Description
Emulsions, heterogeneous mixtures of two immiscible liquids to which surfactants are added, are a very widespread material encountered in various fields, from cosmetics to food and biotechnology. Microfluidics has proven to be an ideal tool for designing custom emulsions with controlled size and composition. However, while micrometre size droplets are an ideal template or microreactor for the creation of advanced products dedicated to applications in life science, their production in large quantity is still challenging.
We developed a microsystem composed of thousands of shallow microchannels emerging in a deeper one and where droplets are formed through the so-called step-emulsification mechanism. The addition of an intermediate channel depth is shown to homogenize droplet production rate and thus droplet size which is here below 10 micrometres. This micro-emulsification system is demonstrated to be valuable for producing calibrated magnetic microspheres, shaped from the emulsion droplets, that finds application in biomolecules or cells extraction from biological samples.
It is also a suitable platform for investigating emulsion stability against coalescence linked to transport of surfactants from the continuous phase towards the freshly created interfaces. The surfactants transport is a combination of advection, related to droplet motion, and diffusion that depends on the geometrical features of the microsystem. A critical surface concentration of surfactant below which coalescence occurs is estimated for various oils. The stability criterion is a function of chemical nature of the oil as well as its viscosity. This microsystem also allows to investigate the role played by the nature of the surfactant, the presence of micelles and their dissociation rate as well as the effect of ions on the stability against coalescence during the emulsification of a concentrated emulsion.
Moreover, the capability to produce monodisperse emulsions with a finely tuned droplet size at a high throughput allowed us to study the rheological behaviour of model concentrated emulsions. We find that droplet size distribution has a major impact on the flowing features of concentrated emulsions. Shear-bands take place in such a system without requiring attractive forces between droplets as previously observed. We attribute this intriguing phenomenon to a variation of the orientation of the crystalline structure that is formed by the monodisperse emulsion droplets, which is related to a different friction between droplets layers.