Formation and Stability of Surface/Bulk Nanobubbles Produced by Decompression at Lower Gas Concentration

Nanobubbles have many fascinating properties and the mechanism of their formation and stability still needs further exploration. According to the conventional theory and experiences, it was suggested that surface/bulk nanobubbles would only be produced at conditions of high gas concentration and stabilized in a gas oversaturated state. However, we showed here that both surface and bulk nanobubbles could be formed at conditions of low gas concentration and exist in an unsaturated gas environment in water for a long time. In our experiments, the surface/bulk nanobubbles were produced by a new method of water decompression started from the normal pressure. Sufficient surface/bulk nanobubbles could be generated during the decompression within a certain time and were quite stable even after the pressure was recovered to the normal pressure. The evolution process of the bulk nanobubbles with the time of decompression was studied and it was found that the concentration of the bulk nanobubbles was firstly increased and then decreased. In addition, the size of the bulk nanobubbles was increased during this decompression process. Our results revealed new information on the unique behavior of nanobubbles and should be helpful to understand their formation and stability mechanism as well as their applications.