The Dynamics of Thin Films on Reaction Surfaces

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Abstract
A model is proposed for heterogeneous ignition and extinction in terms of the dynamics of a thin inert layer covering the surfaces of condensed-phase components in a combustion reaction, the layer consisting of reaction products or impurities which collect in the reaction surface. Heterogeneous ignition then is associated with the formation and the spreading of ruptures and punctures in the surface layer, whereas extinction is associated with the formation and the spreading of layer fragments in the form of “caps” or “islands”. For a liquid layer, the model describes the dynamics of a liquid edge where the layer thickness changes from macroscopic to microscopic values. Such a description is in terms of ordinary hydrodynamics only for layer thicknesses which are large enough. Consequently, an intermediate “mesoscopic” range exists which separates macroscopic from microscopic thicknesses, designating the transition range where ordinary hydrodynamics changes over into different forms of transport. Those parts of the liquid edge which are primarily affected by these different forms of transport comprise the socalled “precursor” for which the layer thickness assumes mesoscopic or microscopic values, Recently, major progress in the field of liquid films lead to a much improved understanding of the wetting dynamics for nonvolatile precursors on inert isothermal surfaces. These results are generalized to include nonisothermal reacting surfaces covered by films for which formation and evaporation is admitted. Conceptually, heterogeneous ignition and extinction are shown to be associated with the nucleation and spreading of precursors with thickness profiles representing self-similar surface waves.