Low-surface-energy (low critical surface tension) coatings with biofouling-release, low-adhesion surfaces dominated by closely packed methyl groups, were first shown by our research team in 1984 to yield reductions in drag in a stagnation point flow cell (Gucinski, et al., 1984). Our continued research now suggests that the presence of a particular range of surface microroughnesses, characterizable by microscopy and stylus profilometry, improves said drag reductions, by the probable mechanism of passive, recurrent microbubble nucleation, growth and release into the boundary layer of the flow. In our extensions of the laboratory systems to large-scale trials, the maximum amount of drag reduction was achieved by coatings combining surface modifications of both chemistry and texture. Our proposed mechanism for the inherent drag reduction of coatings is the presence of conditions that both nucleate and stabilize "microbubbles" at the surface. A number of studies have been reported pertaining to the effects of small bubbles on the turbulent boundary layer, noticing that the presence of microbubbles reduces the skin friction on surfaces (Madavan et al., 1984, 1985a and b; Legner, 1984). Other methods for drag reduction of inanimate objects proposed over the years have been heating of the wall (which reduces skin friction but has no effect on separation), ion wind and near-wall vortex generators, all of which were aimed at boundary layer modification (Lumley, 1977a). A proton magnetic resonance study of water structure near methylated (hydrophobic) vs clean (hydrophilic) glass walls illustrated that boundary layer stabilization via hydrophobically structured vicinal water might be another mechanism involved in the drag reduction of methylsilicone coatings (Fruci, 1997). Two-stage control methods also have been studied, whereby two techniques were combined, such as suction and riblets, and polymers and riblets. The beneficial effects of these prior combinations on reduction of drag, though, were minimal (Lumley, 1977b).
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