Screening the Ion Compositions on Crystal Morphology Transitions by a Microfluidic Chip with a Well-Defined Concentration Gradient

The effects of organic and inorganic additives on crystal morphology have been extensively studied in various materials. However, the basic fundamentals behind the morphology control are still under debate due to the differences in experimental observations and lack of technical means to control the microenvironments where the crystal grows. Microfluidic technology with convenient operation and precise control of related parameters provides a unique tool for the investigation of crystallization processes on the micrometer and even nanometer scale. A concentration-gradient microfluidic chip was designed to screen the ion compositions to investigate the factors influencing the crystal morphology, and calcium carbonate was chosen for this study. Unexpectedly, the CO32-:Ca2+ ratio is discovered to induce a calcite morphology transition from regular rhombohedral to irregular rhombohedral and nanoparticle aggregations. In contrast, the morphology transition is believed to be caused by variations of the Mg:Ca ratio and supersaturation. The incorporation of different mole ratios of magnesium ions is also found to give different morphologies. The experimentally observed crystal morphology transition indicates that the crystal morphology is influenced by the CO32-:Ca2+ ratio when magnesium ions are present in the system. The proposed microfluidic method, providing an exquisite control of microenvironments during crystallization, can be applied in other materials to analyze the origin of unique morphologies. In addition, the discovery extends our understanding of the morphology control of biominerals.