Bulk Nanobubbles Fabricated by Repeated Compression of Microbubbles.

Nanobubbles (NBs), given its extraordinary properties, have drawn keen attention in the field of nanotechnology worldwide. However, compared to that of surface NBs, generation of stable bulk NBs remains an arduous task with the prevailing method. In this study, we developed a pressure-driven method to prepare bulk NBs by repeatedly compressing sulfur hexafluoride (SF6) gas into water. The results show that NBs with a mean diameter of 240 ± 9 nm and a polydispersity index of 0.25 were successfully prepared. The generated NBs had a high negative zeta potential (-40 ± 2 mV) with stability of more than 48 h. Under the condition of 600 times repeated compression, the NB concentration could reach about 1.92 × 1010 bubbles/mL. Furthermore, we examine the possible formation mechanism involved in NB generation by virtue of optical microscopy and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The microscopic results showed that microbubbles about 10-50 μm formed first and then decreased to be nanoscale-sized. A stronger hydrogen bond was detected by ATR-FTIR spectroscopy during the shrinking of microbubbles into NBs. It is speculated that the disappearance of microbubbles contributes to the formation of NBs, and the strong hydrogen bond at the gas-water interface prompts the stability of NBs. Therefore, repeated compression of the gas in aqueous solution could be a new method to prepare stable nanosized bubbles for wide applications in the future.