ENTROPY AND AGING IN BIOLOGICAL SYSTEMS

The Second Law of Thermodynamics (which is often referred to as “times arrow” because it dictates the direction of increasing chronological time), should have a direct bearing on the aging process in biological systems. Basic aging related phenomena are discussed from the point of view of an entropy rate balance, and the known effects of body temperature, metabolic rate and food consumption rate on lifespan are considered. The problems of quantifying the rate of internal entropy production of complex systems is investigated using the fundamentals of non-equilibrium thermodynamics. Entropy rate balance data over the lifespan of the annual fish Nothobranchius guentheri are used to illustrate basic aging phenomena.     

AN ENTROPY MODEL FOR BIOLOGICAL SYSTEMS ENTROPY

The similarity between the entropy balance equation of the Second Law of Thermodynamics and the classical balance equations of chemical reaction kinetics or allometric biological growth models is used to construct an entropy model for biological systems. In this model the entropy transport rate is assumed to be proportional to surface area and the entropy production rate is assumed to be proportional to system volume. It is also hypothesize that, at any time (age), the entropy transport and production rates depend directly upon the instantaneous value of the system total entropy. The resulting entropy rate balance can then be solved uniquely for the total system entropy as a function of time (age)

Also, it is postulated that all living systems are characterized by a continuously, decreasing total entropy level, and that biological death occurs at some minimum total entropy value. The time required to reach this minimum is the lifespan of the biological system.     

“GENERALIZED” ENTROPY PRODUCTION RATE ASSOCIATED TO ONE-DIMENSIONAL HEAT CONDUCTION- EXTREMUM POINTS AND APPLICATIONS

In previous papers, the author considered the critical or the extremum points of the entropy production rate in insulations with one-dimensional heat conduction, either alone or associated with other refrigerating systems necessary to maintain the steady state.

The aim of this paper is to obtain a general formulation for the presence of critical or extremum points of the (generalized) entropy production rate in a more ample domain that includes the previous cases as particular ones.

Homogeneous and more than one component systems are considered; for homogeneous systems, the necessary and sufficient condition for the minimum of the (generalized) entropy production rate in the system is obtained, while for two or more component systems only the necessary condition for a relative extremum is stated. In the latter case one can have also a particular state which is not an extremum and can be known as a “saddle point” state from analogy.

This general formulation can be used for the thermodynamic optimization of cryogenic plants, extensively used in large-scale superconducting plants operating at liquid helium temperatures.     

Molecular kinetic theory of the glass transition

An overview of a new molecular kinetic theory of glass-transition phenomena is presented and experimental comparisons of its prediction for a variety of thermal and stress histories reviewed. The theory, which was developed in accordance with the balance of nonequilibrium statistical entropy, is shown to provide a unified interpretation of some recent models. The volume-relaxation process in amorphous polymers over the glass-transition region is regarded as the result of the collapse of a series of free volumes having different levels of energies of hole formation. An applied stress is shown to contribute to the variation of the entropy. An activation volume is introduced as a new tensorial extensive variable. The theory is applied to the phenomenon of physical aging in polymer glasses and shown to provide good quantitative agreement with the results of a well-known experiment on volume recovery of poly(vinyl acetate). This supports the underlying postulate of a fundamental link between the apparent relaxation time and the mean energy of hole formation, the distribution of relaxation times and the free-volume fractions. In contrast to the prevalent thinking toward free volume theories, an explicit expression between T_g and stress is presented and reveals that T_g does not continue to increase at all pressures but levels off to a semi universal asymptote at very high pressure. The calculated effect of stress rate is found to be in good agreement with dynamic viscoelastic measurements.     

Wave Phenomena in Low Angle Peeling

The angular dependence of peeling has been investigated over a wide range of peel angles for a rubber strip peeled from glass. At low peel angles the peel front becomes “V”-shaped, wave phenomena are often observed and the peel energy can increase by an order of magnitude or more. A tentative theory, which appears to give the correct magnitude in a worked example, is advanced to account for the energy increase. The influence of factors such as electrostatic charge, deformation and rate on the observed phenomena are discussed.     

DYNAMIC BEHAVIOR OF REACTIVE DISTILLATION COLUMNS. EQUILIBRIUM SYSTEMS

This paper deals with the dynamic behavior of simultaneous reaction-separation systems which operate at or near the chemical equilibrium in the liquid phase. The process under study comprises a whole set of “instantaneous”, and very fast reversible reactions where the difference in volatilities favors both the progress of reaction and product separation. The main aim of our study is to gain a deeper understanding of the dynamic behavior of distillation columns by using a model that is simple and efficient, yet informative. This kind of model is outstanding for synthesis and design of control schemes which require a careful modelling and understanding of process response to different changes in the environment. We use a suitable transformation of variables (after Barbosa and Doherty, 1988b) in order to define a new set of state variables; as a result, the balance equations become identical to those for conventional distillation. Also, an efficient physicochemical algorithm that can handle both the original and new state variables is used. Thus, a composition-holdup dynamic model is simulated in the “transformed field” using a stage-by-stage approach. To further reduce computational time, the transformed problem has also been solved by means of a reduction procedure based on approximating by orthogonal polynomials the transformed composition and flow profiles in the column. The performance of the two proposed methods are compared by using the top section of a quaternary reactive column. The results obtained shown that reactive distillation dynamics has certain peculiarities derived from superimposing reaction and separation phenomena.     

A thermodynamically consistent, nonlinear viscoelastic approach for modeling glassy polymers

A thermodynamically consistent nonlinear viscoelastic constitutive theory is derived to capture the wide range of behavior observed in glassy polymers, including such phenomena as yield, stress/volume/enthalpy relaxation, nonlinear stress-strain behavior in complex loading histories, and physical aging. The Helmholtz free energy for an isotropic, thermorheologically simple, viscoelastic material is constructed, and quantities such as the stress and entropy are determined from the Helmholtz potential using Rational Mechanics. The constitutive theory employs a generalized strain measure and a material clock, where the rate of relaxation is controlled by the internal energy that is likewise determined consistently from the viscoelastic Helmholtz potential. This is perhaps the simplest model consistent with the basic requirements of continuum physics, where the rate of relaxation depends upon the thermodynamic state of the polymer. The predictions of the model are compared with extensive experimental data in the following companion paper.     

JNK Signaling in Drosophila Aging and Longevity

The evolutionarily conserved c-Jun N-terminal kinase (JNK) signaling pathway is a critical genetic determinant in the control of longevity. In response to extrinsic and intrinsic stresses, JNK signaling is activated to protect cells from stress damage and promote survival. In Drosophila, global JNK upregulation can delay aging and extend lifespan, whereas tissue/organ-specific manipulation of JNK signaling impacts lifespan in a context-dependent manner. In this review, focusing on several tissues/organs that are highly associated with age-related diseases—including metabolic organs (intestine and fat body), neurons, and muscles—we summarize the distinct effects of tissue/organ-specific JNK signaling on aging and lifespan. We also highlight recent progress in elucidating the molecular mechanisms underlying the tissue-specific effects of JNK activity. Together, these studies highlight an important and comprehensive role for JNK signaling in the regulation of longevity in Drosophila.     

Deactivation handling in a high-throughput kinetic study of o-xylene hydrogenation

The objective of the study is to develop an HT methodology to enable kinetic modeling of diverse catalysts. The reaction of o-xylene hydrogenation is selected as a probe reaction to evaluate the “metallic” features of a library of about 80 diverse bimetallic catalysts. A major issue to overcome is deactivation phenomena which are catalyst dependent and which cannot a priori be predicted. A prerequisite is therefore to handle correctly deactivation processes for accessing intrinsic kinetic parameters. For this purpose, an adapted screening strategy is developed, using a proprietary 16 channels multi-tubular reactor which enables to test catalysts at the same time-on-stream. Original data treatment procedures are implemented in order to correct observed data from deactivation phenomena for the calculation of kinetic parameters. Effects of metal nature, dopants and supports on deactivation rate are analyzed using a statistical approach, and a tentative classification of deactivation processes based on coke analyses performed on aged materials is provided.     

Predicting Thermal Efficiency in Timber Radio Frequency Vacuum Drying

In this work, the efficiency of transforming dielectric energy into evaporated water is analyzed for the case of timber radio frequency vacuum drying. Based on well-known heat and mass transfer equations, a simplified mathematical model is proposed that estimates the drying efficacy in regards to the thermo-physical properties of wood. Although not exact, the theoretical results are close to the experimental observations and elucidate some phenomena like the tendency of the timber to dry from inside to outside, and the drying rate increase with the rise of the timber gas permeability. The theoretical efficiency model also predicts a range of wood permeability values for which the drying efficiency changes from 100 to 0%, thus providing a quantitative scale for classifying the spectrum of “difficult-to-dry” all the way to “easy-to-dry” wood species when using radio frequency vacuum technology.     

复合材料老化方法研究进展

概述了复合材料自然老化及人工气候老化研究方法的进展，介绍了根据老化研究结果确定复合材料使用寿命的方法，并就如何全面地、合理地评价复合材料的使用性能提出了几点建议。     

COMBINED MICROWAVE AND CONVECTIVE DRYING OF A POROUS MATERIAL

A model is formulated to describe the drying of a slab of porous material in a combined microwave and convective environment. The model describes the evolution of temperature, pressure, moisture and power distributions that occur during the drying process. The microwave internal heat source is calculated from electromagnetic theory with varying dielectric properties. The inclusion of pressure in the model allows the physical phenomena of “water pumping”, often observed in microwave drying systems, to be accounted for. The influence of sample size; on the drying kinetics 1s examined and found to be an important parameter during the drying process. In particular the effect of resonance on the moisture and temperature profiles and the need for careful consideration of surface mass transfer coefficients are investigated. Simulation results are presented for the combined microwave and convective drying of a homogeneous, isotropic porous material.     

Modelling and Refining Neuronal Circuits with Guidance Cues: Involvement of Semaphorins

The establishment of neuronal circuits requires neurons to develop and maintain appropriate connections with cellular partners in and out the central nervous system. These phenomena include elaboration of dendritic arborization and formation of synaptic contacts, initially made in excess. Subsequently, refinement occurs, and pruning takes places both at axonal and synaptic level, defining a homeostatic balance maintained throughout the lifespan. All these events require genetic regulations which happens cell-autonomously and are strongly influenced by environmental factors. This review aims to discuss the involvement of guidance cues from the Semaphorin family.     

SUBLIMATION OF PURE SUBSTANCES IN GAS FLUIDIZED BEDS AT ATMOSHPERIC PRESSURE.

This paper deals with the sublimation of large bodies, or “objects”, made up from a pure substance in a bubbling gas fluidized bed of considerably smaller particles, or “fines”. The influence of such parameters as the gas velocity, the bed temperature, the size and the adsorption capacity of the fines has been investigated.

The results obtained clearly show that the rate of sublimation in fluidized beds is far higher than in air alone. It increases with increasing bed temperature, decreasing particle size, increasing powder mass capacity, and roughly varies as a parabolic function of time. It has also been observed that the temperature difference between the bed and the object surface, or “temperature depression”, depends on the fines characteristics as well as on bed temperature, but is independent of gas velocity when good solid mixing conditions are achieved.

Bed-to-object heat and mass transfer coefficients have been deduced from data points and attempts have been made to provide a reasonable theory to account for them. After a complete examination, the idea of interpreting transport phenomena based on a well-adapted “surface renewal model” has been proposed.     

High shear wet granulation modelling—a mechanistic approach using population balances

A population balance approach based on splitting the coalescence kernel into two factors, the first describing the collision frequency of particles and the second describing the collision efficiency, is applied to modelling wet granulation in a high shear mixer. Four different expressions for the collision frequency are compared and discussed. The kernels are the size independent kernel, the shear kernel proposed by Smoluchowski [M. Smoluchowski, “Versuch einer matematischen Theorie der Koagulationskinetic kolloider Lösungen”, Z. Physik. Chem. 92, (1917) 129–168.] and the two kernels proposed by Hounslow [M.J. Hounslow, “The population balance as a tool for understanding particle rate processes”, Kona (1998) 179–193.], i.e. the EKE kernel and the less used kernel based on equipartition of fluctuating translational momentum (ETM kernel). Microcrystalline cellulose (mcc) is granulated under different process conditions and it is found that the ETM kernel best describes the granulation at higher impeller speeds, whereas the EKE kernel gives better agreement at lower impeller speeds. The collision efficiency is assumed to be a function of the liquid saturation. By using this assumption, it was possible to detect similar trends for the remaining part of the collision efficiency regardless of process conditions.     

THERMODYNAMIC ANALYSIS OF DRYING FOODS

Thermodynamic functions computed for food systems showing hysteresis indicate considerable differences. There appears to exist no qualitative agreement between the values obtained. Large differences in values between the adsorption an desorp-tion branches of isotherms make the absolute magnitude of the entropy and enthalpy changes uncertain. The desorption iso-steric heat was found to decrease with temperature while the adsorption branch of the isotherm for a few foods were found to exhibit temperature reversal effect giving higher values at low temperatures. Such trend may indicate either transition in the form of binding energy or the effect of temperature on structural alterations. In designing drying equipment, the isosteric heat of desorption will provide “a worst case” analysis as the true isosteric heat will lie somewhere between the adsorption and desorption isosteric heat. Any overcalculation in designing equipment by using the desorption branch of hysteresis data can only be in favor of increased throughput capability. The use of isosteric heat in freeze drying and in air drying calculations were undertaken and found to rise at a faster rate than the energy expended in moisture removal.     

An entropy-based formulation of irreversible processes based on contact structures

In this paper we propose an analytical formulation of the dynamical behaviour of complex and open physical systems which is formulated on the total thermodynamic phase space using the contact form associated with Gibbs’ relation. Starting from balance equations we construct control contact systems by using the entropy function to represent the thermodynamic properties. The contact Hamiltonian function generating the dynamical behaviour has then the units of an entropy variation. We consider complex thermodynamic systems, described by compartmental systems, and we construct the associated control contact system by composing the control contact formulation of every compartment. The contact Hamiltonian functions generating the dynamical behaviour are discussed with respect to two alternative formalisms used for describing coupled sets of reversible and irreversible processes, namely the GENERIC formulation and the Matrix formulation. This analysis is then illustrated on the elementary example of a coupled mechanical and thermodynamic system.     

BOUNDS FOR ACCEPTABLE VALUES OF ADSORPTION ENTROPY

The 'components“ of a system of an adsorbing—desorbing gas on a surface consist of the pure gas, adsorbed species and the empty site. The standard stales for these components in a system at thermodynamic equilibrium are examined in general, and rational expressions for the entropy of adsorption are obtained for the statistical-thermodynamic limiting cases of completely localized adsorption and adsorption as a two-dimensional gas. These values are suggested as alternatives to those empirically offered by Boudart and coworkers for evaluating the validity of adsorption equilibrium coefficients obtained from kinetic rate data.     

A physico-chemical method to calculate time-dependent reaction mixtures

A method is proposed to calculate multicomponent chemical reaction mixtures as a series of sequential, time-dependent thermochemical states. The procedure is based on the two general principles of physical chemistry, viz., decreasing Gibbs-energy for a natural process and the exponential Arrhenius rate law for the overall reaction kinetics. The calculation is performed in repeated sequences for a closed system element with known heat transfer to its surroundings. The temperature of each sequential reactor element is determined by a Gibbs-energy minimization routine (SOLGASMIX) connected to the closed system enthalpy balance iteration. The extent of overall reaction is limited by the Arrhenius kinetics, while the possible side reactions may reach equilibrium. The mass balance relations forming the subsidiary constraints of the Lagrange method are modified to facilitate the Gibbs-energy minimization for the intermediate “reactor states”. The well-known reaction of titanium (IV) chloride oxidation to titanium dioxide with such side reactions as chlorine dissociation and oxychloride formation is given as a calculational example.     

Diffusion effects in catalytic gas oil cracking

Wojciechowski's four parameter model of catalyst decay is applied to conversion data for gas oil cracking over a diffusion limited catalyst. The parameters thus obtained are compared with those obtained previously for a diffusion free form of the same catalyst, cracking the same feed stock under identical experimental conditions. The comparison shows that the presence of diffusion affects not only the rate of catalytic cracking but also the rate of aging. It is also shown that the introduction of diffusion phenomena has no effect on the mechanism of aging. The presence of diffusion limitations however has the effect of making the feed stock appear more homogeneous in reactivity