Characterization of a 20 kHz sonoreactor. Part II: analysis of chemical effects by classical and electrochemical methods

A new electrochemical redox probe has been investigated in order to characterize the local production of radicals during the cavitation events. The results have been compared with those obtained with Fe(CN)6(3-)/Fe(CN)6(4-) (electrochemical probe for local mechanical effects) and classical chemical methods such as iodide and Fricke dosimeters (chemical probes for global effects).     

First-order Fréedericksz transition and front propagation in a liquid crystal light valve with feedback

Fréedericksz transition can become subcritical in the presence of a feedback mechanism that leads to the dependence of the local electric field onto the liquid crystal re-orientation angle. We have characterized experimentally the first-order Fréedericksz transition in a Liquid Crystal Light Valve with optical feedback. The bistability region is determined, together with the Fréedericksz transition point and the Maxwell point. We show the propagation of fronts connecting the different metastable states and we estimate the front velocity. Theoretically, we derive an amplitude equation, valid close to the Fréedericksz transition point, which accounts for the subcritical character of the bifurcation.Received: 21 October 2003, Published online: 6 January 2004PACS: 05.45.-a Nonlinear dynamics and nonlinear dynamical systems - 64.60.-i General studies of phase transitions     

Modular amino alcohol ligands containing bulky alkyl groups as chiral controllers for Et2Zn addition to aldehydes: illustration of a design principle

A new family of enantiomerically pure (1S,2R)-1-alkyl-2-(dialkylamino)-3-(R-oxy)-1-propanols containing a very bulky alkyl substituent (tert-butyl or 1-adamantyl) on their hydrocarbon chains has been synthesized from the corresponding enantiopure epoxy alcohols, arising from the catalytic Sharpless epoxidation, by protection of the primary hydroxy group and subsequent regioselective ring opening of the epoxide by a secondary cyclic amine (C-2 attack). The performance of these amino alcohols as ligands for the catalytic enantioselective addition of diethylzinc to benzaldehyde has been studied, with enantioselectivities of 92-96% being recorded. The best performing ligands, those with a bulky R-oxy group, also depict a convenient activity and selectivity profile in the addition of Et(2)Zn to a representative family of aldehydes. An anomalous structure/enantioselectivity relationship of some ligands in the tert-butyl series has been studied using PM3 calculations, and conclusions have been drawn on the possible effects of including in modular designs structural fragments giving rise to a variety of rotameric transition states.     

On the use of Monte Carlo simulation,cache and splitting techniques to improve the Clarke and Wright savings heuristics

This paper presents the SR-GCWS-CS probabilistic algorithm that combines Monte Carlo simulation with splitting techniques and the Clarke and Wright savings heuristic to find competitive quasi-optimal solutions to the Capacitated Vehicle Routing Problem (CVRP) in reasonable response times. The algorithm, which does not require complex fine-tuning processes, can be used as an alternative to other metaheuristics—such as Simulated Annealing, Tabu Search, Genetic Algorithms, Ant Colony Optimization or GRASP, which might be more difficult to implement and which might require non-trivial fine-tuning processes—when solving CVRP instances. As discussed in the paper, the probabilistic approach presented here aims to provide a relatively simple and yet flexible algorithm which benefits from: (a) the use of the geometric distribution to guide the random search process, and (b) efficient cache and splitting techniques that contribute to significantly reduce computational times. The algorithm is validated through a set of CVRP standard benchmarks and competitive results are obtained in all tested cases. Future work regarding the use of parallel programming to efficiently solve large-scale CVRP instances is discussed. Finally, it is important to notice that some of the principles of the approach presented here might serve as a base to develop similar algorithms for other routing and scheduling combinatorial problems.     

Evaluation of a glycerol derived biofuel by thermal analysis

This work describes thermal analysis evaluation of a glycerol derived compound (fatty acid esters of (2,2-dimethyl-1,3-dioxolan-4-yl) methanol) developed to work as a biofuel. Mixtures of these ketal–glyceryl esters with fatty acid methyl esters typical of soybean biodiesel were prepared and evaluated in relation to biodiesel critical thermal properties such as temperature of crystallization, thermal stability and volatilization measured by differential scanning calorimetry and thermogravimetric analysis. The volatility of the products containing fatty acid methyl esters and (2,2-dimethyl-1,3-dioxolan-4-yl) methyl esters could be predicted by thermogravimetric analyses conducted in nitrogen that avoided time consuming distillation and greatly reduced material expenditure.     

Energies and radial couplings for the Σ and Σ states of the NaHe quasimolecule

We present the energies and radial couplings for the five ¹Σ and four ³Σ states of lowest energy of the NaHe⁺ quasimolecule, with a precision which is sufficient to treat Na/He⁺ collisions. The characteristics of energies and couplings are studied in detail with the aid of qualitative orbital and state-correlation diagrams.