Reaction erosion fronts

A co-operative diffusion of water molecules in and oligomers out gives rise to sharp reaction-erosion fronts close to the surface in a mechanism similar to that for the creation of the surface layer as proposed by Li et al. 

When a hydrophobic polymer with a physically dispersed acidic excipient is placed into an aqueous environment, water will diffuse into the polymer, dissolving the acidic excipient, and consequently the lowered pH will accelerate hydrolysis of the ortho ester bonds. The process is shown schematically in Fig. 6 (18). It is clear that the erosional behavior of the device will be determined by the relative movements of the hydration front Vj and that of the erosion front V2- If Vj > V2, the thickness of the reaction zone will gradually increase and at some point the matrix will be completely permeated with water, thus leading to an eventual bulk erosion process. On the other hand, if V2 = Vj, a surface erosion process wiU take place, and the rate of polymer erosion will be completely determined by the rate at which water intrudes into the matrix. 

A pyrite decomposition process is an instationary process since the solid phase reactant is being consumed under pyrite-oxidizing conditions causing changes in both reactive surface and mineral mass. Only in a fictitious system, in which the velocity of the depyritization front penetration by chance equals the surface erosion stationary conditions may be found. Diffusion under instationary conditions may be described by numerical solutions of the transport reaction equation considering Picks second law. Reactions can be integrated quite simply, if transport and reaction are decoupled in a... 

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