Dynamic tracking of bulk nanobubbles from microbubbles shrinkage to collapse

Nanobubbles (NBs) have attracted great attention because of their potential role in interfacial science and application. Compared to surface NBs, bulk NBs have not been fully investigated due to lack of proper imaging techniques. Here, light scattering under dark-field microscopy (DFM) was used to light up the dynamic evolution of bulk NBs in situ, which interpretes the formation process, the motion in water and the collapse phenomenon at the sample edge on glass slide. It provided direct evidence that NBs formed after microbubble shrinkage. Microbubbles carrying different type of gas (xenon, air and sulfur hexafluoride) displayed different shrinkage rates because of the different gas permeation resistance properly. However, the final stable NBs show no significant difference in size and surface charge. And the motion analysis of individual NB showed passive Browninan motion of all the NBs independent with the inner gas type. Importantly, NBs are easily to collapse and produce radiation force and hydroxyl radical ((OH)-O-center dot) at the sample edge, and these behaviors can be used to discriminate NBs from droplets and solid particles. Since there is little effect of inner gas on NBs, the common solvent water molecules with strong hydrogen-bonding ability is speculated a crucial factor to stable NBs in water.