfacial elasticity and viscosity were shown to improve film stability. When the interfacial rheological properties are extremely large or small, the maximum perturbation growth coefficient is shown to reduce to those for immobile and mobile interfaces respectively. Calculated values of maximum growth coefficient for thin film stabilized by 0.5% beta-lactoglobulin approached those of mobile films for thick (>2000 nm) and those for immobile films for thin (<100 nm) films respectively with the values lying between the two limits for intermediate film thicknesses.

Air↗
PubMed1978

Spatial structures in a reaction-diffusion system--detailed analysis of the "Brusselator".

Continuous dependence of spatially nonuniform concentration profiles for the 'Brussellator" reaction mechanims on the characteristic length of the system is given both for zero flux and fixed boundary conditions. Branches of solutions arising through primary bifurcation form closed curves. Secondary bifurcations giving rise to spatially asymmetric solutions exist for fixed boundary conditions. Results of a stability analysis of individual solutions are discussed. A method of composing complex spatial profiles for higher lengths from elementary solutions for smaller lenghts is suggested and tested in the case of zero flux boundary conditions. Emergence of subsequently more complex stable patterns in dependence on increasing length of the system suggests many similarities to gradual build up of complex morphogenetic patterns.

Biological Transport↗
PubMed2004

Instability of the interface between thin fluid films subjected to electric fields.

The effect of an externally applied electric field on the stability of the interface between two thin leaky dielectric fluid films of thickness ratio and viscosity ratio ris analyzed using a linear stability analysis in the long-wave limit. A systematic asymptotic expansion is employed in this limit to derive the coupled nonlinear differential equations describing the evolution of the position of the interface between the fluids and the interfacial free charge distribution. The linearized stability of these equations is determined and the effect of the ratio of the conductivities, dielectric constants, thicknesses, and viscosities on the wavenumber of the fastest growing mode, kmax, and the growth rate of the most unstable mode, smax, is examined in detail. Specific configurations considered in previous studies, such as a perfect dielectric-air interface, leaky dielectric-air interface, etc., emerge as limiting cases from the general formulation developed in this paper. Our results show that the viscosity ratio, mur, does not have any significant effect on kmax for the interface between perfect and leaky dielectric fluids. In marked contrast, however, mur is shown to have a significant effect on the interface between two leaky dielectrics. Increasing mur from 0.1 to 10 could decrease kmax up to a factor of 5. In general, our results show that the presence of nonzero conductivity in either one or both of the fluids has a profound influence on the length-scale characteristic of the linear instability: a reduction even by a factor of 1/50 in the length scale can be effected when compared to the interface between two perfect dielectrics. These predictions could have important implications in pattern formation applications in thin fluid films that employ electric fields. The variation of kmax and smax on the thickness ratio, beta, indicates in general that kmaxalpha(beta-apha), and smaxalpha(beta-theta), where the exponents alpha and theta (both >0) are found to depend only on the ratio of conductivities, and are largely independent of other system parameters.

Journal Article↗
PubMed2006

Design and stabilization of sampled-data neural-network-based control systems.

This paper presents the design and stability analysis of a sampled-data neural-network-based control system. A continuous-time nonlinear plant and a sampled-data three-layer fully connected feedforward neural-network-based controller are connected in a closed loop to perform the control task. Stability conditions will be derived to guarantee the closed-loop system stability. Linear-matrix-inequality- and genetic-algorithm-based approaches will be employed to obtain the largest sampling period and the connection weights of the neural network subject to the considerations of the system stability and performance. An application example will be given to illustrate the design procedure and effectiveness of the proposed approach.

Algorithms↗
PubMedDate not supplied

A contribution of sparse matrices tools to matrix population model analysis.

When one wants to study the dynamics of structured populations, matrix type models are widely used. The complexity of such models increases with the number of classes of individuals considered, and a mathematical analysis becomes difficult to perform when a further heterogeneous spatial structure is introduced. The use of efficient computational tools is also required for numerical simulations. In this note we deal with the contribution of sparse matrices tools which allow to numerically simulate and to numerically perform stability analysis of large sized structured population models including density dependence and spatial heterogeneities. Our main motivation through this work is to show that such numerical tools can be useful when mathematical methods become difficult to employ.

Age Factors↗
PubMed2000

Influence of stable Floquet exponents on time-delayed feedback control

The performance of time-delayed feedback control is studied by linear stability analysis. Analytical approximations for the resulting eigenvalue spectrum are proposed. Our investigations demonstrate that eigenbranches that develop from the stable Lyapunov exponents of the free system also have a strong influence on the control properties, either by hybridization or by a crossing of branches which interchanges the role of the leading eigenvalue. Our findings are confirmed by numerical analysis of two particular examples, the Toda and the Rossler models. More important is the verification by actual electronic circuit experiments. Here, the observed reduction of control domains can be attributed to these additional eigenvalue branches. The investigations lead to a thorough analytical understanding of the stability properties in time-delayed feedback systems.

Journal Article↗
PubMed2004

Resistive wall mode in collisionless quasistationary plasmas.

The stability analysis of the n=1 resistive wall mode is carried out for a simplified model of collisionless tokamak plasma. It is found that the trapped particle compressibility and the resonance between the mode and the precession drift frequency lead to a significant improvement of the beta stability limits. It is shown that, within the frame of the simplified model, the resistive wall mode can be fully suppressed and the plasma can be stable up to the wall beta limits for a slow plasma rotation.

Journal Article↗
PubMed2002

Stable drop shapes under disjoining pressure. II. Multiplicity and stability.

The stability of the contact line region as affected by the disjoining pressure has been analyzed by solving the augmented Young-Laplace equation. Because of the results in Part I (Zhang, X., Neogi, P., and Ybarra, R. M., J. Colloid Interface Sci.), we have concentrated on obtaining multiple solutions for the same set of conditions. As many as five solutions were obtained: drops that end in a thin film with uniform thickness and where the film shape oscillates, drops that end with microscopic contact angles, as well as uniform thin films of two different thicknesses. The results of linear stability analysis were used to show that most cases were unstable to infinitesimal disturbances. Only two stable drop shapes for the particular disjoining pressure investigated are stable, a thin film of constant thickness and a thin drop that ends in a film of same thickness. Both multiplicity and stability have been discussed here for the first time and shed considerable light on the role of the attractive and repulsive forces.

Journal Article↗
PubMed2006

Effect of predator density dependent dispersal of prey on stability of a predator-prey system.

This work presents a predator-prey Lotka-Volterra model in a two patch environment. The model is a set of four ordinary differential equations that govern the prey and predator population densities on each patch. Predators disperse with constant migration rates, while prey dispersal is predator density-dependent. When the predator density is large, the dispersal of prey is more likely to occur. We assume that prey and predator dispersal is faster than the local predator-prey interaction on each patch. Thus, we take advantage of two time scales in order to reduce the complete model to a system of two equations governing the total prey and predator densities. The stability analysis of the aggregated model shows that a unique strictly positive equilibrium exists. This equilibrium may be stable or unstable. A Hopf bifurcation may occur, leading the equilibrium to be a centre. If the two patches are similar, the predator density dependent dispersal of prey has a stabilizing effect on the predator-prey system.

Algorithms↗
PubMed2003

Investigation of a pathogenic mtDNA microdeletion reveals a translation-dependent deadenylation decay pathway in human mitochondria.

Human mtDNA is transcribed from both strands, producing polycistronic RNA species that are immediately processed. Discrete RNA units are matured by the addition of nucleotides at their 3' termini: -CCA trinucleotide is added to mt-tRNAs, whilst mt-rRNAs and mt-mRNAs are oligo- or polyadenylated, respectively. The cis-acting elements, enzymes and indeed the mechanisms involved in these processes are still largely uncharacterized. Further, the function of polyadenylation in promoting stability, translation or decay of human mt-mRNA is unclear. A microdeletion has been identified in a patient presenting with mtDNA disease. Loss of these two residues removes the termination codon for MTATP6 and sets MTCO3 immediately in frame. Accurate processing at this site still occurs, but there is a markedly decreased steady-state level of RNA14, the ATPase 8- and 6-encoding bi-cistronic mRNA unit, establishing that an mtDNA mutation can cause dysregulation of mRNA stability. Analysis of the polyadenylation profile of the processed RNA14 at steady state revealed substantial abnormalities. The majority of mutated RNA14 terminated with short poly (A) extensions and a second, partially truncated population, was also present. Initial maturation of mutated RNA14 was unaffected, but deadenylation occurred rapidly. Inhibition of mitochondrial protein synthesis showed that the deadenylation was dependent on translation. Finally, deadenylation was shown to enhance mRNA decay, explaining the decrease in steady-state RNA14. An hypothesis is presented to describe how an mtDNA mutation that results in the loss of a termination codon causes enhanced mt-mRNA decay by translation-dependent deadenylation.

Adenosine Triphosphatases↗
PubMed2004

Effect of sighs on breathing memory and dynamics in healthy infants.

Deep inspirations (sighs) play a significant role in altering lung mechanical and airway wall function; however, their role in respiratory control remains unclear. We examined whether sighs act via a resetting mechanism to improve control of the respiratory regulatory system. Effects of sighs on system variability, short- and long-range memory, and stability were assessed in 25 healthy full-term infants at 1 mo of age [mean 36 (range 28-57) days] during quiet sleep. Variability was examined using moving-window coefficient of variation, short-range memory using autocorrelation function, and long-range memory using detrended fluctuation analysis. Stability was examined by studying the behavior of the attractor with use of phase-space plots. Variability of tidal volume (VT) and minute ventilation (VE) increased during the initial 15 breaths after a sigh. Short-range memory of VT decreased during the 50 breaths preceding a sigh, becoming uncorrelated (random) during the 10-breath presigh window. Short-range memory increased after a sigh for the entire 50 breaths compared with the randomized data set and for 20 breaths compared with the presigh window. Similar, but shorter duration, changes were noted in VE. No change in long-range memory was seen after a sigh. Coefficient of variation and range of points located within a defined attractor segment increased after a sigh. Thus control of breathing in healthy infants shows long-range stability and improvement in short-range memory and variability after a sigh. These results add new evidence that the role of sighs is not purely mechanical.

Carbon Dioxide↗
PubMed2003

Determination of methylparaben, propylparaben, triamcinolone acetonide and its degradation product in a topical cream by RP-HPLC.

A novel reversed-phase high-performance liquid chromatographic (RP-HPLC) method was developed and validated for the determination of active component triamcinolone acetonide, its degradation product triamcinolone (occurring in formulation after long-term stability tests) and two preservatives presented in the cream-methylparaben and propylparaben, using hydrocortisone as an internal standard. The chromatographic separation was performed on a Supelco Discovery C18 column; the mobile phase for separation of all compounds consists of a mixture of acetonitrile and water (40:60 v/v). The analysis time was less than 9 min, at a flow rate of 0.6 mL min(-1) and detection at 240 nm. The method was found to be applicable for routine analysis (stability tests, homogeneity) in the pharmaceutical product topical cream Triamcinolon cream 0.1%.

Administration, Topical↗
PubMed2001

Linear and nonlinear information flow in spatially extended systems.

Infinitesimal and finite amplitude error propagation in spatially extended systems are numerically and theoretically investigated. The information transport in these systems can be characterized in terms of the propagation velocity of perturbations Vp. A linear stability analysis is sufficient to capture all the relevant aspects associated to propagation of infinitesimal disturbances. In particular, this analysis gives the propagation velocity VL of infinitesimal errors. If linear mechanisms prevail on the nonlinear ones Vp=VL. On the contrary, if nonlinear effects are predominant finite amplitude disturbances can eventually propagate faster than infinitesimal ones (i.e., Vp>VL). The finite size Lyapunov exponent can be successfully employed to discriminate the linear or nonlinear origin of information flow. A generalization of the finite size Lyapunov exponent to a comoving reference frame allows us to state a marginal stability criterion able to provide Vp both in the linear and in the nonlinear case. Strong analogies are found between information spreading and propagation of fronts connecting steady states in reaction-diffusion systems. The analysis of the common characteristics of these two phenomena leads to a better understanding of the role played by linear and nonlinear mechanisms for the flow of information in spatially extended systems.

Journal Article↗
PubMed1987

Environmental sampling and analysis for zeranol.

Due to concern over health effects associated with the use of zeranol, a sampling and analytical method to monitor zeranol in the work environment was developed. The collection strategy included air samples on FHLP filters, area samples on gauze pads, hand/dermal samples on gauze pads and bulk samples. The sampling method evaluation included choice of media, flow rate and duration for air samples, sample preparation, extraction efficiencies and sample stability. Analysis of the samples was accomplished by high performance liquid chromatography with identification and quantification by UV and fluorescence detection. The analytical method evaluation included separation of possible interfering compounds, calibration curves, limits of detection (LOD), limits of quantitation (LOQ), precision and accuracy. The tested analytical range was 0.01 to 150 micrograms/mL with a LOD of 0.007 microgram/mL and a LOQ of 0.02 microgram/mL using UV detection. For the fluorescence detector, the LOD was 0.05 microgram/mL and the LOQ was 0.15 microgram/mL. Peak confirmation was achieved by retention time and comparing the two detector responses. Use of this method provided the sensitivity and specificity to analyze environmental samples for zeranol in the workplace environment.

Air Pollutants, Occupational↗
PubMed1997

Reliability of delayed INR determination: implications for decentralized anticoagulant care with off-site blood sampling.

In order to develop decentralized anticoagulant care by off-site blood sampling and transport of samples to a centralized laboratory for International Normalized Ratio (INR) determination we have performed a direct comparative study of INR stability. Analysis was performed daily for 5 d using nine thromboplastins. The overall mean difference of INR after 3d was only 0.05 INR units for samples with a therapeutic INR. After 5 d there was a mean difference of 0.11 INR units with 'non-Manchester' reagents and 0.44 INR units with 'Manchester' reagents. With over-anticoagulated samples mean differences of 0.55-0.72 INR units were observed after 3 d and 1.16-2.46 INR units after 5 d. Although there was some variation in stability of results with different thromboplastins, the difference over time with each thromboplastin was much less than the difference between thromboplastins. In conclusion, there is no clinically significant change in INR when analysis is delayed for up to 3 d. Off-site blood sampling can accommodate a large increase in patient workload without a major revenue increase in primary care and with continued total quality management and central expert advice.

Blood Coagulation Disorders↗
PubMed2000

Structural stability of binding sites: consequences for binding affinity and allosteric effects.

During the course of biological function, proteins interact with other proteins, ligands, substrates, inhibitors, etc. These interactions occur at precisely defined locations within the protein but their effects are sometimes propagated to distal regions, triggering highly specific responses. These effects can be used as signals directed to activate or inhibit other sites, modulate interactions with other molecules, and/or establish inter-molecular communication networks. During the past decade, it has become evident that the energy of stabilization of the protein structure is not evenly distributed throughout the molecule and that, under native conditions, proteins lack global cooperativity and are characterized by the occurrence of multiple independent local unfolding events. From a biological point of view, it is important to assess if this uneven distribution reflects specific functional requirements. For example, are binding sites more likely to be found in well structured regions, unstable regions, or mixed regions? In this article, we have addressed these questions by performing a structure-based thermodynamic stability analysis of non-structurally homologous proteins for which high resolution structures of their complexes with specific ligands are available. The results of these studies indicate that for all 16 proteins considered, the binding sites have a dual character and are characterized by the presence of regions with very low structural stability and regions with high stability. In many cases the low stability regions are loops that become stable and cover a significant portion of low molecular weight ligands upon binding. For enzymes, catalytic residues are usually, but not always, located in regions with high structural stability. It is shown that this arrangement provides significant advantages for the optimization of binding affinity of small ligands. In allosteric enzymes, low stability regions in the regulatory site are shown to play a crucial role in the transmission of information to the catalytic site.

Allosteric Regulation↗
PubMed1992

Role of lamellar membrane structure in tether formation from bilayer vesicles.

A theoretical analysis is presented of the formation of membrane tethers from micropipette-aspirated phospholipid vesicles. In particular, it is taken into account that the phospholipid membrane is composed of two layers which are in contact but unconnected. The elastic energy of the bilayer is taken to be the sum of contributions from area expansivity, relative expansivity of the two monolayers, and bending. The vesicle is aspirated into a pipette and a constant point force is applied at the opposite side in the direction away from the pipette. The shape of the vesicle in approximated as a cylindrical projection into the pipette with a hemispherical cap, a spherical section, and a cylindrical tether with a hemispherical cap. The dimensions of the different regions of the vesicle are obtained by minimizing its elastic energy subject to the condition that the volume of the vesicle is fixed. The range of values for the parameters of the system is determined at which the existence of a tether is possible. Stability analysis is performed showing which of these configurations are stable. The importance of the relative expansion and compression of the constituent monolayers is established by recognizing that local bending energy by itself does not stabilize the vesicle geometry, and that in the limit as the relative expansivity modulus becomes infinitely large, a tether cannot be formed. Predictions are made for the functional relationships among experimentally observable quantities. In a companion report, the results of this analysis are applied to experimental measurements of tether formation, and used to calculate values for the membrane material coefficients.

Biophysical Phenomena↗
PubMed2001

Stability of periodically focused intense particle beams.

A stability analysis of periodically focused intense particle beams based on the beam envelope equation is performed. We show that (i) the scenario, as the focusing field increases, is not the existence of a single threshold above which stable matched (equilibrium) solutions are absent, as generally believed, but the existence of successive regions of stability interrupted by gaps of instability; (ii) the beam can be focused to tighter radii using new stable matched solutions found for focusing field strengths greater than the previous threshold. Self-consistent simulations validate the findings.

Journal Article↗