Monodisperse Water‐in‐Water‐in‐Oil Emulsion Droplets

Abstract

Since first being described by Seifriz in 1925, [1] a double emulsion (also referred to as a multiple emulsion), water-in-oil-inwater (W/O/W) and oil-in-water-in-oil (O/W/O) type double emulsions, have attracted significant research interest because of their potential applications in various fields, including agriculture, cosmetics, the food industry, extraction and drug delivery. [2][3][4][5][6][7][8][9][10][11][12] In addition, double emulsions are often used as a template for the preparation of microcapsules with liquid cores. [13][14][15] Microcapsules can be used to encapsulate drugs, [16] living cells [17] and are a promising technology for food, [18] pharmaceutical [16,17,19] and energy-saving [20] applications. Recently, efficient techniques for producing highly monodisperse W/O/W and O/W/O emulsions have been developed. [21][22][23][24] Microfluidics techniques that utilize microchips, flow-focusing devices and coaxial microcapillary fluidic devices can enable precise control of the outer and internal drop sizes. Monodispersibility should make double emulsions highly desirable for the applications mentioned above. There are still unattained requirements for the production of double emulsions and for the composition or properties of the interior phase. In production, the requirement is to simplify the emulsification process. To fabricate monodisperse double emulsions using microfluidic devices, one must precisely and continuously control the flow rate of the innermost, middle, and outermost fluids because the size of the outer and inner droplets is fixed by the shear stress arising from each fluid flow. In general, different thicknesses of the internal phase occur in double-emulsion globules because it is difficult to maintain the internal phase at the center of the globules.Herein, we report on a new class of double emulsion that can overcome the above disadvantages. The double emulsion is prepared via an aqueous two-phase system. Aqueous twophase systems were originally observed in polymer solutions and are generally composed of a water solution of two particular polymers. At high concentrations of these components, spontaneous phase separation takes place. One phase is rich in one polymer and the second phase is rich in the other polymer. The phase separation phenomenon is interpreted in terms of the incompatibility of polymers. When two entirely aqueous solutions containing different water-soluble molecules are mixed together, a water-in-water (W/W) emulsion is generated, which is a system that consists of a polymer suspended as water-solvated droplets dispersed in a solution of another polymer whose solvent is also water. [25][26][27][28] Two types of W/W emulsion are shown in Figure S1 (Supporting Information), made from aqueous solutions of polyethylene glycol (PEG) and Dextran (DEX) at different concentrations. To clearly demonstrate each phase, a water-soluble dye, reactive blue 180, which preferably partitions into the PEG-rich phase, was added to the system. In a W/W emulsion, the polymer solut...

Liquid–liquid phase separation in artificial cells

“…W/o emulsions can be used to provide individual, diffusion-limited droplets that contain multiple internal phases with microscale total volumes, rendering the system closer to biological cells while at the same time conserving precious reagents (figure 5a). The oil phase formulations used for these emulsions are varied, ranging from mineral oil [119,120,123] and other hydrocarbons [124,125] to fluorocarbons [121,126,127]. Choosing a specific oil base depends on a variety of factors, including viscosity, density and chemical characteristics.…”

Microcapsules with a permeable hydrogel shell and an aqueous core continuously produced in a 3D microdevice by all-aqueous microfluidics

“…[15][16][17] Recently, we and others explored the use of ATPS for the continuous generation of single and double emulsion microdroplets in a microuidic device. 14,[18][19][20][21][22][23][24][25][26][27][28] However, the lack of methods to stabilize the shell of all-aqueous double emulsion microdroplets under continuous ow, until now, precluded the ATPS approach from being applied for the production of microcapsules with an aqueous core and permeable hydrogel shell.…”

Micro segmented flow-functional elements and biotechnical applications

“…The generation of double emulsion droplets is fundamentally the same as that for single emulsion droplets, i.e., either by the cross-channel or T-junctions geometry, as shown in Figure 2 (27,65,98). A slight variation of the T-junctions geometry is the Y-junction geometry (116). However, the generation needs to be repeated with phase inverted to produce double emulsions.…”