Microfluidic synthesis of atto-liter scale double emulsions toward ultrafine hollow silica spheres with hierarchical pore networks

Abstract

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.

A microdevice for producing monodispersed droplets under a jetting flow

“…Gañán-Calvo and Gordillo 22 produced highly monodispersed gas bubbles using a technique called capillary ow focusing. 31,32 However, it should be noted that the operating range for produce uniform microdroplets under jetting in FF devices is still narrow. The capillary ow focusing device was implemented by Anna to form droplets in planar microchannels.…”

Tailoring of High‐Order Multiple Emulsions by the Liquid–Liquid Phase Separation of Ternary Mixtures

“…A simpler process that leads to the spontaneous formation of multiple emulsions consists of the liquid-liquid phase separation of ternary mixtures. [17][18][19] This method enables the stabilization of submicron double emulsions, [20] which are difficult to construct mechanically. These recent results call for an understanding of the basic principles of phase separation inside emulsion droplets.…”

One-step production of multiple emulsions: microfluidic, polymer-stabilized and particle-stabilized approaches

“…One solution to this problem is currently growing in popularity: simplified flow conditions can be combined with a phase separation transition to create a diverse range of multicomponent droplets. [31][32][33][34][35][36][37] The phase separation is triggered either by a change in temperature or a change in composition due to one or more components entering or leaving the droplet.…”

“…4a-d). 32 The smallest droplets they fabricated, which may well be multiple emulsions, were diameter 50 nm! Relatively small droplets (down to 10 mm) were also prepared by Haase and Brujic who triggered phase separation by the movement of two components out of the droplet while the surfactant diffused in from the continuous phase, see Fig.…”

“…The flow fields required to prepare multiple emulsions are typically very simple provided that a phase separation transition is harnessed to produce the internal architecture of the droplets. [31][32][33][34][35][36][37] Such an approach could be implemented on a large scale via, for example, dynamic membrane emulsification. 43…”