Journal of Colloid and Interface Science 415 (2014) 26–31

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Journal of Colloid and Interface Science

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Controlled formation of double-emulsion drops in sudden expansionchannels

0021-9797/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2013.10.020

⇑ Corresponding author. Fax: +82 42 350 3910.E-mail address: kim.sh@kaist.ac.kr (S.-H. Kim).

Shin-Hyun Kim ⇑, Bomi KimDepartment 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 2013Accepted 15 October 2013Available online 26 October 2013

Keywords:MicrofluidicsDouble emulsionsPolymersomesWeber numberCapillary

a b s t r a c t

Double-emulsion drops or drops-in-drop have provided useful templates for production of microcapsulesdue to their core–shell geometry. Here, we introduce new capillary microfluidic geometry for the creationof double-emulsion drops, which is composed of a narrowing channel followed by sudden expansionchannel. Drops injected through the narrowing channel are highly accelerated to flow, inducing highinertia force. When rear interface of the drops arrives at the sudden expansion channel, the high inertiaforce deforms the interface and leads to its breakup into a drop in the interior of the injected drop. Thisinsertion is driven by inertia force against capillary force: High linear velocity and low interfacial tensionfacilitate the insertion. We also apply this emulsification method to double-emulsion drops with singleinnermost drop; insertion of a water drop creates the double-emulsion drops with two distinct innermostdrops. The resultant double-emulsion drops with single- or double-innermost drops provide usefultemplates to produce polymersomes which encapsulate same fluid to the continuous phase; this willbe potentially useful for sampling of the continuous phase and its isolation in a wide range of applicationsfor micro-total analysis system (l-TAS).

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Emulsions have been used as templates to produce microparti-cles and capsules for a wide range of applications including drugdelivery vehicles and active-display pigments [1,2]. Recently,double- or multiple-emulsion drops have been prepared to achieveadvanced structural complexity and functionality [3]. Throughselective solidification of middle phases, the emulsion drops aretransformed into capsule structures with high stability; formationof a robust compartment isolating the innermost drop from thecontinuous phase enables long-term maintenance of their core–shell structure. For example, polymerization of monomers or pre-polymers in the middle phase can produce crosslinked polymericmembranes which exhibit high mechanical stability and chemicalresistance [4–6]. Evaporation of the middle phase, containing poly-mers or colloids, can also produce solid membranes which can befunctionalized to release of encapsulants [7–10]. In addition, thedouble-emulsion drops can be used as templates to create capsuleswhose membranes are composed of molecular bilayer structuressuch as liposomes and polymersomes; by employing mixtures oftwo different organic solvents containing amphiphiles as middlephase, a bilayer membrane can be prepared through dewetting ofthe middle phase on the surface of the innermost drops [11–14].

Recent advances in microfluidics have enabled the controlledformation of double- or multiple-emulsion drops. Microfluidicdevices, made of either polydimethylsiloxane (PDMS) or glasscapillaries, have been employed to provide unprecedented control-lability in size and structure of the drops and high efficiency ofencapsulation. This production of double-emulsion drops has beenachieved in two distinct ways: Sequential emulsification andone-step emulsification. Sequential emulsification is accomplishedby using two serial drop makers [4,15–18]; single-emulsion dropsare produced in the first junction and then, they are encapsulatedin the second level of emulsion drops in the second junction,resulting in drops-in-drops. This method provides high controlla-bility in number of innermost drops. One-step emulsificationproduces double-emulsion drops through simultaneous breakupof two coaxial interfaces in one junction; this method provideshigher controllability in size of both innermost and middle drops[3,19–21]. Both approaches require independent control of threedistinct flows.

In this paper, we report new microfluidic approach to createdouble-emulsion drops through insertion of a drop of continuousphase into single-emulsion drops. To accomplish this, we designa capillary microfluidic device which consists of a narrowingchannel followed by sudden expansion. By flowing oil-in-water(O/W) drops through the channel, we can deform rear interfaceof the drop by high inertia force and insert water drop of continu-ous 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 27

interface, thereby creating water-in-oil-in-water (W/O/W) double-emulsion drops. This insertion of water drops occurs only for theoil drops larger than certain size which is determined by interfacial

Fig. 1. (a) Schematic illustration of the microfluidic device composed of a series oftwo junctions for insertion of water drop into oil drops, making water-in-oil-in-water (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 intoeach oil drop at the second junction, and (d) flow of the double-emulsion dropsthrough collection capillary, where flow rates of oil and water phases, Q1 and Q2, aremaintained at values of 1500 ll/h and 1800 ll/h, respectively.

Fig. 2. (a) Time-dependence of flow velocities of front (j) and rear (N) interfaces of drovelocity (d), Q/A, is shown for comparison, where Q is total volumetric flow rates, Q1

microscope images showing deformation of the rear interface and subsequent breakup, inand red arrows denote position of front and rear interfaces, respectively. (For interpretatversion of this article.)

tension and flow rates for a given geometry; sufficient inertia forcerelative to capillary force is required to deform the rear interfaceand break into a drop. With this method, therefore, we can producedouble-emulsion drops with independent control of two flows. Inthe similar fashion, water drop can be inserted into double-emul-sion drops, making double-emulsion drops with two distinctinnermost drops. The core–shell structures of the resultantdouble-emulsion drops can be stabilized by creating bilayer mem-branes of amphiphilic block-copolymers, thereby providing stableisolation of inner volume of aqueous phase; this is potentially use-ful for sampling of fluids to make further analysis.

2. Materials and methods

2.1. Device preparation

We integrate two junctions into one capillary microfluidic de-vice as shown schematically in Fig. 1a: One for making oil dropsand the other for inserting a water drop into each oil drop. The de-sign of the device comprises three tapered cylindrical capillaries,the left, middle, and right capillaries, which are inserted in twosquare capillaries; the middle cylindrical capillary is tapered atboth sides, whereas the other two cylindrical capillaries aretapered only at one side. The left capillary is tapered to have a20-lm-diameter orifice and treated with n-octadecyltrimethoxylsilane (Sigma–Aldrich) to render it hydrophobic; this left capillaryis not used for production of single oil drops in the first junction,but used for production of double-emulsion drops. The left sideof the middle capillary is tapered to have a 190-lm-diameter ori-fice, whereas the right side is tapered to have a 68-lm-diameterorifice. The orifice diameter of the right side of the middle capillaryis carefully selected to make successful insertion; too small orificeexperiences high flow resistance and frequently leads to a breakupof interfaces within the injection channel, while too large one im-poses small inertia. The right capillary is carefully tapered to have a306-lm-diameter orifice; too small orifice makes negligible relax-ation of drops, while too large orifice induces very low velocity inthe channel which can result in coalescence. The middle and right

p flowing through a tapered capillary, followed by sudden expansion; average flow+ Q2, and A is cross-sectional area of the tapered capillary channel. (b–e) Opticalserting water drop into oil drop. Images are taken at denoted times in (a) and black

ion of the references to color in this figure legend, the reader is referred to the web

28 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 leftcapillary and the left side of the middle capillary are coaxiallyaligned in the first square capillary, whereas the right side of themiddle capillary and the right capillary are coaxially aligned inthe second square capillary. An image of device is shown inFig. S1 in the Supporting Information.

2.2. Materials

As an oil phase, we use a mixture of chloroform and hexanecontaining 5 mg/ml amphiphilic diblock-copolymer of poly(ethyl-ene glycol) (PEG, Mw 5,000)-b-poly (lactic acid) (PLA, Mw10,000) and 2.5 mg/ml hydrophobic homopolymer of PLA (Mw15,000); volume ratio of chloroform in the mixture is controlledin 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 intothe continuous phase in some experiments. For production of dou-ble-emulsion drops in the first junction, we use 10 wt% aqueoussolution of PEG (Mw 6000) containing green dye molecules, 8-hy-droxyl-1,3,6-pyrenetrisulfonic acid trisodium salt, as an innermostphase.

Fig. 3. (a) Influence of volumetric flow rate and diameter of oil drop on insertion ofwater 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 fractionof chloroform in the oil phase determines the interfacial tension: The volumefractions 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 (�).

3. Results and discussion

3.1. Insertion of water drop into oil drops

We inject the oil phase, the mixture of chloroform and hexanein a volume ratio of 40:60, through the interstices of the left cylin-drical capillary and the first square capillary at volumetric flowrate, Q1, of 1500 ll/h, whereas the continuous phase through theinterstices of the middle cylindrical capillary and the first squarecapillary at flow rate, Q2, of 1800 ll/h; syringe pumps are usedfor the injection. The other inlets are all closed. In the first junction,monodisperse oil drops with diameter of 275 lm are generated ina dripping mode and then flow through the middle capillary in azigzag form as shown in Fig. 1b. As the drops pass through the sec-ond junction which is a narrowing channel, followed by suddenexpansion, they transform into water-in-oil-in-water (W/O/W)double-emulsion drops as shown in Fig. 1c and the double-emul-sion 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 (PhantomV9) at 2000 frames per second. The displacement of the frontand rear interfaces between two sequential frames is obtained byimage analysis and their linear velocities are calculated by dividingthe displacement with 1/2000 s as shown in Fig. 2a; still images ta-ken 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 flowthrough the small tapered capillary without contacting the innerwall due to its hydrophilic nature. During this injection of drops,the linear velocities of front and rear interfaces, ufront and urear, dra-matically increase because cross-sectional area, A = pR2, of the nar-rowing channel decreases along the flow direction, where R isinner radius of the channel. In addition, because the oil drops,much larger than the orifice, block the channel before they passthrough the small tip, pressure is accumulated; this further accel-erates the drop migration. We can confirm this by comparison oflinear 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 inertiaforce, qu2

rear , on the rear interface of the drop due to high linearvelocity, where q is density of water phase. At the tip, ratio of iner-

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 contin-uous phase, 13.5 cP. The rear interface is highly deformed in thenarrowing channel due to parabolic velocity profile along thecross-section of the channel, as shown in Fig. 2b. As the interfacearrives at the expansion channel, the high inertia leads to deepindentation of the interfaces into the interior of the drop, whilethe front interface moves backward due to relaxation of the dropto spherical shape, as shown in Fig. 2c and d; negative linear veloc-ity of the front interface during the relaxation is also confirmed inFig. 2a. The sufficient inertia force at the rear interface leads to theformation of a drop from the rear indentation, inserting water dropinto the oil drop and forming double-emulsion drops, as shown inFig. 2e. Therefore, innermost water drop of the resultant double-emulsion drops is exactly the same fluid to the continuous phase.By contrast, insufficient inertia force leads to only indentationwhich cannot transform into the water drops; capillary force, 2c/dtip, prevents the breakup, thereby making the oil drops recoverspherical shape in the right capillary, where c is interfacial tension.Therefore, ratio of the inertia force to the capillary force,We ¼ qu2

reardtip=2c (Weber number), is important parameter to in-sert the water drop into the oil drop [22]; the ratio is estimated

S.-H. Kim, B. Kim / Journal of Colloid and Interface Science 415 (2014) 26–31 29

as 5.0 for this insertion. We find that the Weber number should belarger 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 Supple-mentary movie 2.

3.2. Minimum diameter of oil drop for insertion of water drop

Sufficient inertia force relative to capillary force can be achievedby increasing linear velocity or decreasing interfacial tension. Thelinear velocity of the rear interfaces depends on total flow rateand size of the oil drop; individual flow rates of Q1 and Q2 haveinsignificant effect in the linear velocity for the same total flow rateand larger drop gains higher accumulation of pressure in the nar-rowing channel, leading to higher linear velocity of the rear inter-face for the same total flow rate. Therefore, there is a minimumdiameter of the oil drop to successfully insert water drop into itfor a constant flow rate, as shown in Fig. 3a. At low total volumetricflow rate as 2000 ll/h, the minimum diameter is as large as325 lm, where the critical Weber number is estimated as 1.9. Aswe increase the flow rate to 2600 and 3300 ll/h, the minimumdiameter is reduced to 300 lm and 275 lm, respectively; the crit-ical Weber numbers at the flow rate of 2600 and 3300 ll/h are esti-mated as 2.4 and 3.0, respectively. For further increase of flow rate,by contrast, the minimum diameter increases; for the flow rate of4100 ll/h, the minimum diameter is 290 lm and the critical We-ber number is 4.6 and for the flow rate of 5000 ll/h, the minimumdiameter is 410 lm and the critical Weber number is 8.9. Weattribute this increase of minimum diameter and critical Webernumber to insufficient relaxation of front interface at high flow

Fig. 4. (a) Schematic illustration of the dewetting-induced formation of bilayer membramicroscope images of (b) monodisperse double-emulsion drops immediately taken after cdrops in 5 min. (d and e) Confocal microscope images of polymersomes containing red dythe reader is referred to the web version of this article.)

rate even for higher inertia force at rear interface. At such a highflow rate, the front interface moves forward with insignificantrelaxation in the expansion channel, thereby requiring higherdeformation for breakup of the rear interface. Diameter of the in-serted water drops is linearly proportional to the diameter of theoil drops injected for all flow rates used for successful insertion,as shown in Fig. S2 of the Supporting Information; the relativediameter of the inserted drop to the injected drop is in a range of0.39–0.46. This proves that the diameter of the injected drop sig-nificantly influences on the inertia force.

Interfacial tension between the oil and continuous phases isdetermined by volume ratio of chloroform in the oil phase.Although the interfacial tension decreases as volume fraction ofchloroform increases in the pure mixture of chloroform and hex-ane due to the relative hydrophilicity of chloroform, the tendencybecomes opposite when the mixture contains amphiphilic block-copolymers of PEG-b-PLA. Because chloroform is good solvent forPEG-b-PLA, while hexane is poor solvent, low volume fraction ofchloroform causes strong absorption of the amphiphiles at theinterface, reducing interfacial tension. We measure the interfacialtension between the oil and continuous phases using pendant dropmethods; four different volume fractions of chloroform in the mix-ture of 0.36, 0.40, 0.44, and 0.48 show the interfacial tensions of2.9, 3.4, 3.7, and 3.9 mN/m, respectively. We determine the mini-mum diameter of the oil drops for insertion of water drop usingthese four different oil phases, as shown in Fig. 3b. As the volumefraction of chloroform increases from 0.40 to 0.44 and 0.48, theminimum diameter increases from 278 lm to 298 lm and308 lm, where the total flow rate is maintained at constant value

ne which wraps the innermost drops, resulting in polymersomes. (b and c) Opticalollection and (c) subsequent formation of polymersomes from the double-emulsion

es in their interior. (For interpretation of the references to color in this figure legend,

30 S.-H. Kim, B. Kim / Journal of Colloid and Interface Science 415 (2014) 26–31

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 doesnot allow the insertion of water drops for all diameters of oil dropswe use; this is caused by a breakup of rear interfaces into smalldroplets due to small interfacial tension when they are elongatedin the narrowing capillary, as shown in Fig. S3 of the SupportingInformation.

3.3. Sampling of continuous phase with polymersomes

We can isolate the small volume of the continuous phase withthis insertion approach, which is potentially useful for analysis inl-TAS applications. However, double-emulsion drops are unstableagainst coalescence of innermost drop to continuous phase, fre-quently resulting in release of the isolated volume before analysisis completed. To stabilize the structure, we create a membranecomposed of bilayers of amphiphilic block-copolymers from shellphase of the double-emulsion drops; such vesicle structure isknown as polymersome [23]. We use the middle oil phase of amixture of chloroform and hexane, in a volume ratio of 40:60, con-taining 5 mg/ml PEG-b-PLA and 2.5 mg/ml PLA to form the mem-brane; the PLA homopolymers are added to enhance the stabilityof resultant polymersomes [12]. To confirm the encapsulation,we use the continuous phase containing red dye, sulforhodamineB. After double-emulsion drops are formed at the sudden expan-sion channel, they are collected in 50 mM aqueous solution ofNaCl. As chloroform evaporates, the double-emulsion drops exhibitdewetting of middle phase on the surface of the innermost drop,forming a bilayer membrane composed of PEG-b-PLA whereasPLA homopolymers are incorporated in hydrophobic part of the bi-layer, as shown schematically in Fig. 4a [13]. Optical microscopeimages of the double-emulsion drops taken immediately after col-lection and in 5 minutes are shown in Fig. 4b and c, respectively;

Fig. 5. (a) Schematic illustration of the microfluidic device for insertion of water drop intcores. (b) Optical microscope images showing insertion of water drop into double-emencapsulating two different materials by enclosing each material in its own internal sheinnermost drop and red core contains 10% poly(vinyl alcohol) and red dye from insertedreferred to the web version of this article.)

the double-emulsion drops transform into polymersomes throughthe dewetting as denoted with arrows in Fig. 4c. The oil drops arespontaneously separated, resulting in polymersomes as shown inFig. 4d and e; the resultant polymersomes contain red dye at highconcentration, while the continuous phase has the dye at very lowconcentration due to dilution with collection liquid, 50 mM aque-ous solution of NaCl.

3.4. Insertion of water drop into double-emulsion drops

In the fashion similar to the insertion of water drop into oildrops, we can insert water drop into double-emulsion drops, asshown schematically in Fig. 5a. To do this, we prepare W/O/W dou-ble-emulsion drops in the first junction by injecting 10 wt% aque-ous solution of PEG (Mw 6000) through the left cylindricalcapillary at flow rate of 100 ll/h, while maintaining the injectionof the oil and continuous phases at flow rates of 1500 and2500 ll/h, respectively. The double-emulsion drops with smallinnermost drops are prepared in the first junction and then, theyare injected through the narrowing capillary channel, followedby sudden expansion in the second junction, as shown in Fig. 5b;as they flow through the second junction, small water drop of con-tinuous phase is inserted into the double-emulsion drops, therebyforming double-emulsion drops with two distinct innermost drops.This insertion process is shown in Supplementary movie 3. Whenthe size of innermost drops which are prepared in the first junctionis much larger than the tip of the tapered capillary, the drops blockthe narrowing channel and induce a breakup of the middle oil dropat the leading edge of the innermost drop, dividing the double-emulsion drop into single oil drop and smaller double-emulsiondrop as shown in Fig. S4; such division and breakup of double-emulsion drops in the converging channels are reported by H. A.Stone and coworkers [24,25]. Therefore, small innermost drops

o W/O/W double-emulsion drops, making double-emulsion drops with two distinctulsion drops. (c) Confocal microscope images of dumbbell-shaped polymersomesll, where green core contains 10% poly(ethylene glycol) and green dye from originaldrop. (For interpretation of the references to color in this figure legend, the reader is

S.-H. Kim, B. Kim / Journal of Colloid and Interface Science 415 (2014) 26–31 31

relative to the tip are required to insert water drop into the double-emulsion drops in the second junction. In this experiment, theinnermost drops which are smaller than 2.4 � dtip are allowed topass without the breakup and to insert additional water drops.Using templates of the resultant double-emulsion drops with twodistinct cores, we can produce dumbbell-shaped polymersomesencapsulating two different materials by enclosing each materialin its own internal shell as shown in Fig. 5c [11,26], where greenbulb contains 10% PEG and green dye, 8-hydroxyl-1,3,6-pyrenetri-sulfonic acid trisodium salt, from the original innermost drop andred bulb contains 10% PVA and red dye, sulforhodamin B, from theinserted drop. The two distinct innermost drops are stabilized byamphiphiles as soon as they are generated, which prevents coales-cence between the drops.

4. Conclusions

We report capillary microfluidic geometry composed of a nar-rowing channel, followed by sudden expansion, for insertion of adrop of continuous phase into drops passing through the channel,enabling the controlled formation of double-emulsion drops. Dropsinjected through the channel experience dramatic increase of lin-ear velocity in the narrowing channel, leading to deformation ofthe rear interface of the drops in the expansion channel; sufficientinertia force relative to capillary force can cause a breakup of thedeformed interface into a drop which is confined in the injecteddrop, making drop-in-drops. Therefore, Weber number which de-pends on linear velocity of the rear interface and interfacial tensionshould be larger than certain value to achieve the insertion of drop;the linear velocity depends on volumetric flow rate and size of theinjected drops. The diameter of the inserted drop is linearly pro-portional to that of the injected drops. By applying this insertionto double-emulsion drops, we can also produce double-emulsiondrops with two distinct innermost drops. We demonstrate thatthe resultant double-emulsion drops can be transformed into poly-mersomes through formation of bilayer membranes from middleoil phase, thereby yielding stable vesicular structure which isolatesthe inner volume of aqueous solution from bulk continuous phase.This microfluidic approach provides new method to prepare dou-ble-emulsion drops. In particular, as a drop of continuous phaseis inserted into injected drops, this approach will provide poten-tially useful tools for sampling of small amount of continuousphase and its isolation for applications of l-TAS.

Acknowledgments

This work was supported by the International CollaborationGrant (No. Sunjin-2010-002) and the Industrial strategic technol-ogy development program (No. 10045068) of Korea EvaluationInstitute 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, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.020.

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