Surfaces mobility gradient

Surface contaminants affect mass transfer via hydrodynamic and molecular effects, and it is convenient to consider these separately. Hydrodynamic effects include two phenomena which act in opposition. In the absence of mass transfer, contaminants decrease the mobility of the interface as discussed in Section ILD. In the presence of mass transfer, however, motion at the interface may be enhanced through the action of local surface tension gradients caused by small differences in concentration along the interface. This enhancement of surface... 

The use of the surface concentration gradient as the driving force for surface diffusion is the most popular approach with which to describe the mobility of adsorbed molecules. When each adsorbed species is assumed to be at adsorption equilibrium and transported along the surface independently of the other species, the molar flux of species i due to surface diffusion can be written as... 

A strength of the system is its ability to undertake anode reform, with a comfortable reaction rate, and low thermal stress at 600 °C. The ability of the MCFC to achieve 40 000 h operating life is still being demonstrated by its protagonists. Moreover, changes to the molten carbonate formulation are under consideration. Observed mobility of the molten carbonate, within the matrix conceived as fixing it, has been ascribed by the author to the surface tension gradient associated with concentration and temperature differences. 

The three-phase interface is a well-known concept, notably in connection with the invention by Bacon of bi-porous electrodes (Bacon, 1969). At equilibrium, in such electrodes, gas, liquid electrolyte and catalytic solid are in contact at a convoluted meniscus at the coarse-pore, liquid-containing region interface with the fine-pore, gas-containing region. The picture changes with departure from equilibrium. The meniscus becomes mobile under the influence of surface tension gradients or Marangoni forces - an invisible complex. 

Like the competing IT/SOFC, the MCFC is an incomplete system which has not considered isothermal oxidation with its circulator problem, as defined in this book. The system is also immature as it stands, since the mobility of its electrolyte is a newly recognised fact, for which the author has suggested surface tension gradient as the mechanism. Moreover, a major increase in power density is under consideration, which would interact with electrolyte mobility via higher temperature gradient. 

Fig. 10 Possible model for depth dependence of Tg as a function of Mn, showing the mobility gradient in the surface region. Within the depth region in which chain ends are segregated, the mobility gradient should be strongly dependent on M. This would be not the case far beyond the segregated surface layer of chain ends...

Thermal molecular motion of PS at surfaces and interfaces in films was presented in this review. We clearly show that chain mobility at the surface region is more mobile than in the interior bulk phase and that chain mobility at the interfacial region is less than in the interior phase. This means that there is a mobility gradient in polymer films along the direction normal to the surface. This gradient can be experimentally detected if the ratio of the surface and interfacial areas to the total volume increases, namely in ultrathin films. 

The surface mobility factor is analysed in detail in [36]. This analysis demonstrates that two factors control the degree of surface mobility the surface viscosity and the presence of surface tension gradients. 

The surface mobility coefficient ms is a phenomenological parameter that represents the net mass flow per unit gradient in surface chemical potential. The physical mechanisms of mass transport underlying this parameter are not well understood quantitatively, but some dependencies can be surmised. Surely, mobility must depend on temperature, binding energy and concentration of the diffusing species. A qualitative argument that leads to an estimate of the mobility coefficient proceeds as follows. 

A complicated interaction between chemical reaction and mass transport may occur close to the interface between two phases, particularly when the interface is mobile, such as a gas/liquid or a liquidAiquid interface. Because of the transport of at least one component across the interface, there will be considerable concentration gradients perpendicular to the interface. If the interfacial area is changing, e.g., due to gas bubble (or droplet) coalescence or breakup, or simply by deformation of the bubbles (or droplets), there will also be concentration gradients parallel to the interface, and these will result in surface tension gradients which will influence the rates of coalescence and breakup, which has cpnsequences for the rate of mass transport. The situation becomes even more complicated when local gradients in liquid density and viscosity also play a role. 

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