Formation of Amorphous Calcium Carbonate and Its Transformation Mechanism to Crystalline CaCO3 in Laminar Microfluidics

For traditional experimental methods, the dynamic formation and transformation of amorphous calcium carbonate (ACC) are hard to capture due to its metastability and ease of transforming rapidly to the more stable phase. The emergence of microfluidic technology provides an effective approach on that issue, thus attracting widespread researchers’ interest in crystallization research. In this study, based on laminar microfluidics, we demonstrate a microfluidic approach toward the study of the formation and transformation of ACC. Through the control of the velocity and concentration of CaCl 2 and Na 2 CO 3, the crystallization process was observed on a chip under the microscope. We show by in situ confocal micro Raman spectroscopy and theoretical calculation based on fluid dynamics simulation that the phase transformation pathways are different under the different supersaturation levels in laminar microfluidics. ACC does not always appear in the crystallization process; when supersaturation above the critical supersaturation of ACC occurred, the crystalline CaCO 3 phases appeared after the metastable intermediates ACC through the transformation mechanism of dissolution–recrystallization, while the crystalline CaCO 3 phases formed without the intermediates ACC when supersaturation below the critical supersaturation of ACC occurred. Our work provides an approach for the investigation of metastable intermediates and support for one-step and multistep nucleation mechanisms.