Microfluidic Method

Recently, the droplet microfluidic system has been developed and widely used for generating and manipulating droplets [21–23], which provides a potential way to form Janus droplets. Based on the formation mechanism, the microfluidic method can be divided into three groups: breakup formation [7,8,24–28], evolution from core-shell emulsion [29–36] and phase separation [37–39].

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2.1.2.1 Breakup Formation

The first mechanism, breakup formation, is the most popular one. With this mechanism, Janus droplets can be produced one by one through forcing parallel streams of two immiscible phases to break up into droplets in a microfluidic chip [24,25,40]. Generally, two immiscible dispersed phases flow in the central channels of a microfluidic chip, and the continuous phase flows in the side channels. When the thin stream of the two dispersed phases is forced to flow through a narrow orifice, the shear force generated by the continuous phase breaks up the thin stream and Janus droplets form. The schematic diagrams of the general Janus droplets generators are shown in Figure 2-3 (a) and (b). Maeda et al. [26] extended this method by developing a centrifuge Janus droplet generator to synthesis magnetic anisotropic Janus droplets. The synthesis system comprises two parts: a droplet generator device with built-in capillaries for holding the two immiscible monomers and forming sessile Janus droplets on the capillary orifices, a tabletop centrifuge which is used to provide centrifugal force to drag the sessile Janus droplets away and forms mobile Janus droplets (Figure 2-3 (c)). Furthermore, to increase the volume throughput of the microfluidic method, a lot of attempts have been taken, for example, parallelization devices were utilized to generate multiple Janus droplets at the same time [41–46].

Figure 2-3 Schematic diagrams of the Janus droplets generators. (a) A Y-shaped microfluidic droplet generator (MFDG). Reproduced with permission [24]. Copyright 2007 John Wiley and

Sons. (b) A T-shaped MFDG. Reproduced with permission [40]. Copyright 2016 Elsevier. (c) Centrifuge-based Janus droplets generator. Reproduced with permission [26]. Copyright 2012

John Wiley and Sons.

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With this mechanism, the topology and size of the Janus droplets can be controlled easily. The proportion of different monomers in the droplets can be changed by adjusting the volume flow rates of the two immiscible dispersed phases, and the size of the droplets can be controlled by regulating the volume flow rates of the dispersed phases and the continuous phase [47]. The properties of different sides of the Janus droplets are dependent on the properties of the two immiscible monomers. With the break up mechanism, a variety of Janus droplets has been fabricated, for example, acrylate monomer-silicon oil Janus droplets [24] (Figure 2-4 (a)), 1,6- hexanediol diacrylate (HDDA)-silicone oil Janus droplets [25] (Figure 2-4 (b)) and soybean oil- deionic water Janus droplets [28].

Figure 2-4 Optical images of the Janus droplets. (a) Acrylate monomer-silicon oil Janus droplets developed from breakup mechanism. Reproduced with permission [24]. Copyright 2007 John

Wiley and Sons. (b) 1,6-hexanediol diacrylate (HDDA)-silicone oil Janus droplets developed from breakup mechanism. Reproduced with permission [25]. Copyright 2009 Springer Nature. (c) Tetradecane-tripropylene glycol diacrylate (TPGDA) Janus droplet transferred from O/O/W droplet. Reproduced with permission [33]. Copyright 2008 American Physical Society. (d) Janus liposome prepared from W/O/W droplet through solvent evaporation and dewetting. Reproduced

with permission [34]. Copyright 2011 John Wiley and Sons.

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2.1.2.2 Evolution from Core-shell Emulsion

The Janus droplets can also be produced through the evolution of core-shell emulsion droplets. The core-shell emulsion is consisted with three immiscible phases, i.e., two dispersed immiscible phases and one continuous phase. The core-shell droplets are formed by encapsulating the droplets of one dispersed phase with the other dispersed phase in MFDGs. For oil-in-oil-in-water (O/O/W) droplets, the topology of the core-shell droplets is changeable. With the minimization of the interfacial energy of the droplets, the core-shell droplets transform into Janus droplets. Pannacci et al. [33] introduced three fluids into a microfluidic system to produce double droplets. The core- shell droplets are generated in the upstream of the microchannel. As the translation of the droplets, the complete engulfing droplets transform into Janus droplets in the downstream spontaneously with the minimization of the interfacial energy, as shown in Figure 2-4 (c). The core-shell droplets and the Janus droplets are inter-convertible by using stimuli-responsive surfactants to tune the interfacial tensions. For example, the effectiveness of the stimuli-responsive surfactants changes in response to pH, temperature or light, which further affects the interfacial tensions, and then the topology of the droplets. The Janus droplets can also be fabricated from the water-in-oil-in-water (W/O/W) droplets with the dewetting and evaporation of solvent. The droplets with Janus geometry comprising an aqueous lobe and a solvent lobe show up transiently during the solvent evaporation of W/O/W droplets [36]. Recently, Shum et al. [34] reported a novel method to prepare stable Janus droplet through dewetting. They prepared W/O/W emulsion droplets with two aqueous cores in MFDG. As the evaporation of the middle phase, the amphiphilic diblock copolymers dissolved out and got assembled at the interfaces to form membranes. As a result, the aqueous cores covered with copolymers adhered to each other and a Janus vesicle formed (Figure 2-4 (d)).

2.1.2.3 Phase Separation

The phase separation is another mechanism for generating Janus droplets from single-phase emulsion droplets. In this mechanism, the dispersed phase containing multiple solvents is emulsified into a continuous phase in a MFDG. With the dissolving and evaporation of the co- solvent into the continuous phase, the solvents in the droplet separate out and Janus droplet forms. For example, Zhang et al. [39] injected the ternary mixture (ethanol, water and octanol) and

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fluorinated oil (FC-40) into a T-shape microchannel with the ternary mixture as the dispersed phase and the FC-40 as the continuous phase. The homogeneous ternary mixture droplets were generated in the microchannel. As the FC-40 has high permeability for vapor, the volatile solvent in the ternary mixture droplets, ethanol, entered into the continuous phase and evaporated into the air. Finally, the remaining two solvents, water and octanol, in the droplets separated out to form Janus droplets.