Spatially resolved analysis of calcium–silica tubes in reverse chemical gardens

Abstract

Reverse chemical gardens consist of hollow tubular structures that form in a downward direction from a mechanically held silicate crystal immersed in a metal salt solution. As a model case for this reaction–precipitation system, we investigate the composition, morphology, and microstructure of these tubes in the framework of an experimental model based on preselected reactant concentrations and flow rates. In these experiments, the heavier waterglass solution is being injected downward into large volumes of lighter calcium chloride solution. The resulting hollow precipitation tubes have diameters that range from 0.5 to 3.0 mm. The tube walls have a typical width measurement of 40 μm and are gradient materials. Scanning electron microscopy along with energy-dispersive X-ray spectral data identify calcium and silicon as the major components within the exterior and interior surfaces, respectively. Finally, we compare the behavior, chemical composition, and morphology of tubular precipitation structures created upon the hydrodynamic injection of calcium chloride into a large volume of sodium silicate solution carried out in the upward direction.