Fingering Instability During Mixing-Driven Precipitation Flow

In this study, we experimentally investigate fluid–fluid displacement in a Hele–Shaw cell where the two fluids react, upon mixing, to form solid precipitates. Under the conditions of our experiments, we observe that precipitation reaction along the moving fluid–fluid interface generates solids in the form of mineral particle suspensions. We find that both electrostatic and hydrodynamic forces control the extent of particle–particle agglomeration during the suspension flow. Such particle suspension decreases the overall mobility of the multiphase mixture, thus altering the overall displacement. Although the injected fluids are viscously stable, the precipitation band that forms between the fluids becomes unstable to form finger-like flow channels compartmentalized by solid-deposited walls and clusters. We show that the emergence, growth and decay of the fingering pattern are strongly influenced by the injection rate and the initial fluid chemical concentrations. In addition, we show that precipitation-induced fingering has a strong feedback on fluid–fluid mixing and the subsequent precipitation rate. Lastly, we find that, counter to intuition, a higher injection rate results in a larger amount of precipitates that are securely deposited in the Hele–Shaw cell.