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Abstract
A method to produce monodisperse structured microcapsules in the diameter range from 10–100 µm is here presented. Flow-focusing is a well known technique whereby a steady capillary micro-jet is generated by the action of a highly accelerated co-flowing stream forced through a small orifice. The micro-jet breaks up owing to capillary instability, giving rise to droplets with a narrow size distribution. In the present study, flow-focusing gives rise not to simple but to compound capillary jets. At break-up, under suitable control parameters, such jets give rise to microcapsules where an outer liquid (shell liquid) surrounds a core liquid integrated by one or more vesicles. Furthermore, under adequate stimulation combining a sinusoidal signal with intermitent pulses, the jet break-up can be controlled. Highly monodisperse microcapsules are produced; fundamental geometric parameters (main diameter, shell thickness or number of cores) are reliably controlled. Rather than using a gas flow to focus the concentric stream of two immiscible liquids, this study has investigated in some detail the evolution of a concentric stream of three immiscible liquids forced through a small orifice. The selection of the surface tension coefficients between the three phases ensures the robust production of a microcapsule structure involving a plurality of vesicles homogeneously distributed in the capsule bulk, the number of cores being a freely chosen parameter. Such composite microcapsules find a broad field of technological applications in the pharmaceutical, food or biotechnology industries.