pubs.acs.org/Langmuir © 2009 American Chemical Society
Monodisperse Colloids Synthesized with Nanofluidic Technology Florent Malloggi, Nicolas Pannacci, Rafa€ele Attia, Fabrice Monti, Pascaline Mary, Herve Willaime, and Patrick Tabeling* MMN, Gulliver, ESPCI 10 rue Vauquelin, 75005 Paris, France
Bernard Cabane PMMH, ESPCI 10 rue Vauquelin, 75005 Paris, France
Pascal Poncet Institut Pasteur, LECA, ESPCI, 10 rue Vauquelin, 75005 Paris, France Received July 30, 2009. Revised Manuscript Received September 30, 2009 Limitations in the methods employed to generate micrometric colloidal droplets hinder the emergence of key applications in the fields of material science and drug delivery. Through the use of dedicated nanofluidic devices and by taking advantage of an original physical effect called capillary focusing, we could circumvent some of these limitations. The nanofluidic (i.e., submicrometric) devices introduced herein are made of soft materials, and their fabrication relies upon rapid technologies. The objects that we have generated are simple droplets, multiple droplets, particles, and Janus particles whose sizes lie between 900 nm and 3 μm (i.e., within the colloidal range). Colloidal droplets have been assembled on-chip into clusters and crystals, yielding discrete diffraction patterns. We illustrate potential applications in the field of drug delivery by demonstrating the ability of multiple droplets to be phagocytosed by murine macrophage-type cells.
1. Introduction Colloids are discrete particles undergoing Brownian motion. Their sizes range from a few nanometers to 3 μm. Their large surface areas and small dimensions, combined with the wide variety of volume and surface properties they offer, have generated an impressively high number of applications over the past several decades. At present, many reviews and textbooks are available on the subject.1,2 In this particular class of objects, micrometric liquid colloids (which can either gel or harden to become solid particles) play an important role in a number of fields, such as diagnostics,3 drug research,4 materials science,5 the food industry,6 vaccinations,7 drug delivery,8,9 and condensed matter physics.10 Micrometric colloidal droplets are currently produced by means of ultrasound application,11 high-pressure homogenization,12 high-shear Couette *Corresponding author. E-mail:
[email protected]. (1) An example is Hunter, R. J. Foundations of Colloid Science; Cambridge University Press: New York, 1989. (2) See, for example, the review by Xia, Y.; Gates, B.; Yin, Y.; Lu,Y. Adv. Mater. 2000,12,10. (3) Brigger, I.; Dubernet, C.; Couvreur, P. Adv. Drug Delivery Rev. 2002, 54, 631. (4) Battersby, B. J.; Lawrie, G. A.; Johnston, A. P.; Trau, M. Chem. Commun. 2002, 1435–1441. (5) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Science: San Diego, CA, 1990. (6) Loveday, S. M.; Singh, H. Trends Food Sci. Technol. 2008, 19, 657. (7) Coombes, A. G.; Tasker, S.; Lindbladb, M.; Holmgrenb, J.; Hostec, K.; Tonchevac, V.; Schachtc, E.; Daviesa, M. C.; Illuma, L.; Davis, S. S. Biomaterials 1997, 18, 1153. (8) Couvreur, P.; Dubernet, C.; Puisieux, F. Eur. J. Pharmaceut. Biopharmaceut. 1995, 41, 2. (9) Muller, R. H. Colloidal Carriers for Controlled Drug Delivery and Targeting; CRC Press: Boca Raton, FL, 1991. (10) Dhont, J. K. An Introduction to Dynamics of Colloids; Elsevier: Amsterdam, 1996. (11) Landfester, K. Macromol. Rapid Commun. 2001, 22, 12. (12) Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109. (13) Mason, R. G.; Bibette, J. Langmuir 1997, 13, 4600. (14) Joscelyne, S.; Tragardh, G. J. Membr. Sci. 2000, 169, 107.
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flow,13 and membrane emulsification.14 With these techniques, the widths of size distributions prove to be substantial (unless special formulations are used), and the production conditions for more complex structures (such as double droplets) are poorly controlled. These limitations prevent the use of colloids in applications where precise sizes or complex structures are required. One key example is the synthesis of materials based on colloidal self-assembly.15,16 For such applications, the homogeneity and morphology of the building blocks being assembled become critical to obtaining materials with attractive properties. In this area of study and despite the considerable efforts undertaken, the synthesis of unidirectional photonic band gap materials,17 capable of producing perfect mirrors, efficient sensors, and innovative types of optical sensors,18 remains a tough challenge. Other examples include colloidal lithography,19 an emerging technology dedicated to building 2D and 3D objects, and drug delivery, according to which the synthesis of sophisticated colloids possessing several functionalities would reduce the side effects of current drug administration methods.9 Still further examples, which depend on large colloids with geometric control and substantial complexity, can be found in the fields of printing,20 condensed matter physics,21 biophysics,22 and biomaterials.23 (15) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (16) Manoharan, V. N.; Pine, D. J. MRS Bull. 2004, 91. (17) Subramanian, G.; Manoharan, V. N.; Thorne, J. D.; Pine, D. J. Adv. Mater. 1999, 11, 1261. (18) Soukoulis, C. M., Ed. Photonic Band Gap Materials; NATO ASI Series; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996. (19) Yang, S. M.; Jang, S. G.; Choi, D. G.; Kim, S.; Yu, H. K. Small 2006, 2, 458. (20) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. Nature 1998, 394 (), 253–255. (21) Bianchi, E.; Largo, J.; Tartaglia, P.; Zaccarelli, E.; Sciortino, F. Phys. Rev. Lett. 2006, 97, 168301. (22) Stachowiak, J. C.; Richmond, D. L.; Li, T. H.; Brochard-Wyart, F.; Fletcher, D. A. Lab Chip 2009, 9, 2003. (23) Jang, J.; Dendukuri, D.; Hatton, T. A.; Thomas, E. L.; Doyle, P. Angew. Chem., Int. Ed. 2007, 46, 1.
Published on Web 11/13/2009
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Devising a technique that would generate micrometric colloidal droplets, weakly dependent on the formulation, with substantial throughput and fine control of size and a variety of morphologies, is challenging. With microfluidic technology, monodisperse simple and multiple droplets are currently synthesized under excellent control.24-26 However, the sizes of these objects are tens of micrometers in diameter (i.e., well above the colloidal range). Generating colloidal droplets in standard microfluidic channels (i.e., tens of micrometers large) was recently demonstrated. The idea consisted of collecting small satellites during droplet formation in T junctions27 or collecting tiny droplets generated by tip streaming in focusing geometry.28 However elegant these approaches can be, they rely on particular physicochemical conditions and lead to modest monodispersities and extremely small throughputs. A more straightforward approach consists of using miniaturized systems whose dimensions are comparable to the objects that we wish to synthesize. This leads to operations with nanofluidic systems (i.e., systems incorporating submicrometric channels). Research along this line has been carried out by Kobayashi et al.,29 who succeeded in generating simple colloidal droplets in parallel under largethroughput conditions and without significant formulation constraints; nonetheless, the droplet size distributions that were reported ranged between 10 and 20%, which is not compatible with most of the applications mentioned above.
2. Experimental Section In the present article, by using novel geometries, we succeeded in generating monodisperse colloidal droplets with controlled sizes and various morphologies under large throughput conditions, practically independently of the formulation. The smallest droplets that we synthesize here are around 1 μm in diameter. They are typically generated at extremely high frequencies (up to 15 kHz) and have excellent monodispersity (1% approximately). We can synthesize simple droplets as well as multiple emulsion droplets and Janus droplets with sizes pertaining to the colloidal domain and possessing excellent monodispersity. In our approach, we used soft materials (poly(dimethylsiloxane)) and rapid technologies. We illustrate the potential of our approach in the domains of selfassembly and drug delivery. However, the applications that can be derived from our work are not restricted to these fields. The experimental system that we used is sketched in Figure 1. The devices, made with standard PDMS-based photolithography, include a nanofluidic (i.e., submicrometric) section that comprises a cross junction and a terrace. In this part, the channel depth varies between 420 nm and 1 μm. The inlets of the nanochannel section are U-turn microchannels (rivers), 10 μm in height and between 40 and 100 μm wide, along which pressure differences are imposed (Figure 1A). Small fractions of the flows driven along these rivers are driven into the nanofluidic section. The design of the inlet channels allows us to impose well-controlled pressures at the nanofluidic section entries along with avoiding dust accumulation, which unavoidably clogs the system. In the nanofluidic part, the dispersed phase (yellow) meets with the continuous phase (blue) at the cross junction (Figure 1A). Beyond this junction, for the regimes considered herein, the two fluids flow side by side through a straight, shallow nanochannel (the “terrace”). Over the terrace, the interface between the two fluids takes the form of tongues that thin out in the direction of stream flow. Downstream from the nanofluidic section, there is (24) Squires, T. M.; Quake, S. R. Rev. Mod. Phys. 2005, 77, 977. (25) Tabeling, P. An Introduction to Microfluidics; Oxford University Press; Oxford, U.K., 2005. (26) Shaha, R. K.; Shuma, H. C.; Rowata, A. C.; Leea, D.; Agrestia, J. J.; Utada, A. S.; Chua, L. Y.; Kima, J-W; Fernandez-Nievesa, A.; Martineza, C. J.; Weitz, D. A. Mater. Today 2008, 11, 19. (27) Tan, Y.; Lee, A. P. Lab Chip 2005, 5, 1178–1183. (28) Suryo, R.; Basaran, O. A. Phys. Fluids 2006, 18, 082102. (29) Kobayashi, I.; Uemura, K.; Nakajima, M. Colloids Surf., A 2007, 296, 285.
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Figure 1. (A) Sketch of the experiment showing the dispersed (yellow) and continuous phases (blue) flowing through a nanofluidic channel (terrace) and forming droplets as they arrive at the reservoir. The immiscible fluids are driven into U-turn microchannels (the “rivers”) under fixed pressure differences (where w and o stand for water and oil, respectively). Small fractions of the flows driven along the rivers are directed into the nanochannels. Then they meet in a terrace and flow into a reservoir, generating droplets. In the terrace, the fluids form a tongue with a sharp tip. This is the capillary focusing effect that we discuss in the article and whose theory is given in Supporting Information. This self-focusing effect, not described in the literature yet, enables the controlled generation of monodisperse micrometric droplets. Typical dimensions of the experiment are H = 10 μm, L = 5000 μm, W = 40-100 μm, h = 420 nm-1 μm, L = 50 μm, and W = 5-10 μm. (B) Bright-field pictures of individual droplets generated in the nanofluidic device of different terrace heights h. Diameter of the fluorinated oil droplet from left to right: d = 18 μm-3.6 μm2 μm-900 nm. a microfluidic reservoir, 10 μm in height and between 40 and 100 μm wide. At the step marking the physical frontier between the terrace and the reservoir, the tongue tip becomes unstable, generating droplets that are then conveyed further downstream via the mean flow.30 The emission process is stationary. As shown in Figures 1B and 2A, the droplet sizes obtained with such a device vary between 2 and 3 times the nanochannel depth h over a broad range of flow conditions. The droplet sizes are therefore primarily controlled by the nanochannel depth. By decreasing the nanochannel depth to 420 nm, we are able to produce droplets with diameters as small as 900 nm. The droplet emission frequencies are shown in Figure 2B. We observe that they increase as the terrace height decreases. An important feature is that within the colloidal range, droplets are emitted at frequencies between 5 and 15 kHz (i.e., under high-throughput conditions). To create more complex structures, such as Janus colloidal droplets or colloidal double emulsions, we applied the geometry of Figure 1A, yet under modified fluid entry conditions (Supporting Information). Throughout this research, pressures have ranged between 0.5 and 5 bar, and flow rates have ranged between 1 and 10 μL/min.
3. Results and Discussion Here, we describe the physical origin of the shape of the boundary that separates the two fluids as they flow over the terrace. The physical mechanism at work is the following: as the fluids (30) This is not the only mode of generation of the droplets. We also have situations where droplets are formed at the junction in a way similar to the hydrodynamic focusing or in the jetting regimes described in the microfluidic literature. However, the domains of existence of such regimes are narrow, and they do not deliver high throughput. We thus focused on the regimes where droplets are produced at the step.
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Figure 2. (A) Evolution of the droplet size d as a function of the nanochannel height h in regimes where they are generated at the tongue tips. (B) Emission frequencies of droplets in regimes where they are generated at the tongue tips. (C) Evolution of the relative width of the tongue z = δ/β w as a function of the capillary number Ca. (See eq 2 or Supporting Information for the definitions of these quantities.) The symbols correspond to experimental data, and solid lines correspond to the model (in which we impose l0 = 0.5w, where w is the channel width). The two solid lines correspond to two extreme values (0.01 and 0.65) of the parameter β introduced in the theory (see the text) and achieved in the experiment. Several channel heights h and viscosities μ were used to explore different ranges of the capillary number. (D) Size distribution of a population of 5000 droplets, d = 3 μm in diameter, showing a coefficient of variation CV (i.e., the quantity 100 σ/d) of ∼1%, where σ is the standard deviation and d is the average droplet apparent optical diameter obtained with image processing. Numerical simulations indicate that the optically defined diameter approaches (within (10%) the geometrical diameter.
flow side by side above the terrace (Figure 1A), they are separated by a meniscus that induces a (cross-stream) pressure difference between the two fluids. This effect is particularly acute in nanofluidic channels where meniscus curvatures are large. However, the two fluids are forced to balance their pressures when arriving in the reservoir. To satisfy these two constraints, the inner fluid must move increasingly faster while the outer fluid moves more slowly over the terrace. As a result of mass conservation, this phenomenon forces the boundary between the two fluids to assume a tonguelike shape that thins out along the mean flow. We propose calling this effect capillary focusing to distinguish it from the hydrodynamic focusing effect, which is well known in the microfluidic literature31 and is driven by hydrodynamic conservation constraints. We have developed a quantitative theory for this capillary focusing effect (Supporting Information); the theory leads to the following formula for a tongue width δ at the step qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi w 1 þ Ca - ð1þCaÞ2 -4Caβ δ ¼ 2
ð1Þ
where Ca is defined by Ca ¼
6μ2 U2¥ l0 6μ1 U1¥ l0 ¼ γh γh
ð2Þ
(31) Knight, J. B.; Vishwanath, A.; Brody, J. P.; Austin, R. H. Phys. Rev. Lett. 1998, 80, 173863.
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with μι and Ui¥ being the fluid viscosity and upstream velocity, respectively, of the inner (i = 1) and outer (i = 2) fluids, l0 is a characteristic length comparable to the nanochannel width w, γ is the interfacial tension, h is the nanochannel height, and β is the ratio of the upstream widths occupied by fluids 1 and 2. This theory assumes shallow terraces, applies Laplace’s law, and estimates pressures within the various fluid regions using the Hele-Shaw approximation. Figure 2C shows a comparison between theoretical and experimental findings for various channel heights and a range of flow conditions. The two theoretical lines correspond to the extreme values of β that are achieved experimentally (i.e., 0.01 and 0.65). In the theory, l0 is a characteristic length that is expected to be on the order of the terrace width w and that we set arbitrarily equal to 0.5w. As shown in Figure 2C, good agreement has been found between theory and experiment. In practice, the capillary focusing effect has only a small amplitude in the microfluidic channels currently used and becomes particularly significant in nanofluidic devices, which explains why this effect has not yet been reported in the microfluidic literature. The present study indicates that by decreasing the nanochannel height h the tongues tend to develop sharp tips at the step (eq 11 or Supporting Information). In real systems, the sharpness of the tip (obtained by using the Hele-Shaw approximation) is probably limited by 3D effects. This theoretical study provides elements for discussing the evolution of the droplet size with h, as displayed in Figure 2B. At small capillary numbers, the tips of the tongues form 3D regions whose heights and (presumably) widths are on the order of the channel height h. One may consider that this fluid structure forms a nozzle from which droplets are generated through the Rayleigh-Plateau instability. If this picture is acceptable, then the droplets that are generated will have diameters proportional to the nozzle size (i.e., the terrace height), as observed in Figure 2B. Through the use of our nanofluidic device under the conditions described above, we have succeeded in synthesizing droplets with diameters lying in the range of 0.9-4 μm and larger (Figure 1B). Size dispersivity was measured by first imaging the droplets with a fast camera several milliseconds after their creation and then processing the images. We obtained sharply peaked distributions (Figure 2D). Under stationary emission conditions, diameter monodispersity was found to be on the order of 1%. Our technological approach is a versatile one and allows for synthesizing colloidal particles by means of photocuring droplets composed of photosensitive monomers. An example of such particles, with a diameter of 1 μm, is shown in Figure 3A. We are also able to generate more complex structures, such as monodisperse Janus and multiple droplets with external diameters lying in the range of 2-4 μm (Figure 3B-D). We emphasize here that thus far, to the best of our knowledge, it has not yet been possible to synthesize liquid monodisperse Janus and multiple emulsions of such sizes. We have succeeded in assembling simple droplets into clusters directly within the device. To form clusters, we used an instability that breaks the periodicity of the droplet train formed at the step, leading to local aggregation.32 By exploiting this instability, we have been able to obtain the assemblies depicted in Figure 3E, which correspond well to theoretical models33 (Figure 3E). With the present experimental system, it has not been possible, however, to obtain monodisperse populations of such clusters. Obtaining homogeneous populations of clusters would probably require the addition of a functionality to the chip. This is a (32) Beatus, T.; Tlusty, T.; Bar-Ziv, R. Nat. Phys. 2006, 2, 743.
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Figure 3. Several colloidal objects synthesized in the nanofluidic device. (A) SEM pictures of an Esacure 10%-TPGDA particle; (B) a Janus droplet (silicon oil-rapeseed oil/water þ sodium dodecyl sulfate 1 wt %); (C) a double droplet with complete engulfing (silicon oil-fluorinated oil/water þ SDS 1 wt %); (D) multiple emulsion (silicon oil-fluorinated oil/water þ SDS 1 wt %); and (E) elementary clusters produced in the system and theoretical models.33 Figure 5. Colloidal droplets generated in our nanofluidic chip for vectorizing various entities into macrophages. (A) Bright and (B) confocal images of macrophages with phagocytosed fluorinated oil droplets 4 μm in diameter and vacuoles. (C) Macrophage with phagocytosed Janus droplets. (D) Macrophage with two phagocytosed double droplets (diameter 4 μm) in which tens of submicrometric aqueous droplets filled with fluorescein undergo Brownian motion. (See the video in Supporting Information.)
Figure 4. Range of large assemblies obtained in the nanofluidic device: (A) liquid state (fluorinated oil/water þ SDS 1 wt %); (B) ordered phase (fluorinated oil/water þ SDS 1 wt %); (C) crystal of complete double emulsions (silicon oil-fluorinated oil/water þ SDS 1 wt %); and (D) Janus crystal (fluorinated oil-rapeseed oil/ water þ Pluronic 1 wt %). To the right of each image, we have indicated the corresponding diffraction pattern derived under normal incidence with a green laser (λ = 532 nm), where the zeroth order has been removed. The scale bar corresponds to 5 μm. We also show close-up views of the various patterns.
difficult challenge that nonetheless remains within the reach of the technology that we used. Large populations of colloids (simple, Janus, or double droplets) self-organize into patterns in a manner similar to that described in refs 34-36. Self-assembly takes place directly in the colloidal jet that the droplets form after they are generated. The droplet patterns that we obtain depend, as expected, on the volumetric fraction of the dispersed phase. In our system, owing to mass conservation, the volumetric fraction of the dispersed phase is controlled by the flow rate ratio of incoming phases and therefore can be easily varied. Figure 4A,B shows populations in the millions of colloidal droplets stored in the reservoir for two different volumetric fractions of the dispersed phase. For a volumetric fraction of j = 0.5, we obtained a liquid state (Figure 4A), and a volumetric fraction of around j = 0.74 yielded a hexagonal crystal structure (see the diffraction pattern in Figure 4B) corresponding to the close packing of monodisperse spheres, which is in agreement with the literature on colloids. (33) Sloane, N. J. A.; Hardin, R. H.; Duff, T. D. S.; Conway, J. H. Discrete Comput. Geom. 2003, 14, 237. (34) Hoogenboom, J. P.; Retif, C.; de Bres, E.; Van de Boer, M.; van LangenSuurling, A. K.; Romijn, J.; van Blaaderen, A. Nano Lett. 2004, 4, 205. (35) Priest, C.; Herminghaus, S.; Seemann, R. Appl. Phys. Lett. 2006, 88, 24106. (36) Shui, L.; Kooij, E. S.; Wijnperle, D.; van den Berg, A.; Eijkel, J. C. T. Soft Matter 2009, 5, 2708.
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Ordered patterns were also found to contain doubly encapsulated and Janus droplets (Figure 4C,D). The colored images on the right-hand side of Figure 4 represent the diffraction patterns that we generated by illuminating droplet assemblies stored in the reservoir under normal incidence and with coherent light beams. As expected, a ring is obtained in the colloidal liquid case and discrete peaks are obtained for colloidal solids. These diffraction patterns illustrate the fact that the structures generated in our nanofluidic device are monodisperse with sizes around the visible range. We found that the formation of a large crystal, containing millions of colloidal droplets, occurs extremely quickly (i.e., within a matter of seconds). The high speed at which ordered structures form is favored by both the fast droplet emission rate (several kHz) and seemingly by the flow pattern that develops close to the step. Being apparently controlled by the hydrodynamics, one may perhaps suggest that the self-assembly process occurs at times on the order of advection times (i.e., a few seconds), consistent with the observation. The analysis of the process nonetheless remains to be done. In any case, the fast formation rate that we achieve here opens interesting opportunities for material synthesis. Because the micrometric droplets generated with our nanofluidic technology are able to host all sorts of objects, they can be used as a versatile vessel for vectorizing the entities inside cells. We have illustrated this feature in Figure 5 by means of incubating droplets from various structures with murine macrophage-type cell line RAW 264.7.37,38 Experiments were conducted using simple droplets, multiple droplets, and Janus droplets produced within our nanofluidic device. This process requires three steps: in the first few hours, droplets come into contact with cell membranes under the action of Brownian motion and adhere to the membranes. During a second step, cells engulf these droplets in the cytoplasm. Eventually, after a day or two, lysosomal enzymes digest the droplets, leaving vacuoles inside the cell. Exocytosis (i.e. the release of droplets containing phagosomes) might also occur. (37) Ralph, P.; Nakoinz, I. J. Immunol. 1977, 119, 950–954. (38) Aderem A.; Underhill D. M. Annu. Rev. Immunol. 1999, 17, 593-623.
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Figure 5A,B shows fluorinated droplets, 4 μm in diameter, a few hours after their endocytosis by the macrophage. Figure 5C,D demonstrates that complex objects, such as multiple emulsions including fluorescent submicrometric droplets and droplets with two separate compartments (Janus), are being vectorized into the macrophages. Supporting Information C reveals that the tiny fluorescent droplets in Figure 5D undergo Brownian motion inside the cell until being digested by enzymes as a result of the fusion of lysosomes and phagosomes. This series of experiments thus illustrates that droplets with complex structures may be internalized into cells. As an application of this technology, one may imagine, for example, synthesizing vessels with various compartments, one carrying specific ligands to enhance or target delivery to a number of specific cells and the other transporting active molecules (drug, toxin, antigen, genetic information, etc.) for the therapeutic treatment of a pathophysiological status or vaccination.
4. Conclusions We have demonstrated that by using nanofluidic technology (i.e., systems including submicrometric channels) and exploiting an original self-focusing effect it is possible to generate simple and multiple droplets, particles, Janus droplets, and capsules whose
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minimal size lies within the colloidal range (i.e., with a fraction between 1 and 3 μm). We have assembled simple droplets into variously sized clusters. We have also assembled droplets, Janus particles, and multiple droplets into large crystals that produce discrete diffraction patterns. Smaller than a eukaryotic cell, our droplets can potentially be used as vessels for drug delivery. We have also demonstrated the capacity for multiple droplets to be phagocytosed into macrophages, thus suggesting their use as nonvirally inert vectors for endocytosis-mediated antigen or drug delivery. Acknowledgments. We gratefully acknowledge ESPCI and CNRS for their support of this work. We benefited from fruitful discussions with A. Colin, P. Doyle, J. Eggers, S. Granick, J. Lewiner, J. Prost, S. Quake, H. Stone, A. Viallat, and J. L. Viovy. We thank R. Gohier for his technical help. Supporting Information Available: A confocal microscopy movie showing two multiple emulsions of aqueous fluorescein droplets contained within a fluorinated oil droplet, which are being phagocytosed by a macrophage. This material is available free of charge via the Internet at http://pubs.acs.org.
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