Forced convection diffusion layer formation

Similar processes for producing conducting polymeric films of benzene and its derivatives had been studied earlier [2-4], Necessary conditions for the successful realization of these processes are the use of a platinum electrode and a polar solvent in the presence of catalysts (Lewis acids) and thermostatting of the reactor at -75°C. A poly(para)phenylene polymerizate of the linear structure H-(-C6FLr)n-H with the degree of polymerization n, which varies between 3 and 16, is formed. Forced convection of monomeric molecules facilitates the polymerization reaction in the diffusion layer near the electrode and the formation of a dense film on the electrode surface and prevents the formation of poly(para)phenylene in the bulk. 

It follows from the above discussion and numerical results that even a simple convective-diffusive model of concentration behaviour mechanism gives realistic results and yields a satisfactory description of the formation of the gaseous layer under the anode surface. The model may be improved by adding the electrolyte circulation and electromagnetic forces yet we hope that it will not change the main conclusions. The finite volume method proves to be a flexible and sufficiently accurate numerical technique for solving both the equations for the Galvani potential and the reactant concentrations. The marker-and-cell approach makes it possible to outline the electrode surfaces easily. 

In more complicated models both equations have to be generalised by coupling surface and bulk convective diffusion and hydrodynamics. The situation is finely balanced since the motion of the surface has an effect on the formation of the dynamic adsorption layer, and vice versa. Adsorption increases in the direction of the liquid motion while the surface tension decreases. This results in the appearance of forces directed against the flow and retards the surface motion. Thus, the dynamic layer theory should be based on the common solution of the diffusion equation, which takes into account the effect of surface motion on adsorption-desorption processes and of hydrodynamics equations combined with the effect of adsorption layers on the liquid interfacial motion (Levich 1962). 

Fig. 2. Solute distribution and transport phenomena at the interface of a growing crystal (a) Instability of the crystal-liquid interface and formation of a nonplanar pattern (schematically), (b) Faceted growth. It is assumed that the solute concentration in the liquid far from the interface (Cq) is constant due to forced and natural convection (stirring) whereas a thin solute diffusion layer (S) is quiet and possesses a solute distribution profile depending on the crystallization process type (a) interfacial control or a surface reaction (interfacial kinetics), Ce < C RJ C a difference between Cj and Cj is responsible for the driving force to buUd up the crystal surface (c) diffusion control Cj < Cj, providing a driving force for bulk diffusion in the liquid (b) mixed control.

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