Mechanics of seed-grown planar chemical gardens
In the 17th century, perhaps by accident, the German alchemist Glauber introduced solid crystals of iron(III) chloride into an aqueous solution of potassium silicate. He noted the formation of some strange, plant-like structures, which he recorded in his 1646 book Furni Novi Philosophici as "philosophical trees". Glauber would not have guessed that, three centuries later, his discovery opened up a whole new field of study. The structures he reported are known as "chemical gardens" -- self-assembled semi-permeable precipitates. They are considered laboratory analogues of hydrothermal vents that are thought to be where biological life on Earth self-assembled and originated. In this thesis, in Chapter 1, a review of chemical gardens studied in confined geometries -- Hele-Shaw cells and microfluidic chips -- is presented. Studies in these geometries have been classified into three approaches: seed-growth in Hele-Shaw cells, injection-growth in Hele-Shaw cells, and co-injection-growth in microfluidic chips. Over the past decade, these setups have seen a growing popularity due to lower complexity and easier analysis. Chapters 2-4 focus on the dynamics of seed-grown chemical gardens in Hele-Shaw cells. In Chapter 2, the growth behaviour of chemical garden walls in a horizontal Hele-Shaw cell has been investigated. The experiments have been conducted with pellets of either solid cobalt(II) or manganese(II) chloride contained in aqueous sodium silicate solutions. It is found that the growth of the chemical garden walls can be well described by a simple, diffusion-controlled dynamical model until their eventual osmotic fracture at a reproducible time. Chapter 3 investigates the behaviour of chemical gardens grown in a vertical Hele-Shaw cell. Instead of cobalt chloride, magnesium chloride pellets were used as seeds in aqueous sodium silicate solutions. The chemical gardens exhibited non-isotropic radial growth -- fastest at the bottom and slowest at the top. In each direction, the radial growth can be mathematically modelled by the diffusion-controlled model presented in Chapter 2. The relative pressure at the centre of the chemical garden underwent an initial, gradual growth, followed by a rapid increase that turned into a plateau. Upward tubular growths were observed for higher silicate concentrations, and the onset of upward tubular growth has been found to correlate with the onset of rapid pressurisation. The results may have implications on the formation mechanisms of hydrothermal vent flanges. Building on the findings of Chapter 3, Chapter 4 further studies the behaviour of chemical gardens in a vertical Hele-Shaw cell, focusing on the dynamics of the tubular growth. The chemical gardens were grown from cobalt chloride pellets in aqueous sodium silicate solutions. Tubular growth was again observed to be accompanied by a rapid pressure rise, as found in Chapter 3. Analysis of individual tube dynamics suggests that a reaction-controlled linear model underestimates the growth rate and that a mass transfer-controlled square-root model based on the Hagen-Poiseuille law is inconsistent with experimental results. Together, the results of Chapters 2-4 establish a good quantitative basis for understanding the dynamics of confined chemical garden systems and provide a framework for studying more complex chemical garden systems, such as hydrothermal vents and functional chemobrionic materials.
chemical garden, chemobrionics, Hele-Shaw cell, hydrothermal vent, origin of life, self-assembly
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