Recent advances in microfluidic production of Janus droplets and particles

doi:10.1016/j.cocis.2016.05.003

Current Opinion in Colloid & Interface Science

Volume 25, October 2016, Pages 1-12

https://doi.org/10.1016/j.cocis.2016.05.003 Get rights and content

Highlights

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A review on microfluidic production of Janus emulsion droplets and particles of late.
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Microfluidic Janus droplets and particles with various anisotropies are summarized.
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Recent technologies for scaling-up microfluidic emulsification are also discussed.

Abstract

Microfluidic production of multicompartmental emulsion droplets and particles has received considerable attention of late. In particular, droplets having two physically and chemically distinct segments (so-called Janus droplets) and the anisotropic particles synthesized from these droplets, are becoming increasingly popular because of their novel and promising properties, which make them suitable for use in numerous applications, including for controlled drug release, display devices, and self-assembly. So far, a range of interesting anisotropies have been accorded to Janus droplets and particles via microfluidics; these span from chromatic, magnetic, and hydrophobic–hydrophilic characteristics to selective degradation properties. Here, we summarize and discuss the recent trends related to Janus droplets and particles produced through microfluidic processing. We also review the parallelization technologies being developed for scaling up microfluidic emulsification in the industry.

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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Citation 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].
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Recent advances in microfluidic production of Janus droplets and particles

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Breakup of Janus droplets in a coflow system

From Janus emulsion droplets

Fabrication from Janus droplets

Why is ‘numbering-up’ needed?

Conclusion

Curr Opin Colloid Interface Sci

Particuology

Lab Chip

Chem Soc Rev

RSC Adv

Adv Mater

Soft Matter

Adv Mater

Langmuir

Microfluidic methods for generating continuous droplet streams

J Phys D Appl Phys

Cross-flow microfluidic emulsification from a food perspective

Trends Food Sci Technol

Microstructure devices for preparing controlled multiple emulsions

Chem Eng Technol

Advances in fabricating double-emulsion droplets and their biomedical applications

Microfluid Nanofluid

Polymer chemistry in flow: new polymers, beads, capsules, and fibers

J Polym Sci: Part A: Polym Chem

Microfluidic-assisted synthesis of polymer particles

Chem Eng Technol

The synthesis and assembly of polymeric microparticles using microfluidics

Adv Mater

Droplet microfluidics for producing functional microparticles

Langmuir

Microfluidic fabrication of multifunctional particles and their analytical applications

Talanta

Microfluidic synthesis of barcode particles for multiplex assays

Small

Controllable microfluidic strategies for fabricating microparticles using emulsions as templates

Particuology

Janus particles: synthesis, self-assembly, physical properties, and applications

Chem Rev

Strictly biphasic soft and hard Janus structures: synthesis, properties, and applications

Angew Chem Int Ed

Rational design and synthesis of Janus composites

Adv Mater

Fabrication of polymeric Janus particles by droplet microfluidics

RSC Adv

Novel microreactors for functional polymer beads

Chem Eng J

Synthesis of monodisperse bicolored Janus particles with electrical anisotropy using a microfluidic co-flow system

Adv Mater

Formation of biphasic Janus droplets in a microfabricated channel for the synthesis of shape-controlled polymer microparticles

Adv Mater

Microfluidic conceived drug loaded Janus particles in side-by-side capillaries device

Int J Pharm

A side-by-side capillaries-based microfluidic system for synthesizing size- and morphology-controlled magnetic anisotropy janus beads

Adv Powder Technol

Bioinspired multifunctional Janus particles for droplet manipulation

J Am Chem Soc

A lithography-free procedure for fabricating three-dimensional microchannels using hydrogel molds

Biomed Microdevices

Water–oil Janus emulsions: microfluidic synthesis and morphology design

Soft Matter

A microfluidic cross-flowing emulsion generator for producing biphasic droplets and anisotropically shaped polymer particles

Microfluid Nanofluid

Microfluidic fabrication of oil-filled polymeric microcapsules with independently controllable size and shell thickness via Janus to core–shell evolution of biphasic droplets

Sensors Actuators B Chem

Coalescence of two immiscible liquid drops

Science

Controlled coalescence of two immiscible droplets for Janus emulsions in a microfluidic device

RSC Adv

Biconvex polymer microlenses fabricated from microfluidic Janus droplets

J Jpn Soc Precis Eng

Controlled assembly of jammed colloidal shells on fluid droplets

Nat Mater

Controlled production of monodisperse double emulsions by two-step droplet breakup in microfluidic devices

Langmuir