Production of collimated positronium by positron scattering from CO<Subscript>2</Subscript>, N<Subscript>2</Subscript> and Xe

The efficiency for collimated positronium production by charge-exchange in positron collisions with gaseous targets has been investigated in the range 20–396 eV. At 250 and 396 eV, CO₂ has been found to be approximately twice as efficient as N₂, the previous best neutralising gas at high energies. The efficiency from Xe, whilst lower at low energies, becomes comparable to that from H₂ at around 100–120 eV; at ∼250 eV, it is an order of magnitude lower.     

On the transformation toughening of a crack along an interface between a shape memory alloy and an isotropic medium

In this study, a bilinear cohesive zone model is employed to describe the transformation toughening behavior of a slowly propagating crack along an interface between a shape memory alloy and a linear elastic or elasto-plastic isotropic material. Small scale transformation zones and plane strain conditions are assumed. The crack growth is numerically simulated within a finite element scheme and its transformation toughening is obtained by means of resistance curves. It is found that the choice of the cohesive strength t₀ and the stress intensity factor phase angle φ greatly influence the toughening behavior of the bimaterial. The presented methodology is generalized for the case of an interface crack between a fiber reinforced shape memory alloy composite and a linear elastic, isotropic material. The effect of the cohesive strength t₀, as well as the fiber volume fraction are examined.     

Viscoelastic flow in a two-dimensional collapsible channel

We compute the flow of three viscoelastic fluids (Oldroyd-B, FENE-P, and Owens blood model) in a two-dimensional channel partly bounded by a tensioned membrane, a benchmark geometry for fluid–structure interactions. The predicted flow patterns are compared to those of a Newtonian liquid. We find that computations fail beyond a limiting Weissenberg number. Flow fields and membrane shape differ significantly because of the different degree of shear thinning and molecular extensibility underlying the three different microstructural models.     

Crack growth resistance of shape memory alloys by means of a cohesive zone model

Crack growth resistance of shape memory alloys (SMAs) is dominated by the transformation zone in the vicinity of the crack tip. In this study, the transformation toughening behavior of a slowly propagating crack in an SMA under plane strain conditions and mode I deformation is numerically investigated. A small-scale transformation zone is assumed. A cohesive zone model is implemented to simulate crack growth within a finite element scheme. Resistance curves are obtained for a range of parameters that specify the cohesive traction-separation constitutive law. It is found that the choice of the cohesive strength t₀ has a great influence on the toughening behavior of the material. Moreover, the reversibility of the transformation can significantly reduce the toughening of the alloy. The shape of the initial transformation zone, as well as that of a growing crack is determined. The effect of the Young's moduli ratio of the martensite and austenite phases is examined.     

Behaviour of biaxial nematics in the presence of electric and magnetic fields

The behaviour of a biaxial nematic liquid crystal in the presence of electric and magnetic fields is discussed. In terms of the values of the magnetic susceptibilities and the dielectric permittivities, each biaxial nematic compound can be classified to belong to one of thirty-six different states. These states can be grouped together into three different classes, denoted by us as type A, B and C. The states belonging to each class exhibit a different qualitative behaviour in the presence of perpendicular electric and magnetic fields. While type A biaxial nematics always exhibit the same stable configuration in the presence of the fields, type B and C biaxial nematics exhibit two possible stable equilibrium configurations. Which of these is stable is determined by the magnitudes of the applied fields. The exchange of stability for type B systems can be modelled as a second order transition, while the exchange of stability for type C systems is of first order. In addition, the latter type can develop a bistable behaviour if certain conditions for the magnitudes of the electric and magnetic fields are fulfilled.     

Implications of matrix effects in ultra-fast gradient or fast isocratic liquid chromatography with mass spectrometry in drug discovery.

In the analysis of biological samples it is important to reduce the risk of interferences from the matrix itself, other analytes, the dosing vehicle (commonly PEG), and from the MS/MS transitions used for the analysis. Rapid analysis is essential for drug discovery, and even though the requirements for separation may be minimized for speed, the integrity of the analysis is still dependent on the separation. This paper focuses on the potential for interferences from various endogenous and exogenous matrix components commonly encountered in quantitation of analytes and their metabolites from biological matrices. We demonstrate that neither high organic isocratic nor ballistic gradient ultra-fast HPLC show a clearly defined advantage in regards to complex biological matrices. The critical factor in the resolution of matrix interferences still remains in sample preparation.