Growth Dynamics and Composition of Tubular Structures in a Reaction-Precipitation System

Self-organization in reaction precipitation systems occurs in many physical, chemical, biological, and geological systems. In particular, chemical reactions provide a wealth of examples for this intriguing process. Permanent tubular structures arise from the interplay of chemical and transport phenomena such as diffusion and fluid flow. These astonishing tubular structures are prevalent throughout nature. Examples include black smokers at hydrothermal vents, silica tubes in setting cement, soda-straw stalactites in caves, and biological structures such as the outer skeleton of certain algae. In this work, the aim is to establish and understand a laboratory scale model by examining the, seemingly simple, precipitation reaction between sodium silicate and copper sulfate as well as zinc sulfate. The tubular precipitation structures in so-called silica gardens are known to many scientists and non-scientists alike. However, little is known regarding their growth dynamics and chemical composition. We devised an injection technique which provides control over parameters that are not accessible in the classic silica garden system. For the example of cupric sulfate injection into waterglass solution, we identify three distinct growth regimes (jetting, popping, and budding) and study their concentration dependent transitions. Here we describe the composition and morphology of the tube material using techniques such as electron microscopy and vibrational spectroscopy. Specifically, we find that the tube wall consists of metal hydroxide that is stabilized by a thin, exterior silica layer. After synthesis the tubes can be further modified by using chemical and/or physical means. A second study aims to understand tubule formation under "reverse" conditions. More specifically, waterglass is being injected into lighter cupric sulfate solution. In these experiments, single, downward growing precipitation tubes are created. Four distinct growth regimes are observed and their stability in terms of flow rate and cupric sulfate concentration is investigated. Three of these growth regimes (reverse jetting, reverse popping, and reverse budding) resemble the same behavior for the injection of cupric sulfate into silicate solution. However, the reverse conditions studied herein reveal one novel regime in which the tube is limited by repetitive fracturing. The lengths of the broken-off tube segments and times between subsequent break-off events can be described by log-normal distributions. We also discuss the development of a method for synthesizing highly linear precipitation tubes via gas bubble injection and templating. In this method, an aqueous metal salt is injected into a large reservoir of waterglass. Systematic measurements show that the size of the bubble governs the tube radius. According to this radius, the system selects its growth velocity following volume conservation of the injected metal salt solution. Moreover, scanning electron microscopy reveals intricate ring patterns on the walls. We also show evidence for the existence of minimal and maximal tube radius. Lastly, we report the collapse of tubes at high concentrations of silicate solution, yielding twisted ribbon-like structures. Critical radii and tube collapse are discussed in terms of simple competing forces.