Ocean-ridge-like growth in silica tubes

Complex shapes can occur through successions of instabilities, like in the growth of chemical gardens, where a semi-permeable membrane precipitates at the interface between a sodium silicate solution and a metal salt. Instead of letting the osmotic pressure during the metal salt dissolution lead the dynamics of the growth, we inject a ferric sulfate solution into a sodium silicate solution, both of controlled concentration, controlling also the other hydrodynamical parameters. Although qualitatively distinct regimes can be obtained, we focus here on a previously unobserved regime where the reactive interface grows in tubular fingering patterns with ocean-ridge-like dynamics: the tubes grow evenly on both sides of a central fracture where the silica deposits continuously. Our experiments show that the whole dynamics is intrinsically related to the precipitation occurring at the interface: the tubes elongation rate remains constant even when the injection rate is varying, but strongly depends on the limiting concentration in injected solution, thus on the reaction rate. Copyright (C) EPLA, 2010