Multiscale Optical Imaging Unveils the Distinct Roles of Supersaturation and Substrate in Interfacial Crystallization

Mineral scaling poses severe operational and economic challenges in water treatment and industrial processes. Prevailing models primarily focus on how surface chemistry triggers heterogeneous nucleation. However, a quantitative understanding of how substrate properties govern the more prevalent homogeneous nucleation-bulk deposition pathway remains elusive. Here, we introduce a label-free integrated imaging platform combining bright-field microscopy and low-angle rotational interferometric scattering microscopy (LRISM) to track crystal populations and quantify the three-dimensional orientation during interfacial crystallization. Using calcium sulfate (CaSO₄) and calcium carbonate (CaCO₃) as model scales, we decouple the roles of supersaturation and substrate chemistry. CaSO₄ consistently undergoes heterogeneous nucleation, with morphology transitioning from rod-like morphology at high supersaturation to cruciform morphology at low supersaturation. In contrast, CaCO₃ invariably follows a homogeneous nucleation-bulk solution deposition pathway. Quantitative analysis of this homogeneous nucleation pathway reveals that hydrophilic surfaces capture a greater density of nascent nuclei, limiting their final size through spatial confinement and growth competition. LRISM-based three-dimensional orientation analysis further confirms that hydrophilic surfaces preferentially induce horizontal crystal alignment. These results establish quantitative observables and a general methodology for designing scale-mitigation surfaces by controlling postnucleation capture and orientation rather than nucleation inhibition alone.