Analysis of droplet size during crossflow membrane emulsification using stationary and vibrating micromachined silicon nitride membranes

Crossflow membrane emulsification is a promising method to achieve very small and uniform emulsions. The droplet size produced is controlled mainly by the choice of membrane. Using microengineering technology it is currently possible to produce membranes with precision defined parameters (uniform pore size, shape and inter-pore distance). In the work presented here, individual pore behaviour was studied using micromachined membranes with wider inter-pore distances (100 μm). It was found that the diameter of droplets increased during an initial period of operation. Also, interaction between droplets formed at adjacent pores was observed to enhance the reduction of mean droplet size and negatively correlated with inter-pore distance. A 'push-to-detach' mechanism was proposed to explain the behaviour observed. It was demonstrated that a micromachined membrane with pore diameter of 2 μm and inter-pore distance of 20 μm produced smaller droplets than for membranes with larger inter-pore distances. To facilitate the droplet detachment from the membrane and provide additional control over droplet detachment, the effects of membrane vibration were investigated. Preliminary results showed that smaller droplets could be produced by introducing low frequency (0-100 Hz) membrane vibrations without increasing their size distribution.