The generation and stability of bulk nanobubbles by compression-decompression method: The role of dissolved gas

Nanoscale gas bubbles are paid more and more attention due to their significant applications in different fields including the environmental remediation, plant and animal growth as well as medical diagnosis, etc. As reported, the local gas saturation plays an important role for the formation of surface nanobubbles (NBs) but is less importance for their stability. As for bulk NBs, few researches focused on the influence of dissolved gas on their generation and stability because it is thought generally that a limited amount of gas could dissolve into water. Herein, we reported for the first time the relationship of dissolved gases (Kr, O2 and N2) and the formation and stability of bulk NBs. Firstly, we developed a compression-decompression method to produce the water with super-high concentration of dissolved gas. About 60 mg/L oxygen dissolved gas was created to promote the formation of bulk NBs. It was showed that high bulk NB concentrations were produced using the compression-decompression method by controlling the loading pressure and time at the same time. The evolution process showed that the concentration of dissolved gas would decrease quickly with the deposited time. However, the concentration of formed bulk NBs did not follow the same way as dissolved gas concentration. It exhibited a complicated change over the time. Typically, first sharp increase to one order higher concentration than at the beginning and then decreased with a fluctuation within 72 h. More interestingly, the time of this sharp increase in nanobubble concentration depended on the type of gas, the krypton (Kr) gas system took longer time to reach the highest concentration and the oxygen (O2) as well as nitrogen (N2) gas system reached the highest concentration at about 4 h generally. The change of zeta potential of those NBs followed the same fluctuation as their concentration. Finally, we presumed a theoretical model to explain the evolution mechanism of bulk NBs. It indicated that there is a competition of different bubble behaviors (nucleation, clustering and coalescence) in different time periods. This study provides a new technique to produce high concentration of bulk NBs and dissolved gas in solution. Those results are very significance for further understanding the mechanism of formation and stability of bulk NBs under a super-high concentration of dissolved gas and may be used in some chemical reactions related with gas to promote the reaction efficiency.