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.
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