A percolation context evidenced by thermostimulated currents in oil-resin art media

Degradation over time of home-made thixotropic oil/resin mixtures, similar to those used as natural media by the art painter J.M. William Turner (1775–1851) is investigated by using a combination of dielectric methods, i.e., low-frequency dielectric spectroscopy (LFDS) and the technique of thermostimulated currents (TSC). These address the direct conductivity (d.c.), i.e., the circulation of free charges, and the anelastic orientation of permanent dipoles, respectively. The oil/resin mixtures can be regarded as conducting/insulating-like composite systems with respect to the two conductivities. This provides the opportunity to follow selectively the behavior of the oil, at any composition. In fresh mixtures, LFDS measurements give evidence for three d.c. regimes, delimited by the critical resin-contents (R) R₁30% and R₂60%. This parallels observation with the naked eye of phase separation at R<40% and color change at R>R₂ in four-year-old naturally-aged samples. Such a stable composition-dependent context with two marked thresholds encourages a discussion of the time-behavior in terms of percolation, i.e., relative spatial organization of the parent substances, rather than by using a microscopic approach at the molecular scale. At R>R₁ the surveillance of the TSC signal indicates that the oil clusters tend to coalesce so as to form bigger clusters with aging. The coalescence process appears to be more efficient when the resin forms a continuum (R>R₂) than a dispersion (R<R₂). PACS 82B43     

Modelling of particle paths passing through an ultrasonic standing wave

Within an ultrasonic standing wave particles experience acoustic radiation forces causing agglomeration at the nodal planes of the wave. The technique can be used to agglomerate, suspend, or manipulate particles within a flow. To control agglomeration rate it is important to balance forces on the particles and, in the case where a fluid/particle mix flows across the applied acoustic field, it is also necessary to optimise fluid flow rate. To investigate the acoustic and fluid forces in such a system a particle model has been developed, extending an earlier model used to characterise the 1-dimensional field in a layered resonator. In order to simulate fluid drag forces, CFD software has been used to determine the velocity profile of the fluid/particle mix passing through the acoustic device. The profile is then incorporated into a MATLAB model. Based on particle force components, a numerical approach has been used to determine particle paths. Using particle coordinates, both particle concentration across the fluid channel and concentration through multiple outlets are calculated. Such an approach has been used to analyse the operation of a microfluidic flow-through separator, which uses a half wavelength standing wave across the main channel of the device. This causes particles to converge near the axial plane of the channel, delivering high and low particle concentrated flow through two outlets, respectively. By extending the model to analyse particle separation over a frequency range, it is possible to identify the resonant frequencies of the device and associated separation performance. This approach will also be used to improve the geometric design of the microengineered fluid channels, where the particle model can determine the limiting fluid flow rate for separation to occur, the value of which is then applied to a CFD model of the device geometry.     

On the temperature dependence of the F-coloring efficiency in KCl

A simple model is presented to account for the major features of the temperature dependence of the primary F-coloring efficiency in KCl. It assumes that irradiation creates Frenkel pairs in three basic configurations followed by some correlated diffusion, leading either to annihilation of the pair or formation of a “surviving” primary pair. Steady irradiation as well as fast pulse data are analyzed in terms of the model.     

Optical applications of ion beam machining

It is now apparent that ion beams have a role to play in the machining and polishing of optical components. In this paper we review some of the applications which have been reported so far and also consider future applications. Since ion beam machining is not a passive process we consider the topographies which are generated by the technique and their relevance to device manufacture.