Calcium carbonate amorphous-to-crystalline transition drives complex precipitation patterns in confined fluids

Calcium carbonate undergoes an amorphous-to-crystalline transition during its precipitation, where the morphology of precipitates changes from amorphous aggregates to individual crystal particles. This transition has been extensively investigated in biomineralization and material synthesis while its relevance to fluid dynamics is less explored. Here we demonstrate through microfluidic experiments that this transition drives complex flow behaviors and precipitation patterns during mixing-induced precipitation in confined fluids. In the experiments, Na₂CO₃ and CaCl₂ solutions are co-injected into a submillimeter microfluidic conduit to precipitate CaCO₃. When the two fluids mix, a film of amorphous solids forms along the mixing interface and separates the fluids. However, as the amorphous solids spontaneously dissolve and turn into individual crystal particles, the film thins and ruptures, opening paths for fluid displacement, mixing, reaction and growth of secondary films. The films' rupture/growth modes vary with flow rate and conduit depth, leading to precipitation patterns of distinct spatiotemporal and fluid pressure behaviors and permeability-porosity relationships, which we term as "localized precipitation," "intermittent precipitation," and "homogeneous precipitation." Lattice Boltzmann simulations of the experiments reveal that the patterns originate from the coupled evolution of films rupture/growth and fluid pressure distribution. Our work provides new insights into mineral precipitation in confined fluids and may be useful in research fields of geologic carbon sequestration/mineralization, reactive transport, biomineralization, etc.