Aspects of cell production in mantle tissue of Ciona intestinalis L. (Tunicata, Ascidiacea)

Renewal of cell population is needed in the tunic of ascidians, as the tunic cells are involved in many biological functions. Tunic cells are thought to arrive by migrating across the mantle epithelium into the tunic from the blood lacunae or the mesenchymal space. Electron microscope observations show that the mantle epithelium of Ciona intestinalis shares some proliferative characteristics, releasing cells into the tunic and thus providing an increase renewal of tunical cells in restricted zones of adult animals.     

A statistical fMRI model for differential T2* contrast incorporating T1 and T2* of gray matter

Relaxation parameter estimation and brain activation detection are two main areas of study in magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI). Relaxation parameters can be used to distinguish voxels containing different types of tissue whereas activation determines voxels that are associated with neuronal activity. In fMRI, the standard practice has been to discard the first scans to avoid magnetic saturation effects. However, these first images have important information on the MR relaxivities for the type of tissue contained in voxels, which could provide pathological tissue discrimination. It is also well-known that the voxels located in gray matter (GM) contain neurons that are to be active while the subject is performing a task. As such, GM MR relaxivities can be incorporated into a statistical model in order to better detect brain activation. Moreover, although the MR magnetization physically depends on tissue and imaging parameters in a nonlinear fashion, a linear model is what is conventionally used in fMRI activation studies. In this study, we develop a statistical fMRI model for Differential T₂^? ConTrast Incorporating T₁ and T₂^? of GM, so-called DeTeCT-ING Model, that considers the physical magnetization equation to model MR magnetization; uses complex-valued time courses to estimate T₁ and T₂^? for each voxel; then incorporates gray matter MR relaxivities into the statistical model in order to better detect brain activation, all from a single pulse sequence by utilizing the first scans.     

Generalised model for anisotropic compact stars

In the present investigation an exact generalised model for anisotropic compact stars of embedding class 1 is sought with a general relativistic background. The generic solutions are verified by exploring different physical aspects, viz. energy conditions, mass–radius relation, stability of the models, in connection to their validity. It is observed that the model presented here for compact stars is compatible with all these physical tests and thus physically acceptable as far as the compact star candidates RXJ 1856-37, SAX J 1808.4-3658 (SS1) and SAX J 1808.4-3658 (SS2) are concerned.     

θ renormalization, electron-electron interactions and super universality in the quantum Hall regime

The renormalization theory of the quantum Hall effect relies primarily on the non-perturbative concept of θ renormalization by instantons. Within the generalized non-linear σ model approach initiated by Finkelstein we obtain the physical observables of the interacting electron gas, formulate the general (topological) principles by which the Hall conductance is robustly quantized and derive—for the first time—explicit expressions for the non-perturbative (instanton) contributions to the renormalization group β and γ functions. Our results are in complete agreement with the recently proposed idea of super universality which says that the fundamental aspects of the quantum Hall effect are all generic features the instanton vacuum concept in asymptotically free field theory.     

Effect of the reversibility of the chemical reaction on triple flames

We examine a new aspect of triple flames, namely the effect of the reversibility of the chemical reaction on flame propagation. The study is carried out in the configuration of the two-dimensional strained mixing layer formed between two opposing streams of fuel and oxidiser. The chemical reaction is modelled as a single reversible reaction following an Arrhenius law in the forward and backward directions. The problem is formulated within the constant-density (thermo-diffusive) approximation, the main non-dimensional parameters relevant to this study being a reversibility parameter R (equal to zero in the irreversible case), a non-dimensional measure of the strain rate ? and a stoichiometric parameter S. We provide analytical (asymptotic) expressions for the local burning speed of the triple flame in terms of ?, S, and R. In particular we describe how the propagation speed of the front is decreased by an increase in R and how the location of its leading edge is affected by reversibility. For example, it is found that the leading edge shifts towards the fuel stream for S > 1 and towards the oxidiser if S < 1, as R is increased. A detailed numerical study is conducted covering all propagation regimes ranging from weakly stretched positively propagating (ignition) fronts to thick negatively propagating (extinction) fronts. In the weakly stretched cases we show that the numerics are in good agreement with the asymptotic findings. Furthermore, the results allow the determination of the domains of the distinct propagation regimes, mainly in the terms of R and ?. In line with our physical intuition, it is found that reversibility reduces the domain of ignition fronts and promotes extinction. The results provide a systematic investigation which can be considered as a first step when considering a more realistic chemistry, or poorly explored aspects (such as the existence of a temperature gradient in the unburnt mixture), when analyzing triple flames.     

Structural stability in Aurivillius phases based on ab initio thermodynamics

The Aurivillius oxide family possessess various attractive physical properties and have been commercially applicable in many areas. The regular large-period phases of this family are believed to offer better flexibilities of performance optimization than the commercial ones. However, the experimental synthesis of them is still challenging and the physical mechanisms of their structural stability are not yet clear. We propose a quasi-binary solid solution approach to study the structural stability of Aurivillius phases based on ab initio calculations. Three typical homologous series, CaBi₂Na_m-2Nb_mO_3m+3 and M_m-3Bi₄Ti_mO_3m+3 (M=Ca, Sr) with different in-plane lattice mismatches and chemical compositions, are studied. Our results prove that all the three systems are thermodynamically stable up to m=7 phases. By decomposing the formation process into two separate steps and analyzing the contributions to Gibbs formation energy from different factors, the configurational entropy as well as chemical bond optimization is suggested to play a crucial role in stabilizing the final phases, together with the elastic effect that was solely considered as the dominant factor before.     

Ghost-free non-Abelian gauge theory: renormalization and gauge-invariance

Although it has been known for a long time that the special case n^μA_μ = 0 for an axial gauge of a vector field A_μ, characterized by a direction n_μ, is free from the peculiar loop complications inherent in all other known gauges of non-Abelian gauge theories, practical use of this ghost-free gauge has often met with some reserve. The reasons were always difficulties in the development of the theoretical formalism, all of which can be traced back to a singularity at n^μp_μ = 0 where p is some four-momentum. This paper, which is a sequel to an earlier one by one of the authors, is intended to show that within the functional integration formalism a consistent field theory can be developed. Here we first prove the gauge invariance of the renormalized theory, allowing for the presence of an arbitrary number of scalar and fermion fields with spontaneous symmetry breaking. Then it is shown that all on-shell elements for the physical S-matrix between properly selected physical sources are independent of n_μ (gauge invariant) and so are the renormalized masses.     

Basic concepts for nuclear magnetic resonance imaging

The basic concepts necessary to understand the physical basis of NMR imaging are presented in this didactic article. It is intended as a starting point for the radiologist or medical physicist who is addressing the topic of NMR for the first time. The basis of the NMR phenomena is described with introduction of the concepts of magnetic moment, magnetic fields, magnetic resonance, net magnetic moment of a sample, NMR excitation and NMR emission. The equipment necessary to observe these NMR properties of matter is summarized as well as the procedures for basic pulsed NMR experiments. The physical concepts for spatial localization of NMR emissions are introduced with physical analogies to stringed musical instruments. Several alternative imaging modalities are compared with greatest emphasis on the inversion recovery technique which yields images weighted by tissue T₁ values. The six subsystems of an NMR imaging device (primary magnet, computer, radio equipment, magnetic gradient, data storage and display subsystems) are described in an overview fashion. The paper is followed by a series of study questions to test the reader's comprehension of basic NMR imaging concepts.     

The dynamics of RNA participation in the vitellogenesis of Rhipicephalus sanguineus ticks Latreille 1806 (Acari:Ixodidae). I. Nucleoli or Cajal bodies?

To understand the morphological and histological aspects of internal systems of ticks has become important matter since these arthropods have an impact in the areas of the economy and public health. In this context, this study has provided morphological data on female germinative cells of Rhipicephalus sanguineus ticks, ectoparasites of dogs that maintain a close relationship with human on a daily basis. Oocytes of engorged females were analyzed, through the PAS reaction (detection of polysaccharides) counterstained by methyl green (detection of RNA) revealing information that allowed to infer for the first time the presence of Cajal bodies, in the germinal vesicles (nuclei) of developing oocytes, as well as showing how the RNA and the polysaccharides are involved in the dynamics of the vitellogenesis in this species.     

On the irreducible representations of the lorentz group

All continuous irreducible representations of the SL(2, C) group (as given by Naimark) are obtained by means of methods developed by Harish-Chandra and Kihlberg. The analysis is done in the SU(2) basis and a single closed expression for the matrix elements of the noncompact generators for an arbitrary irreducible representation of SL(2, C) is given. For the unitary irreducible representations the scalar product for each irreducible Hilbert space is found explicitly. The connection between the unitary irreducible representations of SL(2, C) and those of

is discussed by means of Inönü and Wigner contraction procedure and the Gell-Mann formula. Finally, due to physical interest, the addition of a four-vector operator to SL(2, C) unitary irreducible representations in a minimal way is considered; and all group extensions of the parity and time reversal operators by SL(2, C) are explicitly obtained and some aspects of their representations are treated.     

Premiers instants du refroidissement d'une goutte métallique après son impact sur une paroi

This paper presents an experimental study of the evolution of thermal contact conditions (temperature jump at the interface, heat flux and transient thermal contact resistance) during the first stages of solidification of a liquid metal drop on a water-cooled wall. Two complementary approaches were developed. The first method, valid for times longer than 10⁻³ s, is based on the use of one temperature measurement inside the solidifiying drop, using a usual thermocouple, and several temperature measurements in the wall using very fine semi-intrinsic thermocouples installed near the active surface. By solving an inverse heat conduction problem in both regions (drop, wall), this method allows a local thermal characterization. The second approach uses an intrinsic method, based on the use of metal drop and wall as two thermocouple elements, to measure the mean micro-contact temperature of drop and wall. This kind of sensor, without inertia, has approximatively the time delay of fast data acquisition (t_c < 10⁻⁶s). The measured temperature represents the mean surface temperature of all the contact areas of drop and wall. The physical interpretation of this kind of measurement is difficult and needs the development of a new theoretical model. We can already observe that at t = 0⁺, the measured temperature is close to the theoretical one, ie, the interface temperature contact of two semi-infinite media, the liquid metal drop and the wall (effusivity ratio). This means that solidification has not yet taken place.     

The synthesis of cost optimal heat exchanger networks: An industrial review of the state of the art

The industrial heat exchanger network synthesis (HENS) problem is very complex and involves combinatorial problems in the “matching” between hot and cold streams to enhance heat recovery, temperature dependent physical and transport properties, the choice of flow configuration and materials of construction for the heat exchangers, the combination of “hard” and “soft” problem data (some target temperatures must be met, while others may be varied within limits if this is of advantage for the total process economy), various kinds of constraints (forbidden and compulsory matches) and different types of streams (liquid, vapour and mixed phase). Pressure drop limitations and the cost of piping are also important.The design objective includes a quantitative part (cost of heat exchange equipment and external utilities) and a qualitative part (safety, operability, flexibility and controllability). This makes it difficult to establish a single objective function to evaluate the design. Due to topological effects (services are added or removed), the investment cost function exhibits discontinuities since there is a unit cost involved in the equipment. Some of the qualitative aspects mentioned above cannot easily be formulated ahead of time. The cost of flexibility can be calculated, but only for given situations (networks). The global optimum is thus hard to guarantee and the engineer has to resort to simplifications of the model and some heuristic rules that will lead towards a near optimal solution.Research is progressing along three different lines which are the use of thermodynamic concepts, mathematical methods and the use of knowledge based systems for process design. In order to solve real life industrial problems, the engineer should take advantage of all these disciplines. However, the skill and experience of the engineer himself will remain of vital importance.This paper presents the state of the art of HENS methods together with some applications to previous literature problems. An evaluation of the various methods is performed from an industrial point of view. There is also a brief discussion of some of the software tools available to solve such problems. The presentation will emphasize on the design of the heat exchanger network itself although interactions with the rest of the process and the utility system inevitably will be discussed. The aspects of flexibility and operability will also be briefly mentioned.HENS is the most mature field of process synthesis when it comes to systematic methods. The increase of energy prices during the 70s and early 80s has been the major driving force. As things have been developed, however, the emphasis has changed from energy optimal (minimal) structures to cost optimal networks. The latest developed methods can find the proper trade-off between investment cost and operating cost for any price scenario (including regional factors such as the cooling water temperature etc.) ahead of design. Nevertheless, all problems have not yet been solved, and important research is still being conducted addressing all three areas of HENS, which are targeting, synthesis and optimization.Industry spends a significant amount of money to carry out energy analysis of new and existing plants, to support academia in their research and to develop and acquire accurate and efficient computer tools. It is the involvement of our company in these areas that has given us the opportunity, on an industrial basis, to review the field.In the past there have been two schools of HENS. One relies on thermodynamic principles and a few heuristic rules, where the designer manually (or interactively if software is available) synthesizes the network. The other, more automatic, approach relies on mathematical methods like linear and nonlinear programming. The relative merits of these schools will be discussed with reference to case studies. There will also be a short presentation of the historical development within these schools.     

Universality in the transition to chaos of dissipative systems

The period-doubling bifurcation process for two-dimensional transforms exhibits a new class of universality when a small dissipation is taken into account. The effective Jacobian is then defined as a function of both the dissipation and the rank n of the cascade (cycle 2ⁿ). Numerical simulations of a simple mechanical system and numerical calculations on the Hénon mapping show that the decrement lies on a continuous curve as function of the effective Jacobian. A method using this result to understand experimental data is explained and a first order approximation of the renormalization process yields an analytic expression of the curve.Among the different transitions to chaos, the period-doubling bifurcation cascade [1, 2] has been extensively studied. This transition is characterized by an experimental convergence rate of the bifurcation threshold sequence to the accumulation point: the threshold of chaos. It is well known that the decrement of this bifurcation cascade can take different values. Each value corresponds to a specific class of systems which can be characterized by some general features of the system undergoing the transition [3, 4, 5]. We are concerned here with the two values; δ(I) = 4.699… the decrement of the well-known one-dimensional transform with a quadratic maximum [2] and δ(II) = 8.721 the decrement of a two-dimensional non-dissipative transforms [3]. These two classes of systems are generic in physics and the two values δ(I) and δ(II) are therefore relevant values of the decrement. However, these two exponents stand for the infinite dissipation case and the conservative one thus leaving out the general physical situation of a finite dissipation. Only hints of the effect of a small dissipation in a two-dimensional mapping have been given [6] before the work of Zisook [7].A thorough study of the effect of dissipation is set forth here. The first two sections deal with the physical model used to perform the numerical investigation and the “experimental” data thus obtained. A study of the renormalization process enables to generalise the relation δ_n(J)=δ_n−(J²), first given by Zisook in [7], to all transforms where the Jacobian does not depend on the linearization point in the phase space. Furthermore a first order approximation gives an excellent analytic expression of the universal function displaying the crossover of the decrement between δ(II) and δ(I).     

Bianchi type-II universe with linear equation of state

We have studied anisotropic and homogeneous Bianchi type-II cosmological model with linear equation of state (EoS) p = αρ?β, where α and β are constants, in General Relativity. In order to obtain the solutions of the field equations we have assumed the geometrical restriction that expansion scalar θ is proportional to shear scalar σ. The geometrical and physical aspects of the model are also studied.     

Noncommutative lattices as finite approximations

Lattice discretizations of continuous manifolds are common tools used in a variety of physical contexts. Conventional discrete approximations, however, cannot capture all aspects of the original manifold, notably its topology. In this paper we discuss an approximation scheme due to Sorkin (1991) which correctly reproduces important topological aspects of continuum physics. The approximating topological spaces are partially ordered sets (posets), the partial order encoding the topology. Now, the topology of a manifold M can be reconstructed from the commutativè C^*algebra C(M) of continuous functions defined on it. In turn, this algebra is generated by continuous probability densities in ordinary quantum physics on M. The latter also serves to specify the domains of observables like the Hamiltonian. For a poset, the role of this algebra is assumed by a noncommutative C^*-algebra A. This fact makes any poset a genuine ‘noncommutative’ (‘quantum’) space, in the sense that the algebra of its ‘continuous functions’ is a noncommutative C^*-algebra. We therefore also have a remarkable connection between finite approximations to quantum physics and noncommutative geometries. We use this connection to develop various approximation methods for doing quantum physics using A.     

Exact integral positivity and crossing constraints on S and P wave pion-pion scattering amplitudes

S- and P-wave pion-pion scattering amplitudes are constrained by the fact that they have to be compatible with crossing symmetry and the positivity of the absorptive parts of all partial waves. The construction of the necessary and sufficient conditions ensuring the compatibility with positivity and an arbitrary finite set of Balachandran-Nuyts crossing conditions is described. Two sets of constraints are derived explicitly. The first set contains inequalities involving integrals of the S-waves over the unphysical interval 0?s?4m_π². The inequalities of the second set relate integrals of S-waves over the unphysical interval to integrals of S- and P-wave absorptive parts extended to the physical region.     

Spin Hall effect in noncommutative coordinates

A semiclassical constrained Hamiltonian system which was established to study dynamical systems of matrix valued non-Abelian gauge fields is employed to formulate spin Hall effect in noncommuting coordinates at the first order in the constant noncommutativity parameter θ. The method is first illustrated by studying the Hall effect on the noncommutative plane in a gauge independent fashion. Then, the Drude model type and the Hall effect type formulations of spin Hall effect are considered in noncommuting coordinates and θ deformed spin Hall conductivities which they provide are acquired. It is shown that by adjusting θ different formulations of spin Hall conductivity are accomplished. Hence, the noncommutative theory can be envisaged as an effective theory which unifies different approaches to similar physical phenomena.