Migrational separation due to differences in cationic mobility is commonly observed during current passage in molten carbonate mixtures, and this might be responsible for the improved wetting observed upon polarization, as found experimentally according to the literature. To check this, a 2D transport model based on concentrated‐solution theory was applied to analyze the movement of ions in and near the meniscus. The effect of differences in cationic mobility and of ionic transport in general on current distribution, reaction rate, and electrolyte composition in the meniscus region was quantified, and corresponding surface tension gradients over the meniscus surface predicted. The resulting surface tension gradients were found to be too small to account for the experimentally observed meniscus rise. It is, therefore, concluded that the polarization effect on electrode wetting is not due to the gradient of surface tension caused by cationic separation. A plausible alternative explanation is that a gradient of the S/L interfacial tension exists but that this is due to specifically adsorbed intermediate reaction products, in particular oxides. Such a current density dependent adsorption layer would be in dynamic equilibrium with the local melt composition, and, thereby, drive the wetting/dewetting of the electrode surface that is experimentally observed.
A temperature gradient imposed across a binary fluid layer with a nonzero Soret coefficient will induce a solute concentration gradient. The ratio of these two gradients is proportional to the separation ratio χ, a property of the fluid. Similarly, the ratio of the thermal and solutal Marangoni numbers, which are nondimensional increments in surface tension due to changes in temperature and concentration, is also proportional to the separation ratio. As a consequence, the stability of a given binary fluid layer with a free surface under zero gravity depends only on the temperature difference, ΔT, imposed across the layer or, equivalently, on the thermal Marangoni number, M, albeit the dependence, is rather complicated. When the gravity is nonzero but of small magnitude, such that the buoyancy effects are not dominant, the stability characteristics of the layer are functions of two parameters, M and R, the thermal Rayleigh number. In this paper, the stability of such a binary layer under zero and reduced gravity by means of linear stability analysis is studied. Results show that the nature of the instability depends on the product χK, where K is a material constant=(α/αS)(γS/γ), with α and αS denoting the volumetric expansion coefficient due to temperature and solute concentration, respectively, and γ and γS the rate of change of surface tension with respect to temperature and solute concentration, respectively. Both χ and K can assume positive and negative values. Under zero gravity, instability at the critical value of M onsets in steady convection if χK<0 and in oscillating convection if χK≳0. For a layer that is being heated from below and K≳0, the steady instability in the case of χK<0 can be rendered stable by subjecting the layer to a gravity of small magnitude. But for χK≳0, the effect of gravity is always destabilizing.
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