Recent advances in the production of controllable multiple emulsions using microfabricated devices

Abstract

This review focuses on recent developments in fabrication of multiple emulsions in microscale systems, such as membrane, microchannel array and microfluidic emulsification devices.Membrane and microchannel emulsification offer great potential in manufacturing multiple emulsions with uniform drop sizes and high encapsulation efficiency of encapsulated actives.Microfluidic devices enable unprecedented level of control over the number, size and type of internal droplets at each hierarchical level but suffer from low production scales. Microfluidic methods can be exploited to generate high-order multiple emulsions (triple, quadruple and quintuple), non-spherical (discoidal and rod-like) drops and drops with asymmetric properties such as Janus and ternary drops, composed of two or three distinct regions on the surface.Multiple emulsion droplets generated in microfabricated devices can be used as templates for vesicles (polymersomes, liposomes, colloidosomes) with multiple inner compartments for simultaneous encapsulation and release of incompatible actives or reactants.-2-

Microfluidic Production of Multiple Emulsions

“…This method leads to the production of PDMS block with rectangular open channels, which can be sealed by thermal annealing to a glass slide or another PDMS block to produce enclosed fluid paths. Because of the rectangular shape of the channels, the continuous phase cannot completely surround the dispersed phase during the drop generation process [ 69 ]. In addition, since the inlet channel(s), through which the fluids are introduced into the device, have the same depth as the collection channel where the drops are formed, the wall of the latter can be wetted by the generated droplets which may damage the interface of forming droplets and cause failure of the drop formation [ 69 ].…”

“…Because of the rectangular shape of the channels, the continuous phase cannot completely surround the dispersed phase during the drop generation process [ 69 ]. In addition, since the inlet channel(s), through which the fluids are introduced into the device, have the same depth as the collection channel where the drops are formed, the wall of the latter can be wetted by the generated droplets which may damage the interface of forming droplets and cause failure of the drop formation [ 69 ]. Therefore, in order to controllably produce drops in planar devices, it is necessary to tune the wettability of the collection channel so that a greater affinity to the continuous phase rather than the dispersed phase is achieved and maintained.…”

“…Microfluidic devices have been widely used for generation of double emulsion drops with distinct inner fluids [ 69 , 78 , 145 ]. This is useful for keeping mutually incompatible actives physically separated within the same microcapsule to allow synergistic effects upon their triggered release when the contents of the inner drops are combined [ 143 ].…”

Modeling mass transfer and reaction of dilute solutes in a ternary phase system by the lattice Boltzmann method

“…The Janus droplet is one of the special types of microdroplets that two opposite droplets with different chemical properties adhere to each other [74,75]. Microfluidic technology provides a controllable and high throughput method to produce uniform micro-Janus droplets [76] and the hydrodynamic of the Janus droplet in microchannel has been studied [77][78][79].…”

Complex emulsions for shape control based on mass transfer and phase separation

“…An emulsion is a metastable system but can be stabilized by a suitable surfactant, which generally consists of two immiscible liquid phases with one phase dispersed into the second. 1 Recently, complex emulsions, including multiple emulsions 1,2 and Janus emulsions, 3,4 have promising applications in foods, 5 chemical separations, 6 sensors, 7 drug delivery, 8 microcapsules fabrication. 9,10 Multiple emulsions are often generated through a two-step method.…”