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 (tind) 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 tind data. Meanwhile, the scope of research, challenges and future directions on gassing crystallization are also briefly discussed.