Coarsening Behavior of Bulk Nanobubbles in Water

Abstract

\n In recent years, extremely small gas bubbles called bulk nanobubbles (BNBs) have drawn great attention due to their impressive effects and their wide applicability in a variety of technological fields, including biomedical engineering, water treatment, and nanomaterials. However, unsolved questions remain regarding the stability and behavior of BNBs. In the present work, BNBs were generated in water using a gas-liquid mixing method. To investigate the coarsening behavior of BNBs in water over time, particle analysis was performed using a nanoparticle tracking analysis (NTA) method. Over time, the BNB diameter continuously increased (from 88.50 nm to 201.00 nm), and its cubic radius increased linearly (r3 ~ t). While the concentration of BNBs decreased (from 3.47 ×108 particles/mL to 0.61 ×108 particles/mL), the total volume of BNBs remained the same. Moreover, the size distribution broadened over time, and the concentration of larger BNBs gradually increased over time. These results indicate that relatively small BNBs disappear and larger BNBs grow through mass transfer between BNBs instead of dissolution of the gas and coalescence. In other words, BNBs underwent Oswald ripening; that is, gas molecules detached from smaller BNBs, diffused into the continuous phase, and then were absorbed into larger BNBs.