Multiscale modeling of the onset of freckle formation during vacuum arc remelting

A multiscale model for the Vacuum Arc Remelting process (VAR) was developed to simulate dendritic microstructures during solidification and investigate the onset of freckle formation. On the macroscale, a 3D multi-physics model of VAR was used to study complex physical phenomena, including liquid metal flow with turbulence, heat transfer, and magnetohydrodynamics. The results showed that unsteady fluid flow in the liquid pool caused significant thermal perturbation at the solidification front. These results were coupled into a micromodel to simulate dendritic growth controlled by solute diffusion, including local remelting. The changes in Rayleigh number as the microstructure remelts was quantified to provide an indicator of when fluid flow channels (i.e. freckles) will initiate in the mushy zone. By examining the simulated microstructures, it was found that the Rayleigh number increased more than 300 times during remelting, which suggests that thermal perturbation could be responsible for the onset of freckle formation.

The effect of wall-slip formation on the rheological behaviour of lead-free solder pastes

Wall-slip plays an important role in the flow behaviour of solder paste materials. The wall-slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry. These interactions could lead to the presence of a thin liquid layer adjacent to the wall, which causes slippage. The aim of this study is to investigate the influence of the solder paste formulation on wall-slip formation and its effect on the printability of these pastes material. A wall slip model is utilised to calculate the true viscosity and slip velocity for the lead-free solder pastes samples used in this study. The difference in the measured viscosity and the true viscosity could indicate wall-slip formation between the solder pastes and the parallel plate. Sample P1 showed a higher slip velocity compared to sample P2. The slip velocity calculated for the solder pastes could be used as a performance indicator to understand the paste release characteristics in the stencil printing process.

The role of space charge formation in the generation of thermally stimulated current (TSC) spectroscopy data for a model amorphous drug system

Thermally stimulated current (TSC) spectroscopy is attracting increasing attention as a means of materials characterization, particularly in terms of measuring slow relaxation processes in solid samples. However, wider use of the technique within the pharmaceutical field has been inhibited by difficulties associated with the interpretation of TSC data, particularly in terms of deconvoluting dipolar relaxation processes from charge distribution phenomena. Here, we present evidence that space charge and electrode contact effects may play a significant role in the generation of peaks that have thus far proved difficult to interpret. We also introduce the use of a stabilization temperature in order to control the space charge magnitude. We have studied amorphous indometacin as a model drug compound and have varied the measurement parameters (stabilization and polarization temperatures), interpreting the changes in spectral composition in terms of charge redistribution processes. More specifically, we suggested that charge drift and diffusion processes, charge injection from the electrodes and high activation energy charge redistribution processes may all contribute to the appearance of shoulders and 'spurious' peaks. We present recommendations for eliminating or reducing these effects that may allow more confident interpretation of TSC data.

Can Pickering emulsion formation aid the removal of creosote DNAPL from porous media?

The purpose of this investigation was to examine the proposition that creosote, emplaced in an initially water saturated porous system, can be removed from the system through Pickering emulsion formation. Pickering emulsions are dispersions of two immiscible fluids in which coalescence of the dispersed phase droplets is hindered by the presence of colloidal particles adsorbed at the interface between the two immiscible fluid phases. Particle trapping is strongly favoured when the wetting properties of the particles are intermediate between strong water wetting and strong oil wetting. In this investigation the necessary chemical conditions for the formation of physically stable creosote-in-water emulsions protected against coalescence by bentonite particles were examined. It was established that physically stable emulsions could be formed through the judicious addition of small amounts of sodium chloride and the surfactant cetyl-trimethylammonium bromide. The stability of the emulsions was initially established by visual inspection. However, experimental determinations of emulsion stability were also undertaken by use of oscillatory rheology. Measurements of the elastic and viscous responses to shear indicated that physically stable emulsions were obtained when the viscoelastic systems showed a predominantly elastic response to shearing. Once the conditions were established for the formation of physically stable emulsions a "proof-of-concept" chromatographic experiment was carried out which showed that creosote could be successfully removed from a saturated model porous system. (C) 2007 Elsevier Ltd. All rights reserved.