Double-emulsion drops with ultra-thin shells for capsule templates

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Review by Bryan Hassell: AP 255 Fall 11

From: Double-emulsion drops with ultra-thin shells for capsule templates Shin-Hyun Kim,a Jin Woong Kim,b Jun-Cheol Choc and David A. Weitz, Lab on a Chip, 2011

Keywords: emulsion, microfluidics and capillary

Introduction

This paper is about an emulsification technique that creates monodisperse double-emulsion drops (or drops in drops) with a core–shell geometry having an ultra-thin wall as a middle layer. They create a biphasic flow in a microfluidic capillary device by forming a sheath flow consisting of a thin layer of a fluid with high affinity to the capillary wall flowing along the inner wall of the capillary, surrounding the innermost fluid. This creates double-emulsion drops, using a single-step emulsification, having a very thin fluid shell. If the shell is solidified, its thickness can be small as a hundred nanometres or even less. Despite the small thickness of this shell, these structures are nevertheless very stable, giving them great potential for encapsulation.

The robustness of these particles comes from the use of a core-shell geometry where the middle phase can be solidified thus ensuring a solid layer separating the inner and outer sections. This solidification is done in three ways typically: solvent evaporation, polymerization or dewetting of the middle phase onto the surface of the innermost drop. All of these techniques are used respectively for various applications i.e. creating a porous inner phase, an inner phase with cross linked polymer micro capsules or a middle phase of liposomes, yet to prepare a double emulsion drop highly viscous organic solvents is still a challenge. Also, due to interfacial energies, the inner drop tends to coalesce with the outer before consolidation. Thus it was found that a thinner middle phase can reduce the effect, therefore in this paper a novel technique is described to create stable mono disperse double emulsion drops with an ultra thin shell.

Results

A biphasic flow confined in a capillary

Figure 1: (a) Schematic illustration of the microfluidic device for preparation of double-emulsion drops with an ultra-thin shell. (b and c) Optical microscope images showing flow of water drops in the injection capillary with an inner diameter of 580<math>\mu</math>m and their emulsification at the tip of the injection capillary, where flow rates of inner (Q1), middle (Q2), and continuous (Q3) phases are maintained at values of 4000 ml <math>{\mu}l h^{-1}</math>, 1000 <math>{\mu}l h^{-1}</math>, and 5500 <math>{\mu}l h^{-1}</math>, respectively. (d and e) Optical microscope images showing a stable water jet in the injection capillary with an inner diameter of 200<math>\mu</math>m and its emulsification at the tip of the injection capillary, where Q1, and Q2, and Q3 are maintained at values of 2500 <math>{\mu}l h^{-1}</math>, 500 <math>{\mu}l h^{-1}</math>, and 8000 <math>{\mu}l h^{-1}</math>, respectively.
Figure 2: (a and b) Flow behavior as a function of Q1 and Q2, where Q3 is maintained at 4500 <math>{\mu}l h^{-1}</math>. Circles, triangles, and crosses denote a stable jet, an unstable jet and drops, respectively, as shown in (b). The range of stability is quite large. (c–e) Optical microscope images showing (c) the continuous jetting, (d) the continuous dripping, and (e) the discontinuous dripping modes of double-emulsion generation, where Q1 is (c) 1200 <math>{\mu}l h^{-1}</math>, (d) 800 <math>{\mu}l h^{-1}</math>, and (e) 400 <math>{\mu}l h^{-1}</math>, respectively and Q2 and Q3 are maintained at 100 <math>{\mu}l h^{-1}</math> and 4500 <math>{\mu}l h^{-1}</math>, respectively.
Fig. 4 (a and b) Optical microscope images of mono disperse double emulsion drops before consolidation of the middle phase. (c and d) Confocal microscope images of microcapsules with a poly(lactic acid) membrane. A scanning electron microscope image in the inset of (d) shows a cross-section of the membrane.

Two immiscible fluids which flow coaxially and simultaneously through a single capillary can exhibit two distinct flow patterns, consisting of either a coaxial jet or a stream of drops of one fluid in the second. A jet of one liquid in the second is typically unstable to the Rayleigh–Plateau instability which causes a breakup of the jet into drops; this instability can be suppressed by confining the coaxial flow. Further control over the fluid flow can be achieved by exploiting the affinity of the fluid to the capillary; the fluid with higher affinity to the wall will flow along it whereas the second fluid will flow through the center of the capillary. Because of the affinity to the wall, the thickness of the outer fluid can be very thin. They employ such a biphasic flow, using either a stable jet or plug-like drops, to produce double emulsion drops. By controlling the thickness of the fluid with high affinity to the wall, they are able to produce double-emulsion drops with an ultra-thin middle layer using a one-step emulsification process. To make W/O/W double-emulsion drops, they use a capillary microfluidic device comprised of a hydrophobic tapered injection capillary inserted into a second square capillary (AIT glass) whose inner dimension is the same as that of the outer diameter of the injection capillary, which is typically 1 mm, as schematically illustrated in Fig. 1a. Even when the volumetric flow rate of the inner phase is as much as a factor of 4 larger than that of the middle phase and their linear flow velocities are quite high, a train of plug-like drops are observed as shown in Fig. 1b. These plug-like drops are emulsified at the tip of the injection capillary, resulting in monodisperse double-emulsion drops with an ultra-thin middle layer; however there is excess oil in the middle phase which is not incorporated into the emulsion drops, but instead form large independent oil blobs between double-emulsion drops. This discontinuous generation of double-emulsion drops is shown in Fig. 1c. In contrast, a stable water jet is formed in the injection capillary when the inner diameter is 200 mm (AIT glass) as shown in Fig. 1d; this is emulsified at the tip in a continuous fashion, resulting in monodisperse double-emulsion drops, as shown in Fig. 1e. As the inner diameter of the capillary decreases, there is a higher degree of interfacial confinement, thereby reducing deformation of the interface and making a more stable jet.

Emulsification of biphasic flow

The flow rates of inner (Q1), middle (Q2), and continuous (Q3) phases each influence the flow patterns in the injection capillary and the double-emulsion generated, as summarized in Fig. 2a. At any given Q2, a stable water jet is formed in the 200 mm diameter injection capillary for sufficiently large Q1, as denoted by blue circles in Fig. 2a and in the top image of Fig. 2b. However, a double-emulsion is not always produced by the jet; high inertia of the inner stream at high Q1 causes a leakage of the inner stream into the continuous phase near the tip of the injection capillary, as denoted by the violet-colored region in Fig. 2a. In contrast, for Q1 in the region denoted by blue color, double emulsion drops are generated continuously in either the jetting or the dripping modes.14 For high Q1, the jet flows through the orifice of the collection capillary and then breaks into double emulsion drops at the end of the jet, as shown in Fig. 2c. In contrast, for low Q1, the jet is emulsified in the dripping mode, near the tip of the injection capillary, as shown in Fig. 2d. As Q1 is further reduced, as denoted by the green region, the jet becomes unstable and exhibits fluctuations of the interface but does not breakup, as shown in the middle image of Fig. 2b. A further reduction of Q1, as denoted by the red region, yields plug like water drops in the injection capillary as shown in the bottom image of Fig. 2b; this produces a discontinuous flow of double-emulsion drops in the dripping mode, as shown in Fig. 2e.

Diameter and shell thickness of double-emulsion drops

The thickness, t, of the middle layer of the double-emulsion drops is too small to measure directly from an optical microscope image. Therefore, they rupture the double-emulsion drops to form a single emulsion drop and determine the shell thickness from the radius of this single emulsion drop (<math>R_{rup}</math>) and that of the original double-emulsion drop (R):

<math style="vertical-align: -middle"> t = R - (R^{3} - R^{3}_{rup})^{1/3}</math>

For example, two optical microscope images shown in Fig. 4d exhibit rupturing of a double-emulsion drop, resulting in formation of a single emulsion drop as denoted by the dotted circles; to rupture them, they place the double-emulsion drops on top of a glass slide, and tap it several times. For these two images, R = 61 <math>\mu m</math> and <math>R_{rup}</math> = 19 <math>\mu m</math>, and we use these values to calculate t = 620 nm, where the approximately 10% uncertainty of the thickness results from the limits of the resolution of the optical microscope. In the same fashion, we calculate the thickness of the middle layer of double-emulsion drops which are produced at each values of Q1/Q2. Using this method the thickness measured well with the mass balance equation:

<math> \frac{t}{R} = 1 - \left (1+\frac{Q_1}{Q_2} \right)^{1/3}</math>

Biodegradable microcapsules from double-emulsion drops

Their approach can be used to make double-emulsion drops with fluids having a wide range of viscosities; thus, highly concentrated organic solutions of polymers can be used for the middle phase. Moreover, an ultra-thin middle layer provides stability to the double-emulsion drops, preventing coalescence of the innermost drop with the continuous fluid; such stability is otherwise difficult to achieve with a larger thickness. They attribute the enhancement of stability to the significantly increased drag force on the innermost drop because the very thin width of the middle phase puts the fluid in the lubrication regime.

Conclusion

They produced monodisperse double-emulsion drops with an ultrathin middle layer through a one-step emulsification process using a biphasic flow, confined in a microcapillary, operated in either a stable jet or in a flow of drops. This technique enables production of double-emulsion drops even with highly viscous fluids; moreover the thickness of the shell can be reduced to submicron scales, which is difficult to achieve with any other approach. The very thin middle layer confers high stability to the double-emulsion drops, enabling high frequency drop generation while preventing rupturing during consolidation thus facilitating creation of microcapsules with an ultra-thin membrane of as little as a few tens of nanometres thick. This microfluidic approach should be useful for encapsulation and delivery of active materials such as drugs, cosmetics, and nutrients. Furthermore, the resultant microcapsules with their ultra-thin membranes can serve as model systems for studies of buckling phenomena of thin membranes and for studies of phase behavior of polymer blends or block-copolymers that are spatially confined.