Shirasu Porous Glass membrane emulsification: Characterisation of membrane structure by high-resolution X-ray microtomography and microscopic observation of droplet formation in real time

The microstructure of Shirasu Porous Glass (SPG) membrane has been investigated using metallographic microscope and high-resolution X-ray microtomography (XMT) and the obtained results are compared with Hg-porosimetry data. The porosity in 600 cross sections analysed by high-resolution XMT was found to vary in a narrow range between 52.5 and 57.4% with a mean value of 55.1%. The membrane microstructure looks similar on SEM and XMT images with tortuous, interconnected cylindrical pores extending in all directions within the membrane. The formation of O/W and W/O/W emulsion droplets have been observed in real time using SPG or Microporous Glass (MPG) membrane disks with a mean pore size between 10.2 and 16.2 μm. The quality of video recordings was much better when membrane surface was finely polished with diamond paste, although it did not affect the droplet formation behaviour. The droplets formed at the same pore were highly monodispersed and detached in regular time intervals, but this time interval showed significant variations for different pores. The SDS-stabilised droplets were detached from the pores as soon as they were formed, due to strong electrostatic repulsions between anionic droplets and negatively charged SPG surface. The Tween 80-stabilised droplets were kept attached to the membrane surface after formation, before being pushed away by the next droplet formed at the same pore. Under the same conditions the SDS-stabilised droplets were smaller than the droplets stabilised by Tween 80.