Patterns of entropy production in dissolving natural porous media with flowing fluid

The tendency for irreversible processes to generate entropy is the ultimate driving force for structure evolution in nature. In engineering, entropy production is often used as an indicator for loss of usable energy. In this study, we show that the analysis of entropy production patterns can provide insight into the diverse observations from experiments that investigate porous medium dissolution in imposed flow field. We first present a numerical scheme for the analysis of entropy production in dissolving porous media. Our scheme uses a greyscale digital model for chalk (an extremely fine grained rock), that was obtained using X-ray nanotomography. Greyscale models preserve structural heterogeneities with very high fidelity. We focussed on the coupling between two types of entropy production: the percolative entropy, generated by dissipating the kinetic energy of fluid flow, and the reactive entropy, originating from the consumption of chemical free energy. Their temporal patterns pinpoint three stages of microstructural evolution. We then showed that local mixing deteriorates fluid channelisation by reducing local variations of reactant concentration. We also showed that microstructural evolution can be sensitive to the initial transport heterogeneities, when the macroscopic flowrate is low. This dependence on flowrate indicates the need to resolve the structural features of a porous system when fluid residence time is long

Factors affecting the Faradaic efficiency of Fe(0) electrocoagulation

Electrocoagulation (EC) using Fe(0) electrodes is a low cost water treatment technology that relies on efficient production of Fe(II) from the electrolytic dissolution of Fe(0) electrodes (i.e. a high Faradaic efficiency). However, the (electro)chemical factors that favor Fe(0) oxidation rather than O2 evolution during Fe(0) EC have not been identified. In this study, we combined electrochemical methods, electron microscopy and Fe measurements to systematically examine the interdependent effects of current density (i), anodic interface potential (EA) and solution chemistry on the Faradaic efficiency. We found that Fe(0) oxidation was favored (Faradaic efficiency >0.85) in chloride and bromide solutions at all i, whereas carbonate, phosphate, citrate, and nitrate solutions lead to Faradaic efficiencies <0.1. The anodic reaction (i.e. Fe(0) oxidation or O2 evolution) only depended on i in the sulfate and formate solutions. Experiments in binary-anion solutions revealed that molar ratios of [HCO3−]/[Cl−] near 100 and [NO3−]/[Cl−] near 20 separated the electrochemical domains of Fe(0) oxidation and O2 evolution in the EC system. These molar ratios were supported by experiments in synthetic groundwater solutions. We also found that the EA vs i curves for solutions with poor Faradaic efficiency overlapped but were situated 2–4 V vs Ag/AgCl higher than those of solutions with high Faradaic efficiency. Therefore, the position of the EA vs i curve, rather than the EA alone, can be used to determine unambiguously the reaction occurring on the Fe(0) anode during EC treatment

Homogeneous nucleation of colloidal melts under the influence of shearing fields

We study the effect of shear flow on homogeneous crystal nucleation, using Brownian Dynamics simulations in combination with an umbrella sampling like technique. The symmetry breaking due to shear results in anisotropic radial distribution functions. The homogeneous shear rate suppresses crystal nucleation and leads to an increase of the size of the critical nucleus. These observations can be described by a simple, phenomenological extension of classical nucleation theory. In addition, we find that nuclei have a preferential orientation with respect to the direction of shear. On average the longest dimension of a nucleus is along the vorticity direction, while the shortest dimension is preferably perpendicular to that and slightly tilted with respect to the gradient direction.Comment: 10 pages, 8 figures, Submitted to J. Phys.: Condens. Matte

Brands in international and multi‐platform expansion strategies: economic and management issues

Powerful media branding has historically facilitated successful international expansion on the part of magazine and other content forms including film and TV formats. Multi-platform expansion is now increasingly central to the strategies of media companies and, as this chapter argues, effective use of branding in order to engage audiences effectively and to secure a prominent presence across digital platforms forms a core part of this. Drawing on original research into the experience of UK media companies, this chapter highlights some of the key economic, management and socio-cultural issues raised by the ever-increasing role of brands and branding in the strategies of international and multi-platform expansion that are increasingly common- place across media

Competition policy newsletter Volume 1, No 2, Summer 1994

When reactive fluids flow through a dissolving porous medium, conductive channels form, leading to fluid breakthrough. This phenomenon is caused by the reactive infiltration instability and is important in geologic carbon storage where the dissolution of CO₂ in flowing water increases fluid acidity. Using numerical simulations with high resolution digital models of North Sea chalk, we show that the breakthrough porosity is an important indicator of dissolution pattern. Dissolution patterns reflect the balance between the demand and supply of cumulative surface. The demand is determined by the reactive fluid composition while the supply relies on the flow field and the rock’s microstructure. We tested three model scenarios and found that aqueous CO₂ dissolves porous media homogeneously, leading to large breakthrough porosity. In contrast, solutions without CO₂ develop elongated convective channels known as wormholes, with low breakthrough porosity. These different patterns are explained by the different apparent solubility of calcite in free drift systems. Our results indicate that CO₂ increases the reactive subvolume of porous media and reduces the amount of solid residual before reactive fluid can be fully channelized. Consequently, dissolved CO₂ may enhance contaminant mobilization near injection wellbores, undermine the mechanical sustainability of formation rocks and increase the likelihood of buoyance driven leakage through carbonate rich caprocks