No abstract

The crystallization of magnesium ammonium phosphate hexahydrate (struvite) often occurs under conditions of fluid flow, yet the dynamics of struvite growth under these relevant environments has not been previously reported. In this study, we use a microfluidic device to evaluate the anisotropic growth of struvite crystals at variable flow rates and solution supersaturation. We show that bulk crystallization under quiescent conditions yields irreproducible data owing to the propensity of struvite to adopt defects in its crystal lattice, as well as fluctuations in pH that markedly impact crystal growth rates. Studies in microfluidic channels allow for time‐resolved analysis of seeded growth along all three principle crystallographic directions and under highly controlled environments. After having first identified flow rates that differentiate diffusion and reaction limited growth regimes, we operated solely in the latter regime to extract the kinetic rates of struvite growth along the [100], [010], and [001] directions. In situ atomic force microscopy was used to obtain molecular level details of surface growth mechanisms. Our findings reveal a classical pathway of crystallization by monomer addition with the expected transition from growth by screw dislocations at low supersaturation to that of two‐dimensional layer generation and spreading at high supersaturation. Collectively, these studies present a platform for assessing struvite crystallization under flow conditions and demonstrate how this approach is superior to measurements under quiescent conditions.

Crystallization in flow systems is a common occurrence in pipelines, catheters, and commercial processes. This accumulation of mineral scale, such as struvite, poses a problem. Microfluidic devices are an efficient platform to assess crystallization under flow conditions, in which time‐resolved measurements allow the quantification of anisotropic growth rates at variable conditions. When coupled with techniques, such as in situ atomic force microscopy, it is then possible to elucidate growth mechanisms and validate predictions made on the basis of macroscopic analysis. More information can be found in the Full Paper by N. J. Irwin, J. D. Rimer, et al. on page 3555.