Shirasu porous glass (SPG) membrane emulsification in the absence of shear flow at the membrane surface: Influence of surfactant type and concentration, viscosities of dispersed and continuous phases, and transmembrane pressure

There has been an increasing interest in membrane emulsification using Shirasu porous glass (SPG) membranes due to the better control of the droplet size, lower energy consumption, and simplicity of design. In this study, SPG membrane emulsification in the absence of shear flow at the membrane surface was investigated in order to optimize the emulsification process and to gain deep insight into the droplet formation behavior. At transmembrane pressures slightly higher than the capillary pressure, a faster decrease in the interfacial tension resulted in the spontaneous formation of smaller droplets with a narrower droplet size distribution. In this case, the dispersed phase distorted by the tortuous membrane pores was spontaneously transformed into spherical droplets by interfacial tension. The drop diameter increased with a decrease in the ratio of the dispersed-phase viscosity to the continuous-phase viscosity. The droplet formation behavior was changed from the spontaneous droplet formation to the continuous outflow of the dispersed phase at a critical dispersed-phase velocity, which was determined by the capillary number: below the critical capillary number, uniformly sized droplets are spontaneously generated; above the critical capillary number, dispersed phase flowed out continuously, resulting in polydispersed larger droplet formation. The critical capillary number was independent of the equilibrium interfacial tension and the dispersed-phase viscosity.