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The velocity boundary condition that must be imposed at an interface between a porous medium and a free fluid is investigated. A heterogeneous transition zone characterized by rapidly varying properties is introduced between the two homogeneous porous and free fluid regions. The problem is solved using the method of matched asymptotic expansions and boundary conditions between the two homogeneous regions are obtained. The continuity of the velocity is recovered and a jump in the stress built using the viscosity (and not the effective viscosity) appears. This result also provides an explicit dependence of the stress jump coefficient to the internal structure of the transition zone and its sensitivity to this micro structure is recovered.

Marine biofilm communities that developed on artificial substrata were investigated using molecular and microscopic approaches. Polystyrene, Teflon® and four antifouling (AF) paints were immersed for 2 weeks at two contrasting sites near Toulon on the French Mediterranean coast (Toulon military harbour and the natural protected area of Porquerolles Island). Biofilms comprising bacteria and diatoms were detected on all the coatings. The population structure as well as the densities of the microorganisms differed in terms of both sites and coatings. Lower fouling densities were observed at Porquerolles Island compared to Toulon harbour. All bacterial communities (analysed by PCR-DGGE) showed related structure, controlled both by the sites and the type of substrata. Pioneer microalgal communities were dominated by the same two diatom species, viz. Licmophora gracilis and Cylindrotheca closterium, at both sites, irrespective of the substrata involved. However, the density of diatoms followed the same trend at both sites with a significant effect of all the AF coatings compared to Teflon and polystyrene.