One-Step Emulsification of Multiple Concentric Shells with Capillary Microfluidic Devices†

Emulsions have long been utilized to make materials for foods, drugs, cosmetics, and displays because of their efficient encapsulation properties. While they are most commonly made in bulk, recent advances in microfluidics have enabled the design and manipulation of emulsion drops with unprecedented control and flexibility, albeit only on a drop-bydrop basis. Multiple-phase emulsions of very high uniformity in size can be prepared using precise control of flow that can be achieved in tailored poly(dimethylsiloxane) (PDMS) or capillary devices. The monodisperse single and double emulsion drops produced by such microfluidic devices can be employed to make functional microparticles and microcapsules, respectively. For example, nonspherical and Janus microparticles with optical, electrical, and magnetic functionalities can be prepared from monodisperse single emulsion drops. Moreover, polymeric microcapsules, vesicles, and colloidosomes can be prepared from double emulsion drops. Double emulsion drops provide particularly effective encapsulation of active materials with high encapsulation efficiency; in addition, many routes to controlled release are feasible. Even greater structural and functional enhancement of the microcapsules can be achieved with multiple emulsion drops of yet higher order. For example, multiple emulsion drops of high order are useful in the production of complex microcapsules for encapsulation and sequential release of multi-component active materials while avoiding cross-contamination. However, production of such multiple emulsion drops remains a challenge. Most previous approaches utilize sequential emulsification using a series of single drop makers. Thus, the device fabrication entails complex procedures and delicate control of flow rates required to synchronize the frequencies of drop generations in all drop makers. Moreover, the dispersed phases are used as the continuous carrier fluid of the inner drops before the sequential emulsification, making it difficult to achieve full control of a diameter of each layer in the multiple emulsion drops. By contrast, single-step emulsification obviates the shortcomings of sequential emulsification and enhances the controllability and the stability of the generation of multiple emulsion drops using an even simpler device design. Although double emulsion drops have been prepared through single-step emulsification, coaxial introduction of four or more immiscible fluids into a single channel is challenging, making it difficult to produce higher-order multiple emulsion drops through single-step emulsification. Herein, we report a facile one-step emulsification approach to make monodisperse multiple emulsion drops of high order using stable biphasic flows in confining channels. Through controlled surface modification of glass capillary devices, immiscible multiphase streams flow through a single orifice, forming layered coaxial interfaces. Breakup of the interfaces is achieved in dripping or jetting modes, determined by the flow rates. In the dripping mode, breakup is triggered by inserting a drop in the core of the emulsion, facilitating the production of monodisperse triple or quadruple emulsion drops with an onionlike configuration. In addition, by using the jetting breakup mode, the number of drops in the core can be manipulated. The essential strategy of our approach relies on the stable flow of two fluids through a single channel without the formation of drops, which commonly occurs because of Rayleigh-Plateau instability. The flow of two immiscible fluids through a single capillary channel exhibits two distinct patterns consisting of either drops or a jet, depending on several control parameters. In either case, the fluid with the higher affinity to the wall forms the continuous phase while that with the lower affinity flows through the center, without contacting the wall, either as distinct drops or a jet. In most cases, this jet is unstable to formation of drops because of the Rayleigh-Plateau instability. However, the spatial confinement of the interface because of the geometry of capillary can, in fact, provide additional stability to the jet, suppressing the breakup into drops. This is the particularly effective when the width of the inner fluid is large, so that the interface between the two fluids is formed near the capillary wall; then strong confinement of the continuous fluid helps prevent the Rayleigh-Plateau instability and the subsequent breakup into drops. We exploit this feature to produce high-order multiple emulsions through a one-step emulsification process. The design of the device for producing triple emulsion drops of water-in-oil-in-water-in-oil (W1/O2/W3/O4) phases comprises two tapered cylindrical capillaries, one for injection and the other for collection. Each is inserted into a square capillary, the inner dimension of which is slightly larger than the outer diameter of the cylindrical capillary before they are tapered, as shown schematically in Figure 1a. The cylindrical capillaries are treated to make them hydrophobic; the outer square capillary is treated to make it hydrophilic. In addition, a small tapered capillary is inserted into the space between the collection and the square capillaries to simultaneously [*] Dr. S.-H. Kim, Prof. D. A. Weitz School of Engineering and Applied Sciences, Department of Physics Harvard University, Cambridge, MA 02138 (USA) E-mail: weitz@seas.harvard.edu Homepage: http://weitzlab.seas.harvard.edu/