Determination of Caffeic Acid in Red Wine by Voltammetric Method

The electrochemical oxidation of 3,4‐dihydroxycinnamic acid, caffeic acid, leads to a stable electroactive poly(caffeic acid) thin film containing quinone moiety on a preactivated glassy polymeric carbon electrode. The properties of the deposited films as well as the stability study under different experimental conditions were investigated. Taking advantage of the electrochemical behavior, an analytical method based on differential pulse voltammetry for determination of caffeic acid in red wine was proposed.     

Optimization strategies in credit portfolio management

This paper focuses on the application of an original global optimization algorithm, based on the hybridization between a genetic algorithm and a semi-deterministic algorithm, for the resolution of various constrained optimization problems for realistic credit portfolios. Results are analyzed from a financial point of view in order to confirm their relevance.     

Determination of periodic orbits of nonlinear oscillators by means of piecewise-linearization methods

An approximate method based on piecewise linearization is developed for the determination of periodic orbits of nonlinear oscillators. The method is based on Taylor series expansions, provides piecewise analytical solutions in three-point intervals which are continuous everywhere and explicit three-point difference equations which are P-stable and have an infinite interval of periodicity. It is shown that the method presented here reduces to the well-known Störmer technique, is second-order accurate, and yields, upon applying Taylor series expansion and a Padé approximation, another P-stable technique whenever the Jacobian is different from zero. The method is generalized for single degree-of-freedom problems that contain the velocity, and (approximate) analytical solutions are presented. Finally, by introducing the inverse of a vector and the vector product and quotient, and using Taylor series expansions and a Padé approximation, the method has been generalized to multiple degree-of-freedom problems and results in explicit three-point finite difference equations which only involve vector multiplications.     

An almost L‐stable BDF‐type method for the numerical solution of stiff ODEs arising from the method of lines

A new BDF‐type scheme is proposed for the numerical integration of the system of ordinary differential equations that arises in the Method of Lines solution of time‐dependent partial differential equations. This system is usually stiff, so it is desirable for the numerical method to solve it to have good properties concerning stability. The method proposed in this article is almost L‐stable and of algebraic order three. Numerical experiments illustrate the performance of the new method on different stiff systems of ODEs after discretizing in the space variable some PDE problems. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007     

Visualization of capillary boiling

Capillary boiling has been experimentally analyzed using an optical technique, based on the extraction of gray levels at specific locations from video frames. Boiling is achieved by placing a small capillary in a large container filled with water at boiling temperature. A heating wire located inside the capillary provides the additional heat that triggers the evaporation. The phenomenon is videotaped and digitized. A small region of the filmed field is chosen and the gray level of all the digitized frames is stored in a file which is then analyzed. This constitutes in essence the possibility of using a large number of non-intrusive, fixed (Eulerian) virtual sensors. The information extracted concerns the global properties of the phenomenon such as the time between the departure of two subsequent bubbles, and more detailed properties such as the shape and volume of the departing bubbles or even the motion of liquid packets inside the capillary. Moreover, this technique permits the study of time and space correlations. It is observed that the bubbles depart from the capillary tip with a variable frequency that can be approximately grouped around two values. Clear evidence that this is due to two different mechanisms is given. The time between the departure of two bubbles is correlated with the volume of the departing bubbles. Received: 21 April 1999/Accepted: 18 July 2000     

Introduction to Nonlinear Dynamics of Electronic Systems: Tutorial

The study of chaos has generated enormous interest in exploring the complexity of the behavior in nature and in technology. Many of the important features of chaotic dynamical systems can be seen using experimental and computational methods in simple nonlinear mechanical systems or electronic circuits. Starting with the study of a chaotic nonlinear mechanical system (driven damped pendulum) or a nonlinear electronic system (circuit Chua) we introduce the reader into the concepts of chaos order in Sharkovsky's sense, and topological invariants (topological entropy and topological frequencies). The Kirchhoff's circuit laws are a pair of laws that deal with the conservation of charge and energy in electric circuits, and the algebraic theory of graphs characterizes these linear systems in terms of cycles and cocycles (or cuts). Here we discuss methods (topological semiconjugacy to piecewise linear maps and Markov graphs) to find a similar situation for the nonlinear dynamics, to understanding chaotic dynamics. Thus to chaotic dynamics we associate a Markov graph, where the dynamical and topological invariants will be seen as graph theoretical quantities.     

Polymeric and colloidal features of latex particles with surface amino groups obtained by semicontinuous seeded cationic emulsion polymerization*

Monodisperse latex particles with different amounts of surface amino and amidine groups were synthesized by means of a semicontinuous seeded cationic emulsion polymerization of styrene and a cationic monomer. High partial overall conversions for styrene and limited ones for the cationic monomer were achieved. A reliable method for the quantification of surface amidine and amino groups was developed. It was found that the amount of surface amidine groups provided by the cationic initiator was higher when the amount of cationic monomer added increased. The value for the partition coefficient of the cationic monomer indicated that this polymerizes with the same probability in the water phase as in the particle. The colloidal stability, in terms of critical coagulation concentration, shows that the latexes would be useful as polymeric supports in immunoassays. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3878–3886, 2005     

Semicontinuous seeded cationic emulsion polymerization of styrene: The effects of the concentration and type of cationic surfactant

The objective of this work was to analyze the effects of the concentration and type of cationic surfactant on the kinetic features (instantaneous and overall conversions) and colloidal characteristics [mean particle diameter, particle size distribution (PSD), and surface charge density] in the semicontinuous seeded cationic emulsion polymerization of styrene. 2,2′‐Azobis(N,N′‐dimethyleneisobutyramidine)dihydrochloride was used as an initiator. The surfactants were dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (HDTAB). So that the evolution of some polymeric and colloidal characteristics of the synthesized latices could be followed, the overall and instantaneous conversions were defined and determined gravimetrically. The PSDs and average particle diameters were determined by transmission electron microscopy and photon correlation spectroscopy. The surface charge density was determined by conductimetric titration. The evolution of the instantaneous conversions, the total number of particles, and the PSDs of the different reactions were related to the nucleation, growth, and coagulation processes taking place in the semicontinuous seeded emulsion polymerizations. The PSDs obtained from the reactions carried out with the emulsifier DTAB, at a concentration equal to its critical micelle concentration (cmc) and at a concentration twice its cmc, presented more and smaller particles than those obtained by the addition of HDTAB to the polymerization recipe. At lower emulsifier concentrations equal to half of the cmc, the system had lower colloidal stability with DTAB. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2322–2334, 2003