Lattice instability of Ni2MnGa

Anomalies have been detected in the temperature behavior of the physical properties of Ni₂MnGa in the temperature interval preceding the martensitic transformation, which is attributed to TA ₂ phonon mode condensation at T=T _I>T _m (T _m is the martensitic transition temperature). Fiz. Tverd. Tela (St. Petersburg) 39, 557–559 (March 1997)     

From Quantum Probabilities to Quantum Amplitudes

The task of reconstructing the system’s state from the measurements results, known as the Pauli problem, usually requires repetition of two successive steps. Preparation in an initial state to be determined is followed by an accurate measurement of one of the several chosen operators in order to provide the necessary “Pauli data”. We consider a similar yet more general problem of recovering Feynman’s transition (path) amplitudes from the results of at least three consecutive measurements. The three-step histories of a pre- and post-selected quantum system are subjected to a type of interference not available to their two-step counterparts. We show that this interference can be exploited, and if the intermediate measurement is “fuzzy”, the path amplitudes can be successfully recovered. The simplest case of a two-level system is analysed in detail. The “weak measurement” limit and the usefulness of the path amplitudes are also discussed.     

Elastic-bound conditions for energetically optimal elasticity and their implications for biomimetic propulsion systems

Minimising the energy consumption associated with periodic motion is a priority common to a wide range of technologies and organisms. These include many forms of biological and biomimetic propulsion system, such as flying insects. Linear and nonlinear elasticity can play an important role in optimising the energetic behaviour of these systems, via linear or nonlinear resonance. However, existing methods for computing energetically optimal nonlinear elasticities struggle when actuator energy regeneration is imperfect: when the system cannot reuse work performed on the actuator, as occurs in many realistic systems. Here, we develop a new analytical method that overcomes these limitations. Our method provides exact nonlinear elasticities minimising the mechanical power consumption required to generate a target periodic response, under conditions of imperfect energy regeneration. We demonstrate how, in general parallel- and series-elastic actuation systems, imperfect regeneration can lead to a set of non-unique optimal nonlinear elasticities. This solution space generalises the energetic properties of linear resonance, and is described completely via bounds on the system work loop: the elastic-bound conditions. The choice of nonlinear elasticities from within these bounds leads to new tools for systems design, with particular relevance to biomimetic propulsion systems: tools for controlling the trade-off between actuator peak power and duty cycle; for using unidirectional actuators to generate energetically optimal oscillations; and further. More broadly, these results lead to new perspectives on the role of nonlinear elasticity in biological organisms, and new insights into the fundamental relationship between nonlinear resonance, nonlinear elasticity, and energetic optimality.

     

Investigation of the interactions in emulsions stabilized by a polymeric surfactant and its mixtures with an anionic surfactant

 The interaction of a nonionic polymeric surfactant with an anionic surfactant at the oil–water interface has been studied by its effects on the droplet size, stability and rheology of emulsions. Oil-in-water (o/w) emulsions were prepared using isoparaffinic oil and mixtures of a nonionic polymeric surfactant with an anionic surfactant. The macro-molecular surfactant was a graft copolymer with a backbone of polymethyl methacrylate and grafted polyethylene oxide (a graft copolymer with PEO chains of MW=750). The anionic surfactant was sodium dodecyl sulfate (SDS). The stabiliza-tion of the emulsion droplets was found to be different when using one or the other surfactant. The mechanism of stabilization of emulsion droplets by the macro-molecular surfactant is of the steric type while the stabilization by anionic surfactant is of the electrostatic repulsion type. Emulsions stabilized with mixtures present both types of stabilization. Other effects on the preparation and stabilization of emulsions were found to be dependent on properties associated with the surfactant molecular weight such as the Marangoni effect and Gibbs elasticity. The initial droplet size of the emulsions showed a synergistic effect of the surfactant combination, showing a minimum for the mixtures compared to the pure components. Emulsion stability also shows a synergistic interaction of both surfactants. Rheological measurements allow for the estimation of the interparticle interaction when measured as a function of volume fraction. Most of the effects observed can be attributed to the differences in interfacial tension and droplet radius produced by both surfactants and their mixtures. The elastic moduli are well explained on the basis of droplet deformation. Ionic versus steric stabilization produce little difference in the observed rheology, the only important differences observed concerned the extent of the linear viscoelasticity region. Received: 22 November 1996 Accepted: 24 March 1997     

Direct quantitation of volatile organic compounds in packaging materials by headspace solid-phase microextraction-gas chromatography-mass spectrometry

The quantification of volatile organic compounds (VOCs) in flexible multilayer packaging materials using headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) was studied. The analytes imclude 22 compounds such as aldehydes. ketones, carboxylic acids and hydrocarbons formed by thermooxidative degradation of polyethylene during the extrusion coating process in the manufacture of the packaging, and many of them are involved in the unpleasant and undesirable odour of these materials. External standard calibration using a solution of the analytes in an appropriate solvent was the first approach studied. Aqueous solutions of the analytes provided low reproducibility and the reduction of aldehydes to alcohols under the HS-SPME conditions. Hexadecane was chosen as the solvent since its polarity is similar to that of polyethylene and its volatility is lower than that of the analytes. However, hexadecane should be added to the sample before the analysis as it modifies the absorption capacity of the fibre. A 75-microm Carboxen-poly(dimethylsiloxane) fibre was used to extract the VOCs from the headspace above the packaging in a 15-ml sealed vial at 100 degrees C after 5 min of preincubation. The influence of the extraction time on the amount extracted was studied for a standard solution of the analytes in hexadecane, together with the influence of the volume of the standard solution and the amount of the sample placed in the vial. Standard addition and multiple HS-SPME were also studied as calibration methods and the results obtained in the quantitative analysis of a packaging material were compared.     

Lanthanide modulation of the orientation of macromolecules induced by purple membrane.

Addition of Tb3+ to purple membrane (PM) suspensions changes the orientation of the menbrane normal from parallel to perpendicular with respect to the magnetic field. Residual dipolar couplings measured in protein L in the presence of PM are scaled by a factor of -1/2. NMR line broadening and cross-correlation effects induced by the addition of PM are partially reversed by Tb3+ but not by Tm3+ that has no effect on the orientation of PM. This is interpreted as the result of anisotropically restricted motion of protein L in the transiently PM-bound form.     

Regioselective formation of N-alkyl-3,5-pyrazole derived ligands. A synthetic and computational study

New N-alkyl-3,5-pyrazole derived ligands were synthesized by reaction between 3,5-pyrazole derived ligands and the appropriate haloalkane in toluene or THF using NaOEt or NaH as base. When the precursor ligand bears a pyridyl substituent the alkylation reaction presents a large regioselectivity. Theoretical calculations have been carried out to rationalize the experimental observations. It has been shown that regioselectivity is governed by the formation of Na⁺-pyrazolide chelate complexes.