From Balloon to Crystalline Structure in the Calcium Phosphate Flow-Driven Chemical Garden

Abstract

We have studied the calcium phosphate precipitation reaction by producing chemical gardens in a controlled manner using a three-dimensional flow-driven technique. The injection of the phosphate containing solution into the calcium ion reservoir has resulted in structures varying from membranes to crystals. Dynamical phase diagrams are constructed by varying chemical composition and flow rates from which three different growth mechanisms have been revealed. The microstructural analysis by scanning electron microscopy and powder X-ray diffraction confirmed the morphological transition from membrane tubes to crystalline branches upon decreasing pH.

Nonclassical Crystallization Causes Dendritic and Band‐Like Microscale Patterns in Inorganic Precipitates

“…A third, and arguably the most widely known, example is the chemical garden system (Figure S1) [10,19–23,26,32–34] . These plant‐like inorganic structures arise from self‐organizing precipitation reactions between a metal salt and a solution containing silicates, hydroxides, carbonates, phosphates or similar anions [10,26] .…”

“…During these reactions, the precipitates assemble thin membranes that compartmentalize the system and auto‐regulate the exchange of reactants [10,19,21,25,26] . This exchange includes an osmotically driven flow of water towards the dissolving metal salt, which causes the macrostructure to increase in volume and, directed by buoyancy, extend in the upward direction [19,32,33,35,36] . In addition, pH gradients drive the transport of hydroxide ions across the membrane and steadily thicken the precipitate barrier [21–23,36] …”

Nonclassical Crystallization Causes Dendritic and Band‐Like Microscale Patterns in Inorganic Precipitates

“…A third, and arguably the most widely known, example is the chemical garden system (Figure S1). [10,[19][20][21][22][23]26,[32][33][34] These plant-like inorganic structures arise from self-organizing precipitation reactions between a metal salt and a solution containing silicates, hydroxides, carbonates, phosphates or similar anions. [10,26] During these reactions, the precipitates assemble thin membranes that compartmentalize the system and auto-regulate the exchange of reactants.…”

“…[10,19,21,25,26] This exchange includes an osmotically driven flow of water towards the dissolving metal salt, which causes the macrostructure to increase in volume and, directed by buoyancy, extend in the upward direction. [19,32,33,35,36] In addition, pH gradients drive the transport of hydroxide ions across the membrane and steadily thicken the precipitate barrier. [21][22][23]36] Despite these qualitative insights, the mechanisms by which the chemical species combine to create chemical garden membranes have remained unknown.…”