Recent advances in microfluidic production of Janus droplets and particles
Graphical abstract
Introduction
Micro- and nanodroplets as well as particles with a multicompartmental structure have attracted significant interest recently owing to their promising properties, which make them suitable for use in various applications in biotechnology, chemistry, materials science, and other fields. A number of routes for synthesizing such droplets and particles with multiple functionalities have been developed and studied extensively [1].
In the past decade, a new emulsification technology that uses a microfluidic droplet generator (MFDG) has been developed to produce emulsion droplets with an extremely narrow size distribution [2], [3]. An MFDG typically consists of confined geometries such as T- and cross-junctions microfabricated on a planar chip or coaxially assembled three-dimensional (3D) microcapillaries. By infusing a dispersed-phase fluid (e.g., an aqueous solution) into a coflowing continuous-phase fluid (e.g., oil) in these devices under low Reynolds and capillary number conditions, monodisperse emulsion droplets can be formed one by one. Further, their sizes can be tuned by changing the flow conditions, while their breakup rate can be varied in the range of 1 to 104 Hz, depending on the properties, such as the viscosity and interfacial tension, of the fluids used. In addition to allowing for controllability with respect to the size and size distribution, MFDGs can also produce emulsion droplets with precisely controlled internal architectures. For example, microfluidic double emulsification methods can produce different types of monodisperse double and higher-order multilayered emulsions with precisely designed internal compartments [4], [5], [6•]. Various particulate materials with controlled sizes, internal structures, and chemical compositions have been created using these double emulsions as templates [7], [8], [9], [10], [11], [12], [13], [14].
In this review, we focus on the recent studies on the microfluidic production of another kind of multicompartmental droplets and particles, the so-called Janus droplets and particles [15], [16••], [17], [18], [19], which have two physically and chemically distinct exposed segments. We describe the microfluidic devices used for Janus emulsification, the routes available for creating Janus droplets, functional Janus microparticles that exhibit different anisotropies, and the nonspherical particles produced from Janus templates. On the other hand, nonemulsification technologies can also yield Janus particles. Examples of these technologies include electrified cojetting and flow lithography, which have been reviewed in detail recently [7], [8], [9], [11], [12], [13]. We also discuss the recent development of parallelization devices for scaling up production on an industrial scale.
Section snippets
Breakup of Janus droplets in a coflow system
Different types of Janus emulsion droplets have been produced in microfluidic devices by forcing two separately infused dispersed phases to form a parallel stream that is later emulsified by a coflowing continuous phase. In particular, two device configurations are used widely. One is a planar microfluidic chip made of polymers, silicon, and glasses, on which a Y-shaped channel is present to form a parallel stream of the two dispersed phases and which leads to a MFDG where the continuous phase
From Janus emulsion droplets
Various solidification methods have been used to prepare Janus microparticles from Janus droplets generated in MFDGs (Fig. 3). The ultraviolet (UV) irradiation of Janus droplets of monomers containing a photoinitiator is the most widely used method for solidification [23], [24], [25], [27]. In contrast, there have been only a few reports on the thermally induced polymerization of acrylic Janus particles [20], [21]. Further, there have been several reports on polysaccharide Janus hydrogels
Fabrication from Janus droplets
Different types of nonspherical microparticles of controlled sizes and shapes have been produced from microfluidic biphasic Janus droplets comprising two immiscible photocurable and noncurable segments. The methods to obtain such Janus droplets are described in Section 2. Upon on-chip or off-chip photopolymerization, polymeric microparticles with curvatures templated by the noncurable segments have been produced [42], [44••], [50•], [52]. The shape of the particles can also be varied by
Why is ‘numbering-up’ needed?
While MFDGs that can produce anisotropic droplets and particles are garnering significant attention, it is becoming increasingly important to mass-produce such droplets and particles using MFDGs for industrial applications. In this respect, the volume throughput per MFDG is very low, even if their breakup rates are increased for high shear flows. For example, the generation of W/O droplets having a diameter of ~ 100 μm at a breakup rate of 103 drops s− 1 only corresponds to a volume throughput of ~
Conclusion
In this review, we summarized the recent trends in microfluidic Janus emulsification technology and its use in producing functional microparticles having different anisotropic properties. A variety of anisotropies have been incorporated into such droplets and particles by using various types of droplet generators and subjecting the droplets and particles to subsequent treatments. In particular, the number of studies on (a) the phase-separation-assisted fabrication of Janus droplets and
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2023, PolymerCitation Excerpt :In the confined space, controlled properties and structures of anisotropic Janus particles were fabricated via combination of dominant and multiple components with micro-processing [27,28]. Up to date, microfluidic production [29–31] and seeded emulsion polymerization [32,33] were widely utilized to synthesize Janus particles. Among them, seeded emulsion polymerization is a popular technique due to the advantages of controllability and flexibility [34,35].