Hydrodynamic and Magnetostatic Characterization of a Rotating Vortex-Based Nanobubble Generator

Abstract

This study combines experimental characterization, computational fluid dynamics (CFD) and magnetostatic modelling to investigate a rotating vortex-based nanobubble generator incorporating six permanent magnets. The combined approach distinguishes the hydrodynamic generation pathway from the possible role of magnetic fields in interfacial stabilization. Experiments were performed at 600 rpm and 25 °C using geometrically matched impellers containing either demagnetized inserts or NdFeB N45 magnets arranged in an alternating south–north (S–N) configuration. In two independent experiments, the mean NTA-derived apparent hydrodynamic diameter decreased from 211.2 and 141.7 nm in the controls to 88.2 and 76.9 nm under magnetic treatment, respectively, while detected particle concentrations increased from 9.17 × 107 and 7.48 × 107 to 1.70 × 108 and 2.45 × 108 particles mL−1. Both magnetic size distributions were strongly right-skewed, with skewness coefficients of 4.06 and 3.76, indicating a dominant small-size population with a persistent larger-size tail. In a separate measurement series using freshly generated samples, the mean zeta potential shifted from −4.70 ± 3.29 mV for the control to −35.30 ± 1.54 mV for the S–N condition. CFD predicted local velocities up to 1.57 m s−1 and a minimum gauge pressure of −156 Pa. The corresponding absolute pressure remained well above the vapour pressure of water, supporting vortex-driven gas entrainment, a modest reduction in equilibrium gas solubility and shear-driven fragmentation rather than conventional vapour cavitation. Magnetostatic modelling predicted local flux densities up to approximately 0.45 T. Scaling estimates indicated that direct magnetostatic stabilization, bulk magnetohydrodynamic forcing and simple Faraday-induced polarization were insufficient to explain the observed magnetic response. The results support a two-level interpretation in which vortex hydrodynamics drives generation, while magnetic exposure may influence interfacial stabilization, reflected most clearly in the more negative zeta potential. These findings provide a basis for further development of magnetic-assisted nanobubble generators for potential aeration and water-treatment applications.