No abstract

Chemical gardens are mineral aggregates that grow in three dimensions with plant-like forms and share properties with selfassembled structures like nanoscale tubes, brinicles, or chimneys at hydrothermal vents. The analysis of their shapes remains a challenge, as their growth is influenced by osmosis, buoyancy, and reaction-diffusion processes. Here we show that chemical gardens grown by injection of one reactant into the other in confined conditions feature a wealth of new patterns including spirals, flowers, and filaments. The confinement decreases the influence of buoyancy, reduces the spatial degrees of freedom, and allows analysis of the patterns by tools classically used to analyze 2D patterns. Injection moreover allows the study in controlled conditions of the effects of variable concentrations on the selected morphology. We illustrate these innovative aspects by characterizing quantitatively, with a simple geometrical model, a new class of self-similar logarithmic spirals observed in a large zone of the parameter space.C hemical gardens, discovered more than three centuries ago (1), are attracting nowadays increasing interest in disciplines as varied as chemistry, physics, nonlinear dynamics, and materials science. Indeed, they exhibit rich chemical, magnetic, and electrical properties due to the steep pH and electrochemical gradients established across their walls during their growth process (2). Moreover, they share common properties with structures ranging from nanoscale tubes in cement (3), corrosion filaments (4) to larger-scale brinicles (5), or chimneys at hydrothermal vents (6). This explains their success as prototypes to grow complex compartmentalized or layered self-organized materials, as chemical motors, as fuel cells, in microfluidics, as catalysts, and to study the origin of life (7-18). However, despite numerous experimental studies, understanding the properties of the wide variety of possible spatial structures and developing theoretical models of their growth remains a challenge.In 3D systems, only a qualitative basic picture for the formation of these structures is known. Precipitates are typically produced when a solid metal salt seed dissolves in a solution containing anions such as silicate. Initially, a semipermeable membrane forms, across which water is pumped by osmosis from the outer solution into the metal salt solution, further dissolving the salt. Above some internal pressure, the membrane breaks, and a buoyant jet of the generally less dense inner solution then rises and further precipitates in the outer solution, producing a collection of mineral shapes that resembles a garden. The growth of chemical gardens is thus driven in 3D by a complex coupling between osmotic, buoyancy, and reaction-diffusion processes (19,20).Studies have attempted to generate reproducible micro-and nanotubes by reducing the erratic nature of the 3D growth of chemical gardens (10,11,13,15,21). They have for instance been studied in microgravity to suppress buoyancy (22, 23), or by injecting aqueous ...

The growth of chemical gardens is studied experimentally in a horizontal confined geometry when a solution of metallic salt is injected into an alkaline solution at a fixed flow rate. Various precipitate patterns are observed-spirals, flowers, worms or filaments-depending on the reactant concentrations. In order to determine the relative importance of the chemical nature of the reactants and physical processes in the pattern selection, we compare the structures obtained by performing the same experiment using different pairs of reactants of varying concentrations with cations of calcium, cobalt, copper, and nickel, and anions of silicate and carbonate. We show that although the transition zones between different patterns are not sharply defined, the morphological phase diagrams are similar in the various cases. We deduce that the nature of the chemical reactants is not a key factor for the pattern selection in the confined chemical gardens studied here and that the observed morphologies are generic patterns for precipitates possessing a given level of cohesiveness when grown under certain flow conditions.