Mixing intensification by chaotic advection inside droplets for controlled nanoparticle preparation

A novel droplet-based approach for the mixing intensification was proposed in this study, where mixing occurred inside hanging droplets out of a micro-channel. Two miscible liquid streams were first confined into the same micro-channel but remained unmixed until the outlet by controlling flow conditions. When the segregated streams flowed out of the micro-channel, droplets would form spontaneously in the open-air environment. Thus, the simultaneous mixing would be activated by the inherent chaotic advection within each droplet. Meanwhile, the external disturbance could be conveniently applied on the droplets to enhance the inner chaotic flows for a better mixing performance. Planar laser-induced fluorescence (PLIF) technique was employed to visualize the dynamic mixing behavior in each droplet. The results showed that a rich variety of mixing performance at different time scales could be achieved under well-controlled conditions. Additionally, the application of this approach for mixing intensification was demonstrated by the preparation of curcumin nano-particles (i.e., nano-drugs) in the antisolvent precipitation process. High quality products with spherical shape and narrow size distribution could be well obtained, and the initial concentration of the curcumin solution played a dominant role in the average particle size.