Droplet formation in a T-shaped microchannel junction: A model system for membrane emulsification

Droplet formation was studied in a glass microchip with a small channel containing to-be-dispersed phase perpendicular to a large channel with a cross-flowing continuous phase. This resembles the situation during cross-flow membrane emulsification. In this model system, droplets are formed at a T-junction of these two rectangular channels; the droplet formation and detachment process is studied from aside with a microscope connected to a high speed camera. Monodisperse hexadecane droplets were formed in aqueous solutions with various concentrations of ethanol, sodium dodecyl sulfate (SDS) and Tween 20. Just before detachment, the neck diameter was measured and a critical neck diameter of 4 μm was found, which is in the same range as the depth of the channel (5 μm). After detachment, the droplet diameter was determined for the various aqueous solutions. The droplet diameter increased as a function of the oil flow rate. Use of surfactants (SDS, Tween 20) resulted in the formation of smaller droplets than in systems without surfactants. A simple model is proposed to describe the droplet formation process, inspired on the idea that the necking process is a dynamic process that takes a certain time, which explains the influence of both the oil flow rate and the properties of the fluid phases on the final droplet size. Fitting the experimental data with the model results in a necking time of about 11 ms.