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

Abstract

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.

Materials Synthesis and Catalysis in Microfluidic Devices: Prebiotic Chemistry in Mineral Membranes

“…For precipitation reactions, the flow rate decides not only the width of the diffusive layer, but‐in some cases‐also the product composition. For example, Zeng et al . have reported that the crystallization of CaCO 3 is dependent on the volumetric flow rate Q .…”

“…Zeng et al. have studied the transition from amorphous to crystalline calcium carbonate precipitate through dissolution‐recrystallization processes at the interface of equimolar aqueous solutions of CaCl 2 and Na 2 CO 3 . A mixture of vaterite and calcite was observed in the channel and shows distributional variations along the channel, that is, a decreasing ratio of vaterite to calcite from the Y‐junction to the outflow port.…”

Calcium Carbonate Mineralization in a Surface-Tension-Confined Droplets Array

“…Gong et al's research provided a new approach to biomimetic crystallization by using crystal hotels [26]. Zeng et al's work demonstrated a microfluidic approach towards the study of the formation and transformation of ACC (amorphous calcium carbonate) by using microfluidic technology [27]. However, these experiments were mainly carried out in microfluidic chips, which constrained the methods of characterization and hindered the use of high-throughput experiments.…”

Flow-driven crystal growth of lithium phosphate in microchannels

“…37 The confinement due to the microfluidic conditions slows down the conversion of the unstable amorphous calcium carbonate into the crystalline form as opposed to the well-stirred reaction where the stable crystalline form appears within seconds to minutes of mixing. [38][39][40] Building upon the versatile applicability of microfluidics, we first study experimentally the precipitation of lithium phosphate in a microchannel and study the effect of flow on the growth of the precipitate particles. The results are then corroborated by simple calculations.…”