Tubular precipitation structures: materials synthesis under non-equilibrium conditions.

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 µm. 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.

Compositional analysis of copper-silica precipitation tubes.

Silica gardens consist of hollow tubular structures that form from salt crystals seeded into silicate solution. We investigate the structure and elemental composition of these tubes in the context of a recently developed experimental model that allows quantitative analyses based on predetermined reactant concentrations and flow rates. In these experiments, cupric sulfate solution is injected into large volumes of waterglass. The walls of the resulting tubular structures have a typical width of 10 microm and are gradient materials. Micro-Raman spectroscopy along with energy dispersive X-ray fluorescence data identify amorphous silica and copper(ii) hydroxide as the main compounds within the inner and outer tube surfaces, respectively. Upon heating the blueish precipitates to approximately 150 degrees C, the material turns black as copper(ii) hydroxide decomposes to copper(ii) oxide. Moreover, we present high resolution transmission electron micrographs that reveal polycrystalline morphologies.

Bubble guidance of tubular growth in reaction-precipitation systems.

Numerous types of reaction-precipitation systems involve the growth of tubular structures similar to those formed in "silica gardens". As a model case for this phenomenon, we investigate the rapid growth of hollow tubes in the reaction between sodium silicate and cupric sulfate. The latter solution is injected hydrodynamically at constant flow rates of 1-20 mL h(-1) into a large reservoir of waterglass. In this study, the growth is templated and guided by single, buoyant gas bubbles. The resulting tubes can be several decimetres long and have constant radii in the range of 100-600 microm. Systematic measurements show that bubble size governs the tube radius. According to this radius, the system selects its growth velocity following volume conservation of the injected solution. Moreover, scanning electron microscopy reveals intricate ring patterns on the tube walls. We also show evidence for the existence of a minimal and a maximal tube radius. Finally, we report an intriguing collapse of tubes created at high silicate concentrations, which yields twisted ribbon-like structures. Critical radii and tube collapse are discussed in terms of simple competing forces.