Harmonic balancing approach to nonlinear oscillations of a punctual charge in the electric field of charged ring

The harmonic balance method is used to construct approximate frequency-amplitude relations and periodic solutions to an oscillating charge in the electric field of a ring. By combining linearization of the governing equation with the harmonic balance method, we construct analytical approximations to the oscillation frequencies and periodic solutions for the oscillator. To solve the nonlinear differential equation, firstly we make a change of variable and secondly the differential equation is rewritten in a form that does not contain the square-root expression. The approximate frequencies obtained are valid for the complete range of oscillation amplitudes and excellent agreement of the approximate frequencies and periodic solutions with the exact ones are demonstrated and discussed.     

Microwave-assisted rapid and efficient synthesis of C-alkyl imidazoisoquinolinone derivatives

The synthesis of a set of 10-benzyl-2,3-dihydroimidazo[1,2-b]isoquinolin-5(1H)-one and 5-oxo-imidazo[1,2-b]isoquinolin-10-yl)-N-phenylacetamide derivatives was achieved by exposing the corresponding alkylating agent and imidazoisoquinolinone to microwave irradiation and traditional oil bath heating in the presence of K₂CO₃ and DMAP. The microwave technique as well as DMAP as base accelerated the alkylation reaction for 2-6 min giving 79-88% yields.     

Temperature and pressure effects on zeta potential values of reservoir minerals

An experimental study of the effect of temperature and pressure on zeta potential of typical reservoir minerals, including quartz, kaolinite, and calcite, is presented. Experiments included the design and construction of an electrophoretic cell for zeta potential measurements at variable pressure and temperature. Electrolyte concentration was varied in the range from 0.0001 to 0.1 M in the pH range from 2 to 9. For all the minerals it is found that the zeta potential decreases with temperature at a rate characteristic of each mineral; values are around -2.3 mV/degrees C for quartz, -0.96 mV/degrees C for kaolinite, and -2.1 mV/degrees C for calcite for pressure values less than 45 psi. The effect of pressure is found to depend on the mineral nature and pH of the electrolytic solution. In the case of quartz, a systematic increase in the value of the zeta potential with pressure is observed, whereas a decreasing trend is measured for the kaolinite. In the case of calcite, a decreasing trend is observed for pressures up to 45 psi, whereas the experimental data suggest an increasing trend for higher pressure values.     

Microwave-assisted extraction through an aqueous medium and simultaneous cleanup by partition on hexane for determining pesticides in agricultural soils by gas chromatography: a critical study

A simple microwave-assisted extraction and partitioning method (MAEP) using water-acetonitrile and n-hexane for desorption and simultaneous partitioning, respectively, together with gas chromatography (GC) was studied to determine representative pesticides (trifluralin, metolachlor, chlorpyriphos and triadimefon) with a broad range of physico-chemical properties in agricultural soils. Three points were considered crucial in this study: instrumental and sample-associated factors affecting extraction of the target compounds were studied through experimental design; the spiking procedure at trace levels was carried out to reproduce the solute-soil sorption taking place in the environment as closely as possible; and results were analyzed taking into account the adsorption behaviour of the compounds on different kinds of soils. The complete analytical procedure proposed consisted of the MAEP of pesticides from 1.0 g of soil with 1 mL of 1:1 water/acetonitrile mixture, and 5 mL of hexane for trapping. The microwave heating program applied was 2 min at 250 W and 10 min at 900 W, and 130 °C maximum temperature. After extraction, the hexane layer was evaporated to dryness; the residue was re-dissolved and directly analyzed by gas chromatography electron capture detection (GC-ECD). Clean chromatograms were obtained without any additional cleanup step. Besides the four pesticides used to optimise MAEP, the method was applied to determine an additional group of pesticides (triallate, acetochlor, alachlor, endosulphan I and II, endrin, methoxychlor and tetradifon) in different soils. Most of the compounds studied were recovered in good yields with relative standard deviations (R.S.D.s) below 9% and detection limits ranged from 0.004 to 0.036 μg g⁻¹. The described method is efficient and fast to determine hydrophobic pesticides at ng g⁻¹ level in soil with different clay-to-organic matter ratios.     

Determination of copper and nickel in vegetable oils by direct sampling graphite furnace atomic absorption spectrometry

The quality of food products has been receiving great attention due to its influence on human nutrition and health. In this sense, the determination of trace metals in foods has turned an important field on food analysis. Concerning vegetable oils, its metal trace composition is an important criterion for the assessment of their quality once it is known that trace metals affect their rate of oxidation, influencing freshness, keeping properties as well as storage. In the present work an analytical method which enables the direct determination of Cu and Ni in vegetable oils by graphite furnace atomic absorption spectrometry (GFAAS), using a “solid” sample strategy is presented: in nature, samples are directly weighed on the graphite platform boat and inserted in the graphite tube. An adequate temperature program permitted the calibration by external aqueous analytical curves. Good concordance between the proposed procedure and EPA procedures was found in the analysis of real samples. Limits of detection of 0.001 and 0.002 μg g⁻¹ were found for Cu and Ni, respectively, in the original samples, and they were comfortably below the concentrations found.     

Design of a continuous wave blood flow bi-directional Doppler system

This paper describes the design of a continuous wave blood flow bi-directional Doppler system based on an open, high-resolution architecture that is portable and low cost. The system incorporates the advantages of expensive systems with dedicated hardware. The system is composed of a flow detector probe, a signal conditioning stage, a direction detection module, a signal processing stage and a graphic user interface. The direction detection of the signal is achieved using a fast digital phasing filter. The Doppler signal is processed using a short-time Fourier transform-based algorithm. This is commonly used as a reference. Nevertheless, the system allows us to incorporate alternative high-resolution spectral estimation methods that might offer more precise information to the specialist.     

Development and validation of a new LBM-MRT hybrid model with enthalpy formulation for melting with natural convection

This article presents a new model to simulate melting with natural convection of a phase change material. For the phase change problem, the enthalpy formulation is used. Energy equation is solved by a finite difference method, whereas the fluid flow is solved by the multiple relaxation time (MRT) lattice Boltzmann method. The model is first verified and validated using the data from the literature. Then, the model is applied to a tall brick filled with a fatty acid eutectic mixture and the results are presented. The main results are (1) the spatial convergence rate is of second order, (2) the new model is validated against data from the literature and (3) the natural convection plays an important role in the melting process of the fatty acid mixture considered in our work.     

Values for particle-scale properties of biomass briquettes made from agroforestry residues

The production of biomass pellets and briquettes from agroforestry residues has increased rapidly in recent years. The design of machines, equipment and the infrastructure necessary for the handling, transport and storage of these products has been an engineering challenge since, when moving, biomass does not usually flow as easily as other granular materials. The discrete element method(DEM) provides a means of studying the handling and silo discharge behaviour of granular solids as well as the distribution of the pressures exerted by such materials in silos. However, the development of such models requires certain properties of the particles in question to be known. The present work experimentally determines the true density, Young's modulus of elasticity(axial and radial), the particle–particle restitution coefficient and the particle–wall friction coefficient—all variables that must be known in the construction of DEM models—for briquettes made from maize stalk, maize stalk plus pine wood chips, rice husk, vine shoots, rape straw, cereal straw and sawdust plus cereal straw.     

Electronic properties of thienylene vinylene oligomers: synthesis and theoretical study

(E)-1,2-bis(2-thienyl)vinylene (TV), (E)-1,2-bis(3-octyl-2-thienyl)vinylene (TOV), (E)-1,2-bis(3-(2-ethylhexyl)-2-thienyl)vinylene (T2EHV), and (E)-1,2-bis-[2,2??-bithienyl] vinylene (BTV) have been synthesized by a combination of formylation reaction and Mc Murry dimerization. UV?CVis spectra of BTV showed the longest wavelength absorption, TOV and T2EHV showed a bathochromic shift to the red compared with TV, due to an increment of delocalization of the conjugated ??-system as the result of the weakening of the carbon?Ccarbon double bonds of the thienyl rings due to the substitution of one hydrogen by the alkyl group. Based on optical data, the effect of linear and branched alkyl chain and extension of conjugation length on the electronic properties is discussed. ¹H, ¹³C-NMR, UV?CVisible, Fluorescence data are discussed and theoretical DFT and TD-DFT calculations of ground state and excited states have been also considered in the analysis and explanation of results.