Size control of nanobubbles generated from Shirasu-porous-glass (SPG) membranes

We recently discovered that monodispersed nanobubbles/microbubbles could be generated from Shirasu-porous-glass (SPG) membranes with uniform pores in a system composed of dispersed gaseous and continuous water phases containing a surfactant. In the present study, size control of the nanobubbles generated was examined using SPG membranes with less than submicron-size pores. Air was pressurized into a sodium dodecyl sulfate (SDS) solution ranging from 0.05 to 0.5 wt.% flowing through a SPG membrane with a transmembrane/bubble point pressure ratio of 1.1–2.0. Under these conditions, monodispersed nanobubbles with a mean bubble diameter of 360–720 nm were stably produced from membranes with mean pore diameters of 43–85 nm. The mean bubble diameter was shown to be 8.6 times larger than the mean pore diameter; therefore, the nanobubble diameter could be controlled by the membrane pore size. Mean bubble diameter was barely affected by a flow velocity of 0.5–3.7 m s−1 or by the surface tension between air and 0.05–0.5 wt.% SDS solution.