Frequency doubling of phase-modulated chirped ultrashort laser pulses using a deformable mirror

We explore the use of a computer-controlled micro-machined deformable mirror to tailor the spectral phase of an ultrashort laser pulse to control the second-harmonic generation in KDP crystals. The SHG dependence of chirp and phase mask could be observed and adjusted by a simple theory.     

Analysis of a multistate control problem related to food technology

This paper is concerned with an optimal control problem related to the determination of an optimal profile for the steam temperature into the autoclave along the processing of canned foods. The problem studies a system coupling the evolution Navier-Stokes equations with the heat transfer equation by natural convection (the so-called Boussinesq equations), and with the microorganisms removal equation. The essential difficulties in the study of this multistate control problem arise from the lack of uniqueness for the solution of the state system. Here we obtain—after a careful analysis of the problem mathematical formulation—the uniqueness of part of the state, and the existence of optimal solutions.     

Rheo-optic flow-induced crystallisation of polypropylene and polyethylene within confined entry–exit flow geometries

This paper reports experimental observations on the way polypropylene (PP) and high-density polyethylene (HDPE) can crystallise during flow. Both a deep and a shallow slit geometry were chosen for the rheo-optical study. Preliminary linear viscoelastic rheological tests enabled the temperature window for quiescent crystallisation to be established. Flow-induced crystallisation (FIC) studies were performed in a temperature regime above the normal quiescent crystallisation conditions. In the case of HDPE, FIC occurred during flow at the sidewalls of the slit and in localised regions downstream and the processing pressure increased during the piston movement. In the case of PP, flow-induced crystallisation was generally observed after flow cessation, and the processing pressure did not change during flow. For PP, FIC also occurred preferentially at the walls in the form of elongated crystallites, but the fibres gradually emerged after flow cessation. The difference in the FIC behaviour was attributed to differences in both the rheology and the crystal growth kinetics of the two materials at the particular super-cooling used.     

Dimension reduction in multiobjective optimization

In multiobjective optimization there is often the problem of the existence of a large number of objectives. For more than two objectives there is a difficulty with the representation and visualization of the solutions in the objective space. Therefore, it is not clear for the decision maker the trade-off between the different alternative solutions. Thus, this creates enormous difficulties when choosing a solution from the Pareto-optimal set and constitutes a central question in the process of decision making. Based on statistical methods as Principle Component Analysis and Cluster Analysis, the problem of reduction of the number of objectives is addressed. Several test examples with different number of objectives have been studied in order to evaluate the process of decision making through these methods. Preliminary results indicate that this statistical approach can be a valuable tool on decision making in multiobjective optimization. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the nuclear recoil response of a NaI(Tl) scintillator detector with a Cf neutron source

This paper describes the behavior of the scintillation pulses induced in a NaI(Tl) crystal scintillator under excitation of a ²⁵²Cf neutron source and a ¹³⁷Cs gamma source and the possible identification of the incident particle by the study of the pulse shape. Furthermore, the energy dependence of the mean decay time is presented.     

Substituent effects in the migration step of the Baeyer-Villiger rearrangement. A theoretical study

Quantum mechanical calculations have been performed on the migration step of the Baeyer-Villiger (BV) rearrangements of some acetophenones, p-RC6H4COCH3 (R = CN, Cl, H, CH3, CH3O) with m-chloroperbenzoic acid. The energy barriers, charge distributions, and frontier molecular orbitals determined for the aryl migration step explain the effects of substituents on the reactivity of these ketones. A plot of the log of relative oxidation rates of the ketones versus their corresponding calculated energy barriers of the migration stage showed a downward deviation for the p-OCH3 derivative. This result is consistent with a change in the rate-determining step, from the aryl migration to the carbonyl addition, in the case of p-methoxyacetophenone, according to the suggestion that the rate-determining step of the BV oxidation can change with variations in the substituent group.     

Influence of hydration and crosslinking in transdermal delivery of nicotine from chitosan-based gels by thermal analysis

Chitosan is a biopolymer that forms hydrogels after swell in acid medium. The environment of the three-dimensional network of the chitosan-based hydrogels can be modified by its degree of swelling and crosslinking. In this way, nicotine was incorporated in the hydrogel formulations, with or without crosslinking with glutaraldehyde (0.01%), in different swollen states. Transdermal delivery of nicotine by chitosan-based hydrogels was studied in order to achieve the prolonged administration of the drug. Thermal analysis indicated a preliminary stability of these formulations, and the mechanism of drug release from hydrogels was dependent of the swelling degree and crosslinking. These formulations were able to control the transdermal flux of nicotine for up to 48 h following zero-order kinetics. The hydrogels with higher amounts of water or the partially dried crosslinked hydrogels reduced the partition of nicotine into the skin, leading to a minor transdermal flux of the drug (<3.4 µg cm⁻² h⁻¹). On the other hand, the partially dried non-crosslinked hydrogels lead to a major transdermal flux of the drug (20.19 µg cm⁻² h⁻¹) due to modifications of the environment into the hydrogel. In this way, these transdermal formulations were promising vehicles for prolonged administration of nicotine.

     

Fins and turbulence promoters for heat transfer enhancement in latent heat storage systems

One of the serious problems associated with the operation of PCM storage system is the heat transfer in and out of the element containing the PCM. This paper presents the results of an experimental investigation of the effects of radial fins and turbulence promoters on the enhancement of phase change heat transfer external to a horizontal tube submersed in the PCM with the working fluid flowing through it. The experimental measurements were realized on a bare cupper tube and an identical cupper tube fitted with radial fins. The fins investigated are 40, 60, 120 and 180 mm diameters. A turbulence promoter made of stainless steel wire of 1.0 mm diameter coiled in a helical form with a pitch of 25.0 mm was inserted into the cupper tubes. The tests were realized on bare tubes, finned tubes and finned tubes with the turbulence promoter inserted into the finned tubes. The measurements were realized for the working fluid temperatures in the range of −10 °C, to −25 °C and six values of the mass flow rate ranging from 0.013 to 0.031 kg/s. The position of the phase interface was photographed by a high resolution digital camera and scanned to determine the real interface position by comparison with a precision measuring scale. The results of the phase interface position, velocity of the interface, solidified mass fraction and the time for complete solidification are presented in function of the working fluid temperature, the working fluid mass and the tube arrangements. The results are presented and discussed.