Optimisation of stir bar sorptive extraction for the analysis of volatile phenols in wines

An easy, fast and reliable analytical method is proposed for the determination of the concentration of volatile phenols (ethyl- and vinylphenols) in wines. The novel stir bar sorptive extraction (SBSE) technique is employed, following a simple and fast procedure that allows 15 samples to be extracted simultaneously using very small sample volume. Extracts are desorbed in a thermodesorption system (TDS) coupled on-line to a gas chromatograph-mass spectrometry system. The SBSE offers better recovery and linear regression coefficient (r2) for the four volatile phenols than solid-phase extraction (SPE). The mass spectrometric detection in selected ion monitoring mode contributes to the lower detection limit and good sensibility obtained with this method.     

Reproductive solution of a second-grade fluid system

We treat the existence and uniqueness of reproductive solution (weak time-periodic solution) of a second-grade fluid system for small enough source terms, by using the Galerkin approximation method and compactness arguments.     

A parametrisation of the direct correlation function for the square-shoulder fluid

We introduce a parametrisation of the direct correlation function for the square-shoulder fluid and demonstrate that this parametrisation is in quantitative agreement with the numerical solution of the Ornstein–Zernike equation within the Percus–Yevick approximation. Moreover, the radial distribution function obtained from the parametrisation reproduces quantitatively Monte Carlo simulation data. Our results show that the parametrisation is accurate over a large regime of densities for different interaction ranges and potential strengths.     

Multiscale hygroviscoelastic approach to predicting the internal stresses in composite materials

A self-consistent approach is used to estimate the internal stresses induced by moisture absorption in composite materials with a viscoelastic matrix and linear elastic fibers. A strong influence of the behavior of the viscoelastic matrix and humidity on the internal stresses in the structures at macroscopic and local scales is found to exist.     

Development and validation of UPLC for the determination of phenolic compounds and furanic derivatives in Brandy de Jerez

Brandy and other aged distillates are a rich source of polyphenols. For brandies, contact with wood during ageing makes an important contribution to their polyphenols content. This paper describes the use of a previously devised ultra performance LC (UPLC) method to study the polyphenols content of Brandy de Jerez. UPLC is a new technique in LC offering several potential advantages, especially the reduction of time. Analyses of brandy performed by HPLC were repeated by UPLC. A special UPLC analytical column (Acquity UPLC BEH C₁₈ column, 100×2.1 mm), with a particle size of 1.7 μm, forms part of this system. Using the UPLC system enabled the time needed for analysis to be reduced to one tenth of the time needed in the conventional HPLC system. In conclusion, the separation factor results of the UPLC were compared to those obtained using HPLC methods; this demonstrated that simple, high efficiency UPLC gradients are viable and advantageous substitutes for traditional analysis of polyphenols in brandy by HPLC. The method enabled 14 phenolic compounds to be identified and determined in 33 different commercial brandies, and this allowed them to be differentiated in function of quality.     

Finite element approximation of nematic liquid crystal flows using a saddle-point structure

In this work, we propose finite element schemes for the numerical approximation of nematic liquid crystal flows, based on a saddle-point formulation of the director vector sub-problem. It introduces a Lagrange multiplier that allows to enforce the sphere condition. In this setting, we can consider the limit problem (without penalty) and the penalized problem (using a Ginzburg–Landau penalty function) in a unified way. Further, the resulting schemes have a stable behavior with respect to the value of the penalty parameter, a key difference with respect to the existing schemes. Two different methods have been considered for the time integration. First, we have considered an implicit algorithm that is unconditionally stable and energy preserving. The linearization of the problem at every time step value can be performed using a quasi-Newton method that allows to decouple fluid velocity and director vector computations for every tangent problem. Then, we have designed a linear semi-implicit algorithm (i.e. it does not involve nonlinear iterations) and proved that it is unconditionally stable, verifying a discrete energy inequality. Finally, some numerical simulations are provided.     

A multiscale analysis of the mechanical behavior of a thermo-oxidized c/epoxy lamina

The mechanical behavior of carbon-fiber-reinforced polymer matrix composites having undergone a thermo-oxidation process is studied. The purpose is to perform a multiscale analysis of the consequences of oxidation on the intrinsic mechanical properties of the external composite ply and on the internal mechanical states experienced by the structure under mechanical loads. The effective mechanical properties of oxidized composite plies are determined according to the Eshelby–Kr?ner self-consistent homogenization procedure, depending on evolution of the oxidation process. The results obtained are compared with estimates found by the finite-element method. The macroscopic mechanical states are calculated for a unidirectional composite and laminates. The macroscopic stresses in each ply of the structure are determined by the classical lamination theory and the finite-element method, whereas the local stresses in the carbon fiber and epoxy matrix are calculated by using an analytical stress concentration relation.