Double emulsions with ultrathin shell by microfluidic step-emulsification.

Double emulsions with ultrathin shells are important in some biomedical applications, such as controlled drug release. However, the existing production techniques require two or more manipulation steps, or more complicated channel geometry, to form thin-shell double emulsions. This work presents a novel microfluidic tri-phasic step-emulsification device, with an easily fabricated double-layer PDMS channel, for production of oil-in-oil-in-water and water-in-water-in-oil double emulsions in a single step. The shell thickness is controlled by the flow rates and can reach 1.4% of the μm-size droplet diameter. Four distinct emulsification regimes are observed depending on the experimental conditions. A theoretical model for the tri-phasic step-emulsification is proposed to predict the boundaries separating the four regimes of emulsification in plane of two dimensionless capillary numbers, Ca. The theory yields two coupled nonlinear differential equations that can be solved numerically to find the approximate shape of the free interfaces in the shallow (Hele-Shaw) microfluidic channel. This approximation is then used as the initial guess for the more accurate finite element method solution, showing very good agreement with the experimental findings. This study demonstrates the feasibility of co-flow step-emulsification as a promising method to production of double (and multiple) emulsions and micro-capsules with ultrathin shells of controllable thickness.