We investigate the growth of self-organized tubes formed by injection of metal salt solutions into silicate solution. The wall thickness increases strictly in an inward direction and obeys square root functions suggesting the presence of a traveling reaction-diffusion front in the radial direction. We also demonstrate the construction of multi-layered tubes.

Inorganic precipitation reactions are known to self-organize a variety of macroscopic structures, including hollow tubes. We discuss recent advances in this field with an emphasis on experiments similar to 'silica gardens'. These reactions involve metal salts and sodium silicate solution. Reactions triggered from reagent-loaded microbeads can produce tubes with inner radii of down to 3 mm. Distinct wall morphologies are reported. For pump-driven injection, three qualitatively different growth regimes exist. In one of these regimes, tubes assemble around a buoyant jet of reactant solution, which allows the quantitative prediction of the tube radius. Additional topics include relaxation oscillations and the templating of tube growth with pinned gas bubble and mechanical devices. The tube materials and their nano-to-micro architectures are discussed for the cases of silica/Cu(OH) 2 and silica/Zn(OH) 2 /ZnO tubes. The latter case shows photocatalytic activity and photoluminescence.

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Using reaction conditions far from equilibrium, we produce hollow tubes of silica-supported Cu(OH)2. The samples are then processed postsynthetically without compromising the macroscopic tubular structure. We specifically induce an amorphous-crystalline transition and demonstrate the sequential conversion of Cu(OH)2 to CuO, Cu2O, and metallic copper using thermal treatment and wet chemistry.

The paper describes a new phenomenon discovered in the electrolytic analog of a semiconductor diode. As an example, the phenomenon is studied in the 0.1M KOH-0.1M HCl diode where the alkaline and the acidic reservoirs are connected by a hydrogel cylinder. First the traditional, so-called positive salt effect is discussed. In that case some salt is added to the alkaline reservoir of a reverse biased electrolyte diode and as a result, close to a critical concentration of the added salt the electric current increases sharply. The so-called negative salt effect appears as a suppression of the positive one. It is shown by numerical simulations, by approximate analytical formulae, and also by experiments that the high current caused by the salt contamination in the alkaline reservoir can be mostly suppressed by relatively small salt concentrations in the acidic reservoir. Thus a straightforward application of the negative salt effect would be the sensitive detection of nonhydrogen cations in an acidic medium (e.g., in ion chromatography).