Control of crystal nucleation, size and morphology using micro−/nanobubbles as green additives – a review

This review provides a summary of recent studies that addressed the question of whether micro−/nanobubbles provide heterogeneous substrates to impact metastable zone width (MSZW), the nucleation induction time (t_ind) and solubility, by looking at the influence of gassing parameters (gassing duration, gassing supersaturation and gassing flow rate) during gassing crystallization. Gassing crystallization has emerged as a so-called green technique for inhibiting and/or promoting crystal nucleation and growth by the injection of gas micro−/nanobubbles into saturated solutions to induce heterogonous nucleation. The introduction of a moving gas phase improves the mass transfer in the crystallizing suspension leading to enhanced nucleation and crystal growth. We highlight the physicochemical properties that render gas micro- or nanobubbles as environmentally benign, and the role of gas-liquid-solid interfacial properties in crystal nucleation and growth from solubility, MSZW and t_ind data. Meanwhile, the scope of research, challenges and future directions on gassing crystallization are also briefly discussed.