Visualization of droplet break-up in pre-mix membrane emulsification using microfluidic devices

To investigate the break-up of emulsion droplets in pre-mix or dead-end membrane emulsification, a microscopic study was conducted in microfluidic devices. Channels with shapes such as constrictions, junctions, and combinations thereof representing ideal membrane structures where etched in silicon. Through these structures pre-mix emulsion droplets where pushed, which was recorded with high-speed video microscopy. Accumulation of droplets of the dispersed phase before and inside the pore network was found to occur under all conditions. Thus, the effective volume fraction of dispersed phase inside the pores is always high. In a brick-shaped pore structure, an optimum thickness of the layer was found. In these structures, channelling occurs below a critical pressure difference over the structure. In the channelling regime, isolated channels are active, effectively excluding action of branchings and junctions. We found three main categories of break-up mechanisms (snap-off due to localized shear, break-up due to interfacial tension effects, and break-up due to steric hindrance) and were able to quantify some. Break-up due to branchings and junctions do not seem to be essential for break-up to occur. It is, however, clear that the other phenomena (accumulation, channelling) can have major impact on the total behaviour of the system and should not be neglected.