Aragonite crystallization in a Christmas-tree model microfluidic device with the addition of magnesium ions

Aragonite is an important dimorph of calcium carbonate, industrially and biologically. However, aragonite is so thermodynamically unstable that it is difficult to understand its formation mechanism. A continuous microfluidic system was employed, in which crystallization was induced only by diffusion in a micron-scale channel. Calcium carbonate (CaCO₃) formed by liquid-liquid reaction and magnesium ions (Mg²⁺) were used as additives. To assess the influence of Mg²⁺ concentration, the Mg²⁺/Ca²⁺ molar ratio was set to 1, 3, and 5. Laminar streams flowed in the detection channel with different concentration gradients. The initial crystallization time (t_I.C) increased exponentially and the density of crystals decreased as the Mg²⁺ ion concentration increased. Following transformation of all particles into snowman or sphere shapes, they became spinose sphere-shaped crystals, which was the final form in this study.