Bulk Nanobubbles: generation using a two-chamber swirling flow nozzle and long-term stability in water

Research on bulk nanobubbles with various applications has been widely reported in the literature. However, the majority of studies are still limited to small scales with non-continuous generation processes, thus, the results may be difficult to apply on large-scale applications. In this work, a nanobubble generator was developed based on hydrodynamic cavitation using a two-chamber swirling flow nozzle that efficiently produced nanobubbles and had the potential to be applied to continuous flow systems and processes. The bulk nanobubble characteristics were evaluated according to the hydrodynamic diameter, zeta potential, and dissolved oxygen (DO) concentration based on the influence of the gas flow rate ratio (Ql/Qg), generation time, gas type, pH value, and NaCl concentration on the generated nanobubbles. The results show that oxygen and air nano bubbles smaller than 200 nm were successfully generated in pure water. The oxygen and air nanobubbles were negatively charged in pure water. The effects of pH and salt addition were similar to those during nanobubble generation using ultrasonic cavitation. In alkaline medium, nanobubbles were smaller and more stable than in acidic medium. The addition of salt to the nanobubble suspension reduced the repulsive electrostatic forces between the nanobubbles by increasing their size and decreasing their negative zeta potential. The resulting oxygen and air nanobubbles in pure water were verified to be stable for up to 10 and 5 months, respectively, without any significant changes in size or zeta potential. These results corresponded to predictions by the ionic repulsion model based on microbubble shrinkage.