Spontaneous formation behavior of uniform-sized microbubbles from Shirasu porous glass (SPG) membranes in the absence of water-phase flow

Recently, it was found that uniform-sized nanobubbles/microbubbles could be generated from Shirasu porous glass (SPG) membranes with uniform pores in a system composed of a dispersed gaseous phase and a continuous water phase. In this study, microbubble formation was attempted using a plate-like SPG membrane in the absence of water-phase flow. Surfactants and proteins were used as dispersing agents. Air was pressurized into a quiescent water phase containing a dispersing agent and forced through an SPG membrane with a mean pore diameter of 3.07 μm, at a transmembrane pressure 1.1 times larger than the bubble point pressure. Under these conditions, uniform-sized microbubbles with mean diameters ranging from 27.8 to 64.8 μm were produced from the SPG membrane. Microbubble formation appeared to be based on the spontaneous transformation caused by surface tension, as previously shown for liquid–liquid dispersion systems, where the mean bubble diameter increased with increasing water-phase viscosity. For protein solutions, the mean diameters and monodispersities of the microbubbles formed were larger than those for the surfactant solutions. This result is probably due to the adsorption kinetics of the dispersing agent, where the faster adsorbing dispersing-agent molecules (ions) produce smaller microbubbles.