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Controlled formation of double-emulsion drops in sudden expansion channels
Journal of Colloid and Interface Science 415 (2014) 26–31
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Controlled formation of double-emulsion drops in sudden expansion channels Shin-Hyun Kim ⇑, Bomi Kim Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 September 2013 Accepted 15 October 2013 Available online 26 October 2013 Keywords: Microfluidics Double emulsions Polymersomes Weber number Capillary
a b s t r a c t Double-emulsion drops or drops-in-drop have provided useful templates for production of microcapsules due to their core–shell geometry. Here, we introduce new capillary microfluidic geometry for the creation of double-emulsion drops, which is composed of a narrowing channel followed by sudden expansion channel. Drops injected through the narrowing channel are highly accelerated to flow, inducing high inertia force. When rear interface of the drops arrives at the sudden expansion channel, the high inertia force deforms the interface and leads to its breakup into a drop in the interior of the injected drop. This insertion is driven by inertia force against capillary force: High linear velocity and low interfacial tension facilitate the insertion. We also apply this emulsification method to double-emulsion drops with single innermost drop; insertion of a water drop creates the double-emulsion drops with two distinct innermost drops. The resultant double-emulsion drops with single- or double-innermost drops provide useful templates to produce polymersomes which encapsulate same fluid to the continuous phase; this will be potentially useful for sampling of the continuous phase and its isolation in a wide range of applications for micro-total analysis system (l-TAS). Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Emulsions have been used as templates to produce microparticles and capsules for a wide range of applications including drug delivery vehicles and active-display pigments [1,2]. Recently, double- or multiple-emulsion drops have been prepared to achieve advanced structural complexity and functionality [3]. Through selective solidification of middle phases, the emulsion drops are transformed into capsule structures with high stability; formation of a robust compartment isolating the innermost drop from the continuous phase enables long-term maintenance of their core– shell structure. For example, polymerization of monomers or prepolymers in the middle phase can produce crosslinked polymeric membranes which exhibit high mechanical stability and chemical resistance [4–6]. Evaporation of the middle phase, containing polymers or colloids, can also produce solid membranes which can be functionalized to release of encapsulants [7–10]. In addition, the double-emulsion drops can be used as templates to create capsules whose membranes are composed of molecular bilayer structures such as liposomes and polymersomes; by employing mixtures of two different organic solvents containing amphiphiles as middle phase, a bilayer membrane can be prepared through dewetting of the middle phase on the surface of the innermost drops [11–14]. ⇑ Corresponding author. Fax: +82 42 350 3910. E-mail address: [email protected] (S.-H. Kim). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.020
Recent advances in microfluidics have enabled the controlled formation of double- or multiple-emulsion drops. Microfluidic devices, made of either polydimethylsiloxane (PDMS) or glass capillaries, have been employed to provide unprecedented controllability in size and structure of the drops and high efficiency of encapsulation. This production of double-emulsion drops has been achieved in two distinct ways: Sequential emulsification and one-step emulsification. Sequential emulsification is accomplished by using two serial drop makers [4,15–18]; single-emulsion drops are produced in the first junction and then, they are encapsulated in the second level of emulsion drops in the second junction, resulting in drops-in-drops. This method provides high controllability in number of innermost drops. One-step emulsification produces double-emulsion drops through simultaneous breakup of two coaxial interfaces in one junction; this method provides higher controllability in size of both innermost and middle drops [3,19–21]. Both approaches require independent control of three distinct flows. In this paper, we report new microfluidic approach to create double-emulsion drops through insertion of a drop of continuous phase into single-emulsion drops. To accomplish this, we design a capillary microfluidic device which consists of a narrowing channel followed by sudden expansion. By flowing oil-in-water (O/W) drops through the channel, we can deform rear interface of the drop by high inertia force and insert water drop of continuous phase into the oil drop through a breakup of the deformed
S.-H. Kim, B. Kim / Journal of Colloid and Interface Science 415 (2014) 26–31
interface, thereby creating water-in-oil-in-water (W/O/W) doubleemulsion drops. This insertion of water drops occurs only for the oil drops larger than certain size which is determined by interfacial
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tension and flow rates for a given geometry; sufficient inertia force relative to capillary force is required to deform the rear interface and break into a drop. With this method, therefore, we can produce double-emulsion drops with independent control of two flows. In the similar fashion, water drop can be inserted into double-emulsion drops, making double-emulsion drops with two distinct innermost drops. The core–shell structures of the resultant double-emulsion drops can be stabilized by creating bilayer membranes of amphiphilic block-copolymers, thereby providing stable isolation of inner volume of aqueous phase; this is potentially useful for sampling of fluids to make further analysis. 2. Materials and methods 2.1. Device preparation
Fig. 1. (a) Schematic illustration of the microfluidic device composed of a series of two junctions for insertion of water drop into oil drops, making water-in-oil-inwater (W/O/W) double-emulsion drops. (b–d) Optical microscope images showing (b) flow of the oil drops through middle capillary, (c) insertion of water drop into each oil drop at the second junction, and (d) flow of the double-emulsion drops through collection capillary, where flow rates of oil and water phases, Q1 and Q2, are maintained at values of 1500 ll/h and 1800 ll/h, respectively.
We integrate two junctions into one capillary microfluidic device as shown schematically in Fig. 1a: One for making oil drops and the other for inserting a water drop into each oil drop. The design of the device comprises three tapered cylindrical capillaries, the left, middle, and right capillaries, which are inserted in two square capillaries; the middle cylindrical capillary is tapered at both sides, whereas the other two cylindrical capillaries are tapered only at one side. The left capillary is tapered to have a 20-lm-diameter orifice and treated with n-octadecyltrimethoxyl silane (Sigma–Aldrich) to render it hydrophobic; this left capillary is not used for production of single oil drops in the first junction, but used for production of double-emulsion drops. The left side of the middle capillary is tapered to have a 190-lm-diameter orifice, whereas the right side is tapered to have a 68-lm-diameter orifice. The orifice diameter of the right side of the middle capillary is carefully selected to make successful insertion; too small orifice experiences high flow resistance and frequently leads to a breakup of interfaces within the injection channel, while too large one imposes small inertia. The right capillary is carefully tapered to have a 306-lm-diameter orifice; too small orifice makes negligible relaxation of drops, while too large orifice induces very low velocity in the channel which can result in coalescence. The middle and right
Fig. 2. (a) Time-dependence of flow velocities of front (j) and rear (N) interfaces of drop flowing through a tapered capillary, followed by sudden expansion; average flow velocity (d), Q/A, is shown for comparison, where Q is total volumetric flow rates, Q1 + Q2, and A is cross-sectional area of the tapered capillary channel. (b–e) Optical microscope images showing deformation of the rear interface and subsequent breakup, inserting water drop into oil drop. Images are taken at denoted times in (a) and black and red arrows denote position of front and rear interfaces, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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S.-H. Kim, B. Kim / Journal of Colloid and Interface Science 415 (2014) 26–31
capillaries are treated with 2-[methoxy(polyethyleneoxy)propyl] trimethoxy silane (Gelest, Inc.) to render them hydrophilic. The left capillary and the left side of the middle capillary are coaxially aligned in the first square capillary, whereas the right side of the middle capillary and the right capillary are coaxially aligned in the second square capillary. An image of device is shown in Fig. S1 in the Supporting Information. 2.2. Materials As an oil phase, we use a mixture of chloroform and hexane containing 5 mg/ml amphiphilic diblock-copolymer of poly(ethylene glycol) (PEG, Mw 5,000)-b-poly (lactic acid) (PLA, Mw 10,000) and 2.5 mg/ml hydrophobic homopolymer of PLA (Mw 15,000); volume ratio of chloroform in the mixture is controlled in a range of 0.36–0.48. As a continuous phase, we use 10 wt% aqueous solution of poly (vinyl alcohol) (PVA, Mw 13,000– 23,000); red dye molecules, sulforhodamine B, are dissolved into the continuous phase in some experiments. For production of double-emulsion drops in the first junction, we use 10 wt% aqueous solution of PEG (Mw 6000) containing green dye molecules, 8-hydroxyl-1,3,6-pyrenetrisulfonic acid trisodium salt, as an innermost phase.
tia force to viscous force, Re = qdtipurear/g (Reynolds number), is 3.5, where dtip is diameter of the tip, 68 lm and g is viscosity of continuous phase, 13.5 cP. The rear interface is highly deformed in the narrowing channel due to parabolic velocity profile along the cross-section of the channel, as shown in Fig. 2b. As the interface arrives at the expansion channel, the high inertia leads to deep indentation of the interfaces into the interior of the drop, while the front interface moves backward due to relaxation of the drop to spherical shape, as shown in Fig. 2c and d; negative linear velocity of the front interface during the relaxation is also confirmed in Fig. 2a. The sufficient inertia force at the rear interface leads to the formation of a drop from the rear indentation, inserting water drop into the oil drop and forming double-emulsion drops, as shown in Fig. 2e. Therefore, innermost water drop of the resultant doubleemulsion drops is exactly the same fluid to the continuous phase. By contrast, insufficient inertia force leads to only indentation which cannot transform into the water drops; capillary force, 2c/ dtip, prevents the breakup, thereby making the oil drops recover spherical shape in the right capillary, where c is interfacial tension. Therefore, ratio of the inertia force to the capillary force, We ¼ qu2rear dtip =2c (Weber number), is important parameter to insert the water drop into the oil drop [22]; the ratio is estimated
3. Results and discussion 3.1. Insertion of water drop into oil drops We inject the oil phase, the mixture of chloroform and hexane in a volume ratio of 40:60, through the interstices of the left cylindrical capillary and the first square capillary at volumetric flow rate, Q1, of 1500 ll/h, whereas the continuous phase through the interstices of the middle cylindrical capillary and the first square capillary at flow rate, Q2, of 1800 ll/h; syringe pumps are used for the injection. The other inlets are all closed. In the first junction, monodisperse oil drops with diameter of 275 lm are generated in a dripping mode and then flow through the middle capillary in a zigzag form as shown in Fig. 1b. As the drops pass through the second junction which is a narrowing channel, followed by sudden expansion, they transform into water-in-oil-in-water (W/O/W) double-emulsion drops as shown in Fig. 1c and the double-emulsion drops flow through the right capillary as shown in Fig. 1d. Each of these steps is shown in Supplementary movie 1. To study the insertion of small water drop into large oil drop, we record motion of the drops with high speed camera (Phantom V9) at 2000 frames per second. The displacement of the front and rear interfaces between two sequential frames is obtained by image analysis and their linear velocities are calculated by dividing the displacement with 1/2000 s as shown in Fig. 2a; still images taken at the times denoted in Fig. 2a are shown in Fig. 2b–e, where Q1 of 1300 ll/h and Q2 of 2800 ll/h are used. The oil drops flow through the small tapered capillary without contacting the inner wall due to its hydrophilic nature. During this injection of drops, the linear velocities of front and rear interfaces, ufront and urear, dramatically increase because cross-sectional area, A = pR2, of the narrowing channel decreases along the flow direction, where R is inner radius of the channel. In addition, because the oil drops, much larger than the orifice, block the channel before they pass through the small tip, pressure is accumulated; this further accelerates the drop migration. We can confirm this by comparison of linear velocity of the rear interfaces with average flow velocity, Q/A, as shown in Fig. 2a, where Q is total volumetric flow rate, Q1 + Q2. The acceleration of drop migration exerts high inertia force, qu2rear , on the rear interface of the drop due to high linear velocity, where q is density of water phase. At the tip, ratio of iner-
Fig. 3. (a) Influence of volumetric flow rate and diameter of oil drop on insertion of water drop, where oil phase contains chloroform in a volume fraction of 0.4. (b) Influence of interfacial tension and diameter of oil drop on insertion of water drop, where total flow rate is maintained at constant value of 3900 ll/h; volume fraction of chloroform in the oil phase determines the interfacial tension: The volume fractions of 0.36, 0.40, 0.44, and 0.48 correspond to 2.9, 3.4, 3.7, and 3.9 mN/m, respectively. In both plots, insertion of water drop is denoted with unfilled circles (s), whereas failure of the insertion is denoted with crosses ().
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as 5.0 for this insertion. We find that the Weber number should be larger than 4.6 for the insertion at this total flow rate of 4100 ll/h. The insertion of the water drop and its failure are shown in Supplementary movie 2. 3.2. Minimum diameter of oil drop for insertion of water drop Sufficient inertia force relative to capillary force can be achieved by increasing linear velocity or decreasing interfacial tension. The linear velocity of the rear interfaces depends on total flow rate and size of the oil drop; individual flow rates of Q1 and Q2 have insignificant effect in the linear velocity for the same total flow rate and larger drop gains higher accumulation of pressure in the narrowing channel, leading to higher linear velocity of the rear interface for the same total flow rate. Therefore, there is a minimum diameter of the oil drop to successfully insert water drop into it for a constant flow rate, as shown in Fig. 3a. At low total volumetric flow rate as 2000 ll/h, the minimum diameter is as large as 325 lm, where the critical Weber number is estimated as 1.9. As we increase the flow rate to 2600 and 3300 ll/h, the minimum diameter is reduced to 300 lm and 275 lm, respectively; the critical Weber numbers at the flow rate of 2600 and 3300 ll/h are estimated as 2.4 and 3.0, respectively. For further increase of flow rate, by contrast, the minimum diameter increases; for the flow rate of 4100 ll/h, the minimum diameter is 290 lm and the critical Weber number is 4.6 and for the flow rate of 5000 ll/h, the minimum diameter is 410 lm and the critical Weber number is 8.9. We attribute this increase of minimum diameter and critical Weber number to insufficient relaxation of front interface at high flow
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rate even for higher inertia force at rear interface. At such a high flow rate, the front interface moves forward with insignificant relaxation in the expansion channel, thereby requiring higher deformation for breakup of the rear interface. Diameter of the inserted water drops is linearly proportional to the diameter of the oil drops injected for all flow rates used for successful insertion, as shown in Fig. S2 of the Supporting Information; the relative diameter of the inserted drop to the injected drop is in a range of 0.39–0.46. This proves that the diameter of the injected drop significantly influences on the inertia force. Interfacial tension between the oil and continuous phases is determined by volume ratio of chloroform in the oil phase. Although the interfacial tension decreases as volume fraction of chloroform increases in the pure mixture of chloroform and hexane due to the relative hydrophilicity of chloroform, the tendency becomes opposite when the mixture contains amphiphilic blockcopolymers of PEG-b-PLA. Because chloroform is good solvent for PEG-b-PLA, while hexane is poor solvent, low volume fraction of chloroform causes strong absorption of the amphiphiles at the interface, reducing interfacial tension. We measure the interfacial tension between the oil and continuous phases using pendant drop methods; four different volume fractions of chloroform in the mixture of 0.36, 0.40, 0.44, and 0.48 show the interfacial tensions of 2.9, 3.4, 3.7, and 3.9 mN/m, respectively. We determine the minimum diameter of the oil drops for insertion of water drop using these four different oil phases, as shown in Fig. 3b. As the volume fraction of chloroform increases from 0.40 to 0.44 and 0.48, the minimum diameter increases from 278 lm to 298 lm and 308 lm, where the total flow rate is maintained at constant value
Fig. 4. (a) Schematic illustration of the dewetting-induced formation of bilayer membrane which wraps the innermost drops, resulting in polymersomes. (b and c) Optical microscope images of (b) monodisperse double-emulsion drops immediately taken after collection and (c) subsequent formation of polymersomes from the double-emulsion drops in 5 min. (d and e) Confocal microscope images of polymersomes containing red dyes in their interior. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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of 3900 ll/h; the critical Weber number is 3.9 for all three cases. Oil phase with volume fraction of chloroform as small as 0.36 does not allow the insertion of water drops for all diameters of oil drops we use; this is caused by a breakup of rear interfaces into small droplets due to small interfacial tension when they are elongated in the narrowing capillary, as shown in Fig. S3 of the Supporting Information.
the double-emulsion drops transform into polymersomes through the dewetting as denoted with arrows in Fig. 4c. The oil drops are spontaneously separated, resulting in polymersomes as shown in Fig. 4d and e; the resultant polymersomes contain red dye at high concentration, while the continuous phase has the dye at very low concentration due to dilution with collection liquid, 50 mM aqueous solution of NaCl.
3.3. Sampling of continuous phase with polymersomes
3.4. Insertion of water drop into double-emulsion drops
We can isolate the small volume of the continuous phase with this insertion approach, which is potentially useful for analysis in l-TAS applications. However, double-emulsion drops are unstable against coalescence of innermost drop to continuous phase, frequently resulting in release of the isolated volume before analysis is completed. To stabilize the structure, we create a membrane composed of bilayers of amphiphilic block-copolymers from shell phase of the double-emulsion drops; such vesicle structure is known as polymersome [23]. We use the middle oil phase of a mixture of chloroform and hexane, in a volume ratio of 40:60, containing 5 mg/ml PEG-b-PLA and 2.5 mg/ml PLA to form the membrane; the PLA homopolymers are added to enhance the stability of resultant polymersomes [12]. To confirm the encapsulation, we use the continuous phase containing red dye, sulforhodamine B. After double-emulsion drops are formed at the sudden expansion channel, they are collected in 50 mM aqueous solution of NaCl. As chloroform evaporates, the double-emulsion drops exhibit dewetting of middle phase on the surface of the innermost drop, forming a bilayer membrane composed of PEG-b-PLA whereas PLA homopolymers are incorporated in hydrophobic part of the bilayer, as shown schematically in Fig. 4a [13]. Optical microscope images of the double-emulsion drops taken immediately after collection and in 5 minutes are shown in Fig. 4b and c, respectively;
In the fashion similar to the insertion of water drop into oil drops, we can insert water drop into double-emulsion drops, as shown schematically in Fig. 5a. To do this, we prepare W/O/W double-emulsion drops in the first junction by injecting 10 wt% aqueous solution of PEG (Mw 6000) through the left cylindrical capillary at flow rate of 100 ll/h, while maintaining the injection of the oil and continuous phases at flow rates of 1500 and 2500 ll/h, respectively. The double-emulsion drops with small innermost drops are prepared in the first junction and then, they are injected through the narrowing capillary channel, followed by sudden expansion in the second junction, as shown in Fig. 5b; as they flow through the second junction, small water drop of continuous phase is inserted into the double-emulsion drops, thereby forming double-emulsion drops with two distinct innermost drops. This insertion process is shown in Supplementary movie 3. When the size of innermost drops which are prepared in the first junction is much larger than the tip of the tapered capillary, the drops block the narrowing channel and induce a breakup of the middle oil drop at the leading edge of the innermost drop, dividing the doubleemulsion drop into single oil drop and smaller double-emulsion drop as shown in Fig. S4; such division and breakup of doubleemulsion drops in the converging channels are reported by H. A. Stone and coworkers [24,25]. Therefore, small innermost drops
Fig. 5. (a) Schematic illustration of the microfluidic device for insertion of water drop into W/O/W double-emulsion drops, making double-emulsion drops with two distinct cores. (b) Optical microscope images showing insertion of water drop into double-emulsion drops. (c) Confocal microscope images of dumbbell-shaped polymersomes encapsulating two different materials by enclosing each material in its own internal shell, where green core contains 10% poly(ethylene glycol) and green dye from original innermost drop and red core contains 10% poly(vinyl alcohol) and red dye from inserted drop. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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relative to the tip are required to insert water drop into the doubleemulsion drops in the second junction. In this experiment, the innermost drops which are smaller than 2.4 dtip are allowed to pass without the breakup and to insert additional water drops. Using templates of the resultant double-emulsion drops with two distinct cores, we can produce dumbbell-shaped polymersomes encapsulating two different materials by enclosing each material in its own internal shell as shown in Fig. 5c [11,26], where green bulb contains 10% PEG and green dye, 8-hydroxyl-1,3,6-pyrenetrisulfonic acid trisodium salt, from the original innermost drop and red bulb contains 10% PVA and red dye, sulforhodamin B, from the inserted drop. The two distinct innermost drops are stabilized by amphiphiles as soon as they are generated, which prevents coalescence between the drops.
4. Conclusions We report capillary microfluidic geometry composed of a narrowing channel, followed by sudden expansion, for insertion of a drop of continuous phase into drops passing through the channel, enabling the controlled formation of double-emulsion drops. Drops injected through the channel experience dramatic increase of linear velocity in the narrowing channel, leading to deformation of the rear interface of the drops in the expansion channel; sufficient inertia force relative to capillary force can cause a breakup of the deformed interface into a drop which is confined in the injected drop, making drop-in-drops. Therefore, Weber number which depends on linear velocity of the rear interface and interfacial tension should be larger than certain value to achieve the insertion of drop; the linear velocity depends on volumetric flow rate and size of the injected drops. The diameter of the inserted drop is linearly proportional to that of the injected drops. By applying this insertion to double-emulsion drops, we can also produce double-emulsion drops with two distinct innermost drops. We demonstrate that the resultant double-emulsion drops can be transformed into polymersomes through formation of bilayer membranes from middle oil phase, thereby yielding stable vesicular structure which isolates the inner volume of aqueous solution from bulk continuous phase. This microfluidic approach provides new method to prepare double-emulsion drops. In particular, as a drop of continuous phase is inserted into injected drops, this approach will provide potentially useful tools for sampling of small amount of continuous phase and its isolation for applications of l-TAS.
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Acknowledgments This work was supported by the International Collaboration Grant (No. Sunjin-2010-002) and the Industrial strategic technology development program (No. 10045068) of Korea Evaluation Institute of Industrial Technology funded by the Ministry of Trade, Industry, & Energy (MI, Korea). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.020. References [1] Q.B. Xu, M. Hashimoto, T.T. Dang, T. Hoare, D.S. Kohane, G.M. Whitesides, R. Langer, D.G. Anderson, Small 5 (2009) 1575. [2] B. Comiskey, J.D. Albert, H. Yoshizawa, J. Jacobson, Nature 394 (1998) 253. [3] A.S. Utada, E. Lorenceau, D.R. Link, P.D. Kaplan, H.A. Stone, D.A. Weitz, Science 308 (2005) 537. [4] S.H. Kim, S.J. Jeon, S.M. Yang, J. Am. Chem. Soc. 130 (2008) 6040. [5] J.W. Kim, A.S. Utada, A. Fernandez-Nieves, Z.B. Hu, D.A. Weitz, Angew. Chem. Int. Edit. 46 (2007) 1819. [6] Z.H. Nie, S.Q. Xu, M. Seo, P.C. Lewis, E. Kumacheva, J. Am. Chem. Soc. 127 (2005) 8058. [7] M.H. Lee, K.C. Hribar, T. Brugarolas, N.P. Kamat, J.A. Burdick, D. Lee, Adv. Funct. Mater. 22 (2012) 131. [8] S.H. Kim, J.W. Kim, J.C. Cho, D.A. Weitz, Lab Chip 11 (2011) 3162. [9] S.W. Choi, Y. Zhang, Y.N. Xia, Adv. Funct. Mater. 19 (2009) 2943. [10] D. Lee, D.A. Weitz, Adv. Mater. 20 (2008) 3498. [11] A. Perro, C. Nicolet, J. Angy, S. Lecommandoux, J.F. Le Meins, A. Colin, Langmuir 27 (2011) 9034. [12] S.H. Kim, H.C. Shum, J.W. Kim, J.C. Cho, D.A. Weitz, J. Am. Chem. Soc. 133 (2011) 15165. [13] H.C. Shum, J.W. Kim, D.A. Weitz, J. Am. Chem. Soc. 130 (2008) 9543. [14] R.C. Hayward, A.S. Utada, N. Dan, D.A. Weitz, Langmuir 22 (2006) 4457. [15] S.H. Kim, J.W. Shim, S.M. Yang, Angew. Chem. Int. Edit. 50 (2011) 1171. [16] S.H. Kim, H. Hwang, C.H. Lim, J.W. Shim, S.M. Yang, Adv. Funct. Mater. 21 (2011) 1608. [17] L.Y. Chu, A.S. Utada, R.K. Shah, J.W. Kim, D.A. Weitz, Angew. Chem. Int. Edit. 46 (2007) 8970. [18] S. Okushima, T. Nisisako, T. Torii, T. Higuchi, Langmuir 20 (2004) 9905. [19] M.B. Romanowsky, A.R. Abate, A. Rotem, C. Holtze, D.A. Weitz, Lab Chip 12 (2012) 802. [20] S.H. Kim, D.A. Weitz, Angew. Chem. Int. Edit. 50 (2011) 8731. [21] A.R. Abate, J. Thiele, D.A. Weitz, Lab Chip 11 (2011) 253. [22] P.-G. de Gennes, F. Brochard-Wyart, D. Quere, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves, Springer, 2004. [23] D.E. Discher, F. Ahmed, Annu. Rev. Biomed. Eng. 8 (2006) 323. [24] J.A. Li, H.S. Chen, H.A. Stone, Langmuir 27 (2011) 4324. [25] H.S. Chen, J.A. Li, H.C. Shum, H.A. Stone, D.A. Weitz, Soft Matter 7 (2011) 2345. [26] H.C. Shum, Y.J. Zhao, S.H. Kim, D.A. Weitz, Angew. Chem. Int. Edit. 50 (2011) 1648.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Controlled formation of double-emulsion drops in sudden expansion channels Shin-Hyun Kim ⇑, Bomi Kim Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 September 2013 Accepted 15 October 2013 Available online 26 October 2013 Keywords: Microfluidics Double emulsions Polymersomes Weber number Capillary
a b s t r a c t Double-emulsion drops or drops-in-drop have provided useful templates for production of microcapsules due to their core–shell geometry. Here, we introduce new capillary microfluidic geometry for the creation of double-emulsion drops, which is composed of a narrowing channel followed by sudden expansion channel. Drops injected through the narrowing channel are highly accelerated to flow, inducing high inertia force. When rear interface of the drops arrives at the sudden expansion channel, the high inertia force deforms the interface and leads to its breakup into a drop in the interior of the injected drop. This insertion is driven by inertia force against capillary force: High linear velocity and low interfacial tension facilitate the insertion. We also apply this emulsification method to double-emulsion drops with single innermost drop; insertion of a water drop creates the double-emulsion drops with two distinct innermost drops. The resultant double-emulsion drops with single- or double-innermost drops provide useful templates to produce polymersomes which encapsulate same fluid to the continuous phase; this will be potentially useful for sampling of the continuous phase and its isolation in a wide range of applications for micro-total analysis system (l-TAS). Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Emulsions have been used as templates to produce microparticles and capsules for a wide range of applications including drug delivery vehicles and active-display pigments [1,2]. Recently, double- or multiple-emulsion drops have been prepared to achieve advanced structural complexity and functionality [3]. Through selective solidification of middle phases, the emulsion drops are transformed into capsule structures with high stability; formation of a robust compartment isolating the innermost drop from the continuous phase enables long-term maintenance of their core– shell structure. For example, polymerization of monomers or prepolymers in the middle phase can produce crosslinked polymeric membranes which exhibit high mechanical stability and chemical resistance [4–6]. Evaporation of the middle phase, containing polymers or colloids, can also produce solid membranes which can be functionalized to release of encapsulants [7–10]. In addition, the double-emulsion drops can be used as templates to create capsules whose membranes are composed of molecular bilayer structures such as liposomes and polymersomes; by employing mixtures of two different organic solvents containing amphiphiles as middle phase, a bilayer membrane can be prepared through dewetting of the middle phase on the surface of the innermost drops [11–14]. ⇑ Corresponding author. Fax: +82 42 350 3910. E-mail address: [email protected] (S.-H. Kim). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.020
Recent advances in microfluidics have enabled the controlled formation of double- or multiple-emulsion drops. Microfluidic devices, made of either polydimethylsiloxane (PDMS) or glass capillaries, have been employed to provide unprecedented controllability in size and structure of the drops and high efficiency of encapsulation. This production of double-emulsion drops has been achieved in two distinct ways: Sequential emulsification and one-step emulsification. Sequential emulsification is accomplished by using two serial drop makers [4,15–18]; single-emulsion drops are produced in the first junction and then, they are encapsulated in the second level of emulsion drops in the second junction, resulting in drops-in-drops. This method provides high controllability in number of innermost drops. One-step emulsification produces double-emulsion drops through simultaneous breakup of two coaxial interfaces in one junction; this method provides higher controllability in size of both innermost and middle drops [3,19–21]. Both approaches require independent control of three distinct flows. In this paper, we report new microfluidic approach to create double-emulsion drops through insertion of a drop of continuous phase into single-emulsion drops. To accomplish this, we design a capillary microfluidic device which consists of a narrowing channel followed by sudden expansion. By flowing oil-in-water (O/W) drops through the channel, we can deform rear interface of the drop by high inertia force and insert water drop of continuous phase into the oil drop through a breakup of the deformed
S.-H. Kim, B. Kim / Journal of Colloid and Interface Science 415 (2014) 26–31
interface, thereby creating water-in-oil-in-water (W/O/W) doubleemulsion drops. This insertion of water drops occurs only for the oil drops larger than certain size which is determined by interfacial
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tension and flow rates for a given geometry; sufficient inertia force relative to capillary force is required to deform the rear interface and break into a drop. With this method, therefore, we can produce double-emulsion drops with independent control of two flows. In the similar fashion, water drop can be inserted into double-emulsion drops, making double-emulsion drops with two distinct innermost drops. The core–shell structures of the resultant double-emulsion drops can be stabilized by creating bilayer membranes of amphiphilic block-copolymers, thereby providing stable isolation of inner volume of aqueous phase; this is potentially useful for sampling of fluids to make further analysis. 2. Materials and methods 2.1. Device preparation
Fig. 1. (a) Schematic illustration of the microfluidic device composed of a series of two junctions for insertion of water drop into oil drops, making water-in-oil-inwater (W/O/W) double-emulsion drops. (b–d) Optical microscope images showing (b) flow of the oil drops through middle capillary, (c) insertion of water drop into each oil drop at the second junction, and (d) flow of the double-emulsion drops through collection capillary, where flow rates of oil and water phases, Q1 and Q2, are maintained at values of 1500 ll/h and 1800 ll/h, respectively.
We integrate two junctions into one capillary microfluidic device as shown schematically in Fig. 1a: One for making oil drops and the other for inserting a water drop into each oil drop. The design of the device comprises three tapered cylindrical capillaries, the left, middle, and right capillaries, which are inserted in two square capillaries; the middle cylindrical capillary is tapered at both sides, whereas the other two cylindrical capillaries are tapered only at one side. The left capillary is tapered to have a 20-lm-diameter orifice and treated with n-octadecyltrimethoxyl silane (Sigma–Aldrich) to render it hydrophobic; this left capillary is not used for production of single oil drops in the first junction, but used for production of double-emulsion drops. The left side of the middle capillary is tapered to have a 190-lm-diameter orifice, whereas the right side is tapered to have a 68-lm-diameter orifice. The orifice diameter of the right side of the middle capillary is carefully selected to make successful insertion; too small orifice experiences high flow resistance and frequently leads to a breakup of interfaces within the injection channel, while too large one imposes small inertia. The right capillary is carefully tapered to have a 306-lm-diameter orifice; too small orifice makes negligible relaxation of drops, while too large orifice induces very low velocity in the channel which can result in coalescence. The middle and right
Fig. 2. (a) Time-dependence of flow velocities of front (j) and rear (N) interfaces of drop flowing through a tapered capillary, followed by sudden expansion; average flow velocity (d), Q/A, is shown for comparison, where Q is total volumetric flow rates, Q1 + Q2, and A is cross-sectional area of the tapered capillary channel. (b–e) Optical microscope images showing deformation of the rear interface and subsequent breakup, inserting water drop into oil drop. Images are taken at denoted times in (a) and black and red arrows denote position of front and rear interfaces, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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capillaries are treated with 2-[methoxy(polyethyleneoxy)propyl] trimethoxy silane (Gelest, Inc.) to render them hydrophilic. The left capillary and the left side of the middle capillary are coaxially aligned in the first square capillary, whereas the right side of the middle capillary and the right capillary are coaxially aligned in the second square capillary. An image of device is shown in Fig. S1 in the Supporting Information. 2.2. Materials As an oil phase, we use a mixture of chloroform and hexane containing 5 mg/ml amphiphilic diblock-copolymer of poly(ethylene glycol) (PEG, Mw 5,000)-b-poly (lactic acid) (PLA, Mw 10,000) and 2.5 mg/ml hydrophobic homopolymer of PLA (Mw 15,000); volume ratio of chloroform in the mixture is controlled in a range of 0.36–0.48. As a continuous phase, we use 10 wt% aqueous solution of poly (vinyl alcohol) (PVA, Mw 13,000– 23,000); red dye molecules, sulforhodamine B, are dissolved into the continuous phase in some experiments. For production of double-emulsion drops in the first junction, we use 10 wt% aqueous solution of PEG (Mw 6000) containing green dye molecules, 8-hydroxyl-1,3,6-pyrenetrisulfonic acid trisodium salt, as an innermost phase.
tia force to viscous force, Re = qdtipurear/g (Reynolds number), is 3.5, where dtip is diameter of the tip, 68 lm and g is viscosity of continuous phase, 13.5 cP. The rear interface is highly deformed in the narrowing channel due to parabolic velocity profile along the cross-section of the channel, as shown in Fig. 2b. As the interface arrives at the expansion channel, the high inertia leads to deep indentation of the interfaces into the interior of the drop, while the front interface moves backward due to relaxation of the drop to spherical shape, as shown in Fig. 2c and d; negative linear velocity of the front interface during the relaxation is also confirmed in Fig. 2a. The sufficient inertia force at the rear interface leads to the formation of a drop from the rear indentation, inserting water drop into the oil drop and forming double-emulsion drops, as shown in Fig. 2e. Therefore, innermost water drop of the resultant doubleemulsion drops is exactly the same fluid to the continuous phase. By contrast, insufficient inertia force leads to only indentation which cannot transform into the water drops; capillary force, 2c/ dtip, prevents the breakup, thereby making the oil drops recover spherical shape in the right capillary, where c is interfacial tension. Therefore, ratio of the inertia force to the capillary force, We ¼ qu2rear dtip =2c (Weber number), is important parameter to insert the water drop into the oil drop [22]; the ratio is estimated
3. Results and discussion 3.1. Insertion of water drop into oil drops We inject the oil phase, the mixture of chloroform and hexane in a volume ratio of 40:60, through the interstices of the left cylindrical capillary and the first square capillary at volumetric flow rate, Q1, of 1500 ll/h, whereas the continuous phase through the interstices of the middle cylindrical capillary and the first square capillary at flow rate, Q2, of 1800 ll/h; syringe pumps are used for the injection. The other inlets are all closed. In the first junction, monodisperse oil drops with diameter of 275 lm are generated in a dripping mode and then flow through the middle capillary in a zigzag form as shown in Fig. 1b. As the drops pass through the second junction which is a narrowing channel, followed by sudden expansion, they transform into water-in-oil-in-water (W/O/W) double-emulsion drops as shown in Fig. 1c and the double-emulsion drops flow through the right capillary as shown in Fig. 1d. Each of these steps is shown in Supplementary movie 1. To study the insertion of small water drop into large oil drop, we record motion of the drops with high speed camera (Phantom V9) at 2000 frames per second. The displacement of the front and rear interfaces between two sequential frames is obtained by image analysis and their linear velocities are calculated by dividing the displacement with 1/2000 s as shown in Fig. 2a; still images taken at the times denoted in Fig. 2a are shown in Fig. 2b–e, where Q1 of 1300 ll/h and Q2 of 2800 ll/h are used. The oil drops flow through the small tapered capillary without contacting the inner wall due to its hydrophilic nature. During this injection of drops, the linear velocities of front and rear interfaces, ufront and urear, dramatically increase because cross-sectional area, A = pR2, of the narrowing channel decreases along the flow direction, where R is inner radius of the channel. In addition, because the oil drops, much larger than the orifice, block the channel before they pass through the small tip, pressure is accumulated; this further accelerates the drop migration. We can confirm this by comparison of linear velocity of the rear interfaces with average flow velocity, Q/A, as shown in Fig. 2a, where Q is total volumetric flow rate, Q1 + Q2. The acceleration of drop migration exerts high inertia force, qu2rear , on the rear interface of the drop due to high linear velocity, where q is density of water phase. At the tip, ratio of iner-
Fig. 3. (a) Influence of volumetric flow rate and diameter of oil drop on insertion of water drop, where oil phase contains chloroform in a volume fraction of 0.4. (b) Influence of interfacial tension and diameter of oil drop on insertion of water drop, where total flow rate is maintained at constant value of 3900 ll/h; volume fraction of chloroform in the oil phase determines the interfacial tension: The volume fractions of 0.36, 0.40, 0.44, and 0.48 correspond to 2.9, 3.4, 3.7, and 3.9 mN/m, respectively. In both plots, insertion of water drop is denoted with unfilled circles (s), whereas failure of the insertion is denoted with crosses ().
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as 5.0 for this insertion. We find that the Weber number should be larger than 4.6 for the insertion at this total flow rate of 4100 ll/h. The insertion of the water drop and its failure are shown in Supplementary movie 2. 3.2. Minimum diameter of oil drop for insertion of water drop Sufficient inertia force relative to capillary force can be achieved by increasing linear velocity or decreasing interfacial tension. The linear velocity of the rear interfaces depends on total flow rate and size of the oil drop; individual flow rates of Q1 and Q2 have insignificant effect in the linear velocity for the same total flow rate and larger drop gains higher accumulation of pressure in the narrowing channel, leading to higher linear velocity of the rear interface for the same total flow rate. Therefore, there is a minimum diameter of the oil drop to successfully insert water drop into it for a constant flow rate, as shown in Fig. 3a. At low total volumetric flow rate as 2000 ll/h, the minimum diameter is as large as 325 lm, where the critical Weber number is estimated as 1.9. As we increase the flow rate to 2600 and 3300 ll/h, the minimum diameter is reduced to 300 lm and 275 lm, respectively; the critical Weber numbers at the flow rate of 2600 and 3300 ll/h are estimated as 2.4 and 3.0, respectively. For further increase of flow rate, by contrast, the minimum diameter increases; for the flow rate of 4100 ll/h, the minimum diameter is 290 lm and the critical Weber number is 4.6 and for the flow rate of 5000 ll/h, the minimum diameter is 410 lm and the critical Weber number is 8.9. We attribute this increase of minimum diameter and critical Weber number to insufficient relaxation of front interface at high flow
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rate even for higher inertia force at rear interface. At such a high flow rate, the front interface moves forward with insignificant relaxation in the expansion channel, thereby requiring higher deformation for breakup of the rear interface. Diameter of the inserted water drops is linearly proportional to the diameter of the oil drops injected for all flow rates used for successful insertion, as shown in Fig. S2 of the Supporting Information; the relative diameter of the inserted drop to the injected drop is in a range of 0.39–0.46. This proves that the diameter of the injected drop significantly influences on the inertia force. Interfacial tension between the oil and continuous phases is determined by volume ratio of chloroform in the oil phase. Although the interfacial tension decreases as volume fraction of chloroform increases in the pure mixture of chloroform and hexane due to the relative hydrophilicity of chloroform, the tendency becomes opposite when the mixture contains amphiphilic blockcopolymers of PEG-b-PLA. Because chloroform is good solvent for PEG-b-PLA, while hexane is poor solvent, low volume fraction of chloroform causes strong absorption of the amphiphiles at the interface, reducing interfacial tension. We measure the interfacial tension between the oil and continuous phases using pendant drop methods; four different volume fractions of chloroform in the mixture of 0.36, 0.40, 0.44, and 0.48 show the interfacial tensions of 2.9, 3.4, 3.7, and 3.9 mN/m, respectively. We determine the minimum diameter of the oil drops for insertion of water drop using these four different oil phases, as shown in Fig. 3b. As the volume fraction of chloroform increases from 0.40 to 0.44 and 0.48, the minimum diameter increases from 278 lm to 298 lm and 308 lm, where the total flow rate is maintained at constant value
Fig. 4. (a) Schematic illustration of the dewetting-induced formation of bilayer membrane which wraps the innermost drops, resulting in polymersomes. (b and c) Optical microscope images of (b) monodisperse double-emulsion drops immediately taken after collection and (c) subsequent formation of polymersomes from the double-emulsion drops in 5 min. (d and e) Confocal microscope images of polymersomes containing red dyes in their interior. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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of 3900 ll/h; the critical Weber number is 3.9 for all three cases. Oil phase with volume fraction of chloroform as small as 0.36 does not allow the insertion of water drops for all diameters of oil drops we use; this is caused by a breakup of rear interfaces into small droplets due to small interfacial tension when they are elongated in the narrowing capillary, as shown in Fig. S3 of the Supporting Information.
the double-emulsion drops transform into polymersomes through the dewetting as denoted with arrows in Fig. 4c. The oil drops are spontaneously separated, resulting in polymersomes as shown in Fig. 4d and e; the resultant polymersomes contain red dye at high concentration, while the continuous phase has the dye at very low concentration due to dilution with collection liquid, 50 mM aqueous solution of NaCl.
3.3. Sampling of continuous phase with polymersomes
3.4. Insertion of water drop into double-emulsion drops
We can isolate the small volume of the continuous phase with this insertion approach, which is potentially useful for analysis in l-TAS applications. However, double-emulsion drops are unstable against coalescence of innermost drop to continuous phase, frequently resulting in release of the isolated volume before analysis is completed. To stabilize the structure, we create a membrane composed of bilayers of amphiphilic block-copolymers from shell phase of the double-emulsion drops; such vesicle structure is known as polymersome [23]. We use the middle oil phase of a mixture of chloroform and hexane, in a volume ratio of 40:60, containing 5 mg/ml PEG-b-PLA and 2.5 mg/ml PLA to form the membrane; the PLA homopolymers are added to enhance the stability of resultant polymersomes [12]. To confirm the encapsulation, we use the continuous phase containing red dye, sulforhodamine B. After double-emulsion drops are formed at the sudden expansion channel, they are collected in 50 mM aqueous solution of NaCl. As chloroform evaporates, the double-emulsion drops exhibit dewetting of middle phase on the surface of the innermost drop, forming a bilayer membrane composed of PEG-b-PLA whereas PLA homopolymers are incorporated in hydrophobic part of the bilayer, as shown schematically in Fig. 4a [13]. Optical microscope images of the double-emulsion drops taken immediately after collection and in 5 minutes are shown in Fig. 4b and c, respectively;
In the fashion similar to the insertion of water drop into oil drops, we can insert water drop into double-emulsion drops, as shown schematically in Fig. 5a. To do this, we prepare W/O/W double-emulsion drops in the first junction by injecting 10 wt% aqueous solution of PEG (Mw 6000) through the left cylindrical capillary at flow rate of 100 ll/h, while maintaining the injection of the oil and continuous phases at flow rates of 1500 and 2500 ll/h, respectively. The double-emulsion drops with small innermost drops are prepared in the first junction and then, they are injected through the narrowing capillary channel, followed by sudden expansion in the second junction, as shown in Fig. 5b; as they flow through the second junction, small water drop of continuous phase is inserted into the double-emulsion drops, thereby forming double-emulsion drops with two distinct innermost drops. This insertion process is shown in Supplementary movie 3. When the size of innermost drops which are prepared in the first junction is much larger than the tip of the tapered capillary, the drops block the narrowing channel and induce a breakup of the middle oil drop at the leading edge of the innermost drop, dividing the doubleemulsion drop into single oil drop and smaller double-emulsion drop as shown in Fig. S4; such division and breakup of doubleemulsion drops in the converging channels are reported by H. A. Stone and coworkers [24,25]. Therefore, small innermost drops
Fig. 5. (a) Schematic illustration of the microfluidic device for insertion of water drop into W/O/W double-emulsion drops, making double-emulsion drops with two distinct cores. (b) Optical microscope images showing insertion of water drop into double-emulsion drops. (c) Confocal microscope images of dumbbell-shaped polymersomes encapsulating two different materials by enclosing each material in its own internal shell, where green core contains 10% poly(ethylene glycol) and green dye from original innermost drop and red core contains 10% poly(vinyl alcohol) and red dye from inserted drop. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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relative to the tip are required to insert water drop into the doubleemulsion drops in the second junction. In this experiment, the innermost drops which are smaller than 2.4 dtip are allowed to pass without the breakup and to insert additional water drops. Using templates of the resultant double-emulsion drops with two distinct cores, we can produce dumbbell-shaped polymersomes encapsulating two different materials by enclosing each material in its own internal shell as shown in Fig. 5c [11,26], where green bulb contains 10% PEG and green dye, 8-hydroxyl-1,3,6-pyrenetrisulfonic acid trisodium salt, from the original innermost drop and red bulb contains 10% PVA and red dye, sulforhodamin B, from the inserted drop. The two distinct innermost drops are stabilized by amphiphiles as soon as they are generated, which prevents coalescence between the drops.
4. Conclusions We report capillary microfluidic geometry composed of a narrowing channel, followed by sudden expansion, for insertion of a drop of continuous phase into drops passing through the channel, enabling the controlled formation of double-emulsion drops. Drops injected through the channel experience dramatic increase of linear velocity in the narrowing channel, leading to deformation of the rear interface of the drops in the expansion channel; sufficient inertia force relative to capillary force can cause a breakup of the deformed interface into a drop which is confined in the injected drop, making drop-in-drops. Therefore, Weber number which depends on linear velocity of the rear interface and interfacial tension should be larger than certain value to achieve the insertion of drop; the linear velocity depends on volumetric flow rate and size of the injected drops. The diameter of the inserted drop is linearly proportional to that of the injected drops. By applying this insertion to double-emulsion drops, we can also produce double-emulsion drops with two distinct innermost drops. We demonstrate that the resultant double-emulsion drops can be transformed into polymersomes through formation of bilayer membranes from middle oil phase, thereby yielding stable vesicular structure which isolates the inner volume of aqueous solution from bulk continuous phase. This microfluidic approach provides new method to prepare double-emulsion drops. In particular, as a drop of continuous phase is inserted into injected drops, this approach will provide potentially useful tools for sampling of small amount of continuous phase and its isolation for applications of l-TAS.
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Acknowledgments This work was supported by the International Collaboration Grant (No. Sunjin-2010-002) and the Industrial strategic technology development program (No. 10045068) of Korea Evaluation Institute of Industrial Technology funded by the Ministry of Trade, Industry, & Energy (MI, Korea). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.020. References [1] Q.B. Xu, M. Hashimoto, T.T. Dang, T. Hoare, D.S. Kohane, G.M. Whitesides, R. Langer, D.G. Anderson, Small 5 (2009) 1575. [2] B. Comiskey, J.D. Albert, H. Yoshizawa, J. Jacobson, Nature 394 (1998) 253. [3] A.S. Utada, E. Lorenceau, D.R. Link, P.D. Kaplan, H.A. Stone, D.A. Weitz, Science 308 (2005) 537. [4] S.H. Kim, S.J. Jeon, S.M. Yang, J. Am. Chem. Soc. 130 (2008) 6040. [5] J.W. Kim, A.S. Utada, A. Fernandez-Nieves, Z.B. Hu, D.A. Weitz, Angew. Chem. Int. Edit. 46 (2007) 1819. [6] Z.H. Nie, S.Q. Xu, M. Seo, P.C. Lewis, E. Kumacheva, J. Am. Chem. Soc. 127 (2005) 8058. [7] M.H. Lee, K.C. Hribar, T. Brugarolas, N.P. Kamat, J.A. Burdick, D. Lee, Adv. Funct. Mater. 22 (2012) 131. [8] S.H. Kim, J.W. Kim, J.C. Cho, D.A. Weitz, Lab Chip 11 (2011) 3162. [9] S.W. Choi, Y. Zhang, Y.N. Xia, Adv. Funct. Mater. 19 (2009) 2943. [10] D. Lee, D.A. Weitz, Adv. Mater. 20 (2008) 3498. [11] A. Perro, C. Nicolet, J. Angy, S. Lecommandoux, J.F. Le Meins, A. Colin, Langmuir 27 (2011) 9034. [12] S.H. Kim, H.C. Shum, J.W. Kim, J.C. Cho, D.A. Weitz, J. Am. Chem. Soc. 133 (2011) 15165. [13] H.C. Shum, J.W. Kim, D.A. Weitz, J. Am. Chem. Soc. 130 (2008) 9543. [14] R.C. Hayward, A.S. Utada, N. Dan, D.A. Weitz, Langmuir 22 (2006) 4457. [15] S.H. Kim, J.W. Shim, S.M. Yang, Angew. Chem. Int. Edit. 50 (2011) 1171. [16] S.H. Kim, H. Hwang, C.H. Lim, J.W. Shim, S.M. Yang, Adv. Funct. Mater. 21 (2011) 1608. [17] L.Y. Chu, A.S. Utada, R.K. Shah, J.W. Kim, D.A. Weitz, Angew. Chem. Int. Edit. 46 (2007) 8970. [18] S. Okushima, T. Nisisako, T. Torii, T. Higuchi, Langmuir 20 (2004) 9905. [19] M.B. Romanowsky, A.R. Abate, A. Rotem, C. Holtze, D.A. Weitz, Lab Chip 12 (2012) 802. [20] S.H. Kim, D.A. Weitz, Angew. Chem. Int. Edit. 50 (2011) 8731. [21] A.R. Abate, J. Thiele, D.A. Weitz, Lab Chip 11 (2011) 253. [22] P.-G. de Gennes, F. Brochard-Wyart, D. Quere, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves, Springer, 2004. [23] D.E. Discher, F. Ahmed, Annu. Rev. Biomed. Eng. 8 (2006) 323. [24] J.A. Li, H.S. Chen, H.A. Stone, Langmuir 27 (2011) 4324. [25] H.S. Chen, J.A. Li, H.C. Shum, H.A. Stone, D.A. Weitz, Soft Matter 7 (2011) 2345. [26] H.C. Shum, Y.J. Zhao, S.H. Kim, D.A. Weitz, Angew. Chem. Int. Edit. 50 (2011) 1648.