Submicron emulsions could be produced via the tip-streaming process in a flow-focusing microfluidic device. In this article, the stability of the liquid cone and thread for tip-streaming mode could be significantly improved by employing a three-dimensional flow-focusing device, in which the hydraulic resistance was adjusted by modulating the channel heights in the flow focusing area, orifice, downstream and dispersed phase inlet channel. The pressure range for tip-streaming mode was enlarged significantly compared with two-dimensional flow-focusing devices. Therefore, monodisperse emulsions were produced under this tip-streaming mode for as long as 48 hours. Furthermore, we could control the size of emulsion drops by changing the pressure ratio in three-dimensional flow-focusing devices while the liquid cone was easily retracted during the adjustment of pressure ratio in two-dimensional flow-focusing devices. Furthermore, using the uniform submicron emulsion droplets as confining templates, polyethylene glycol (PEG) particles were produced with a narrow size distribution at the sub-micrometre scale. In addition, magnetic nanoparticles were added to the emulsion for magnetic PEG particles, which can respond to magnetic field and would be biocompatible.

We report the use of a simple microfluidic device for producing microcapsules with reversible membrane permeability that can be remotely controlled by application of near-infrared (NIR) light. Water-in-oil-in-water (W/O/W) double-emulsion drops were prepared to serve as templates for the production of mechanically stable microcapsules with a core-shell structure and highly uniform size distribution. A biocompatible ethyl cellulose shell was formed, containing densely packed thermoresponsive poly(N-isopropylacrylamide) (pNIPAAm) particles in which gold nanorods were embedded. Irradiation with a NIR laser resulted in heating of the hydrogel particles due to the photothermal effect of the gold nanorods, which absorb at that wavelength. This localized heating resulted in shrinkage of the particles and formation of macrogaps between them and the matrix of the membrane. Large encapsulated molecules could then pass through these gaps into the surrounding fluid. As the phase transition behavior of pNIPAAm is highly reversible, this light-triggered permeability could be repeatedly switched on and off by removing the laser irradiation for sufficient time to allow the gold nanorods to cool. This reversible and remote control of permeability enabled the programmed release of encapsulants, with the time and period of the open valve state able to be controlled by adjusting the laser exposure. This system thus has the potential for spatiotemporal release of encapsulated drugs.

A facile PDMS-glass hybrid microfluidic device is developed for generating uniform submicrometer-scale double emulsion droplets with unprecedented simplicity and controllability. Compared with planar flow-focusing geometries, our three-dimensional flow-focusing geometry is advantageous for stably producing femto- to atto-liter droplets without the retraction problem of the dispersed phase fluid. In addition, this microfluidic platform can withstand the use of strong organic solvents (e.g. tetrahydrofuran (THF) and toluene) as a dispersed phase without deforming PDMS devices because the dispersed phase containing organic solvents does not directly contact the PDMS wall. In particular, monodisperse double emulsions are generated spontaneously via the internal phase separation of single emulsions driven by the diffusion of a co-solvent (tetrahydrofuran) in microfluidic devices. Finally, we demonstrated that the double emulsions can be used as morphological templates of ultrafine spherical silica capsules with controlled hierarchical pore networks via the evaporation-induced self-assembly (EISA) method. During EISA, triblock copolymers (Pluronic F127) act as a surfactant barrier separating the internal droplet from the continuous oil phase, resulting in the 'inverse' morphology (i.e. hydrophobic polymer-in-water-in-oil emulsions). Depending on the precursor composition and kinetic condition, various structural and morphological features, such as mesoporous hollow silica spheres with a single central core, multi-cores, or a combination of these with robust controllability can be seen. Electron microscopy (SEM, STEM, HR-TEM), small angle X-ray scattering (SAXS), and N(2) adsorption-desorption confirm the well-controlled hierarchical pore structure of the resulting particles.