Effect of magnetic field configuration on hydrodynamic cavitation-generated nanobubbles and their zeta potential in water

Nanobubbles generated by hydrodynamic cavitation are increasingly explored in water treatment and process intensification, yet the factors controlling their size, stability, and electrokinetic properties remain poorly understood. In particular, the influence of magnetic fields on cavitation-driven nanobubble formation in the absence of externally imposed currents has not been systematically investigated. Here, we compare two permanent magnet configurations embedded in a rotating impeller (uniform “S-in” and alternating “S-N”) to a nonmagnetic reference in order to determine how rotating magnetic fields modify nanobubbles generated by impeller-driven hydrodynamic cavitation in water at 600 rpm. We find that the alternating S-N configuration increases nanobubble number density by up to one order of magnitude and shifts the zeta potential distribution toward more negative values (from −4 mV to ∼−17 mV) while reducing the mean bubble size relative to the nonmagnetic reference (∼20% decrease in radius of gyration), whereas the uniform S-in configuration yields larger bubbles. Order-of-magnitude analysis shows that Lorentz-force-driven magnetohydrodynamic effects and Faraday-induced electric fields are negligible compared to inertial hydrodynamic forces, indicating that bulk electromagnetic forcing does not control the observed behavior. Instead, the results are consistent with magnetic-field-induced modifications of interfacial electrokinetic properties, leading to enhanced electrostatic stabilization and reduced coalescence. These findings demonstrate that magnetic field configuration provides a previously underexplored control parameter for tailoring nanobubble populations, with potential implications for cavitation-based water treatment and mineral nucleation processes.