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Abstract Hydrothermal synthesis is an attractive route to make nanoparticles utilizing inexpensive precursors under moderate process conditions. Though it provides flexibility and robustness in controlling particle characteristics, process scale-up for continuous production is a major challenge. A novel ‘infinity-’ shaped spiral continuous flow reactor is proposed here, to exploit the large density difference between the precursor solution and supercritical water to provide rapid mixing, leading to uniform conditions for reaction kinetics and particle growth. Hydrothermal synthesis is simulated by coupling computational fluid dynamics with population balance modeling and appropriate reaction kinetics. Simulations indicate three distinct regimes of declining, recovering, and stable flow fields. These regimes are strongly dependent on the flow ratio between the precursor solution and supercritical water. The infinity reactor provides two distinct reaction environments: initial turns of the spiral which serve as a mixed flow reactor facilitating rapid mixing and uniform reaction, followed by a plug flow reactor stabilizing the particle growth. It produces particles with a relatively small mean diameter and a narrow size distribution in comparison to the conventional batch stirred tank reactor and the T-mixer.