Evidence of the existence and the stability of nano-bubbles in water

Although micro- and nano-bubble technology has been attracting attention in many fields, the state of water after the introduction of those bubbles is still not clear. In this study, the existence and stabilization of nano-bubbles after the generation of bubbles were investigated. The presence of nano-sized particles was detected through dynamic light scattering for days, when pure oxygen was used to generate the bubbles, and for less than 1 h, in the case of air bubbles. NMR spin–lattice relaxation time increased with the introduction of micro- and nano-bubbles in manganese ions solution, indicating the presence of a gas–liquid interface which adsorbed the manganese ions. Furthermore, the zeta potential measured in the water after the introduction of oxygen micro- and nano-bubbles was in the range from − 45 mV to − 34 mV and from − 20 mV to − 17 mV in water bubbled with air, indicating the presence of stable electrically charged particles. This study suggested a strong possibility of the existence of nano-bubbles in water for a long time. The stability of nano-bubbles is supported by the electrically charged liquid–gas interface, which creates repulsion forces that prevent the bubble coalescence, and by the high dissolved gas concentration in the water, which keeps a small concentration gradient between the interface and the bulk liquid.