(425a) Formation and Breakup of Magnetic Ionic Liquid-Water Janus Droplet in Assembled 3D-Printed Microchannel

Droplet-based microfluidic technology which provides an ideal platform for precise, controlled and fast processing of microdroplets in multiphase flow, has attracted enormous attention in a variety of fields, such as drug delivery, functional material synthesis, catalysis and so on. Inherent benefits of droplet-based microfluidics, i.e. large specific surface area and reliable manipulation of microdroplets on the sub-microliter scale, leads to more efficient heat and mass transfer, rapid mixing and safer operations in extreme reaction conditions. Room temperature ionic liquids (ILs) as salts with melting temperature below the boiling point of water, possess appealing characteristics of non-volatile, high thermal stability, considerable electroconductivities and polarity, varied solvation capabilities and highly catalytic activity. And new discovered magnetic ionic liquids (MILs) which contain paramagnetic components such as transition metal ions, rare earth ions or organic paramagnetic structures, combine the advantages of ILs with magnetic properties. MILs not only provide a novel way for efficient and simple recycling of ILs, but also exhibit unique characteristics, which have shown great potential in applications of catalysis, extraction, gas absorption and material synthesis. However, the most of ILs are highly viscous (increasing mass and heat transfer resistance) and economically unfriendly, which hinders practical application of ILs. Droplet-based microfluidics offers an effective approach for overcoming the drawbacks of ILs, which opens up a new area of study of droplet flows of ILs. Our group has investigated hydrodynamics of the droplet formation, mass transfer behavior and Suzuki-Coupling reaction in highly viscous ILs involved biphasic microflow system ^1-4. Exceptional behaviors of IL microdroplet flow are exhibited due to the complex interfacial properties and high viscosity of ILs. Droplet formation dominated by pressure gradient of continuous phase in squeezing regime would not be affected by the viscosity of dispersed phases; however, the critical Ca number of flow regimes and scaling law exponent on Ca for droplet size prediction in dripping and jetting modes are smaller than conventional systems in IL microdroplet flow ¹. Via the micro-LIF technique, the improved mass transferring performance was confirmed in IL microdroplet systems, which shows the considerable potential of droplet-based microfluidics in the intensification of mass transfer in IL-involved multiphase systems ^2,4.

Recently, the complex emulsions, owing to multifunction provided by their unique morphologies and multi-compartments, have gained increasing interests for both scientific research and industrial applications. Especially, Janus droplets which composed of two or more compartments with different physically or chemically properties, exhibit potential applications in a wide range of fields including material science, optics, magnetics, bio- and life sciences and so on. Complex microdroplets involved MILs would further achieve manifold functions of MILs and expanded its application fields. However, studies on the formation of IL-based multiphasic droplets in microchannels are still limited due to the complex interfacial properties and high viscosity of the ILs. And the components and structures of multiphasic droplets are largely subject to the restrictions of the materials and geometries of microdevices. 3D printing technology has been emerged as a fast, simple and flexible manufacturing tool for fabrication of microdevices assisted by design software such as computer-aided design (CAD) or Solidworks. Compared to traditional manufacturing method of soft-lithography for microchannels, 3D printing has superior advantages on 3D structures fabrication, on-demand design and rapid prototyping. However, the challenge of removing the uncured resin or support materials from microchannels in stereolithograpy (SLA) and poly-jetting 3D printing limits its resolution and the complexity of microchannels. Therefore, complex geometries are commonly fabricated by assembling different subunits, which can improve the flexibility of multiphasic droplets generation and also reduce the post-processing cost for 3D-printed microdevices.

Herein, we developed an assembled 3D-printed co-flowing microchannel for the [bmim]FeCl₄ MIL-water Janus droplets generation. No surfactants added for the sake of the recovery of pure MIL. Special attention is paid to characterize the hydrodynamics of MIL involved Janus droplet formation, where the biphasic dispersed phases have a large difference in physical properties such as viscosity and interfacial tensions. Flow patterns of MIL-water Janus droplets are analyzed and compared with experimental reports of low viscosity systems and IL single phasic droplet formation. Besides, droplet size scaling laws, structure control and morphology evolution of MIL-water Janus droplets are investigated to obtain a deep and systematic understanding of reliable and controllable formation of MIL-involved multiphasic droplet via co-flowing microchannels. Passive droplet breakup has become a hotspot of research, which facilitates droplet size control and production of sample replicates, multiplexing of a large number of droplets. To our best knowledge, Janus droplet splitting in microchannel has not yet been reported. We applied a simple 3D-printed splitting subunit into the study of the breakup of MIL-water Janus droplets in three-dimensional microdevice and achieved the recovery of MIL by the split of MIL-water Janus droplets. This work should be useful for obtaining a deep and comprehensive understanding on the formation, breakup and structure control of IL-based Janus microdroplets in multiphase flow. The generated MIL-water Janus microdroplets are promising candidates for use in applications of catalysis and extraction.

References:

1. Bai L, Fu Y, Zhao S, Cheng Y. Droplet formation in a microfluidic T-junction involving highly viscous fluid systems. Chemical Engineering Science. 2016;145:141-148.
2. Bai L, Zhao S, Fu Y, Cheng Y. Experimental study of mass transfer in water/ionic liquid microdroplet systems using micro-LIF technique. Chemical Engineering Journal. Aug 15 2016;298:281-290.
3. Bai L, Fu YH, Cheng Y. Ionic Liquid-Based Suzuki Coupling Reaction: From Batch to Continuous Microflow System. Journal of Flow Chemistry. Jun 2017;7(2):52-56.
4. Bai L, Fu YH, Yao M, Cheng Y. Enhancement of mixing inside ionic liquid droplets through various micro-channels design. Chemical Engineering Journal. Jan 2018;332:537-547.