© Copyright 2005 American Chemical Society
MAY 24, 2005 VOLUME 21, NUMBER 11
Letters Microfluidics: From Dynamic Lattices to Periodic Arrays of Polymer Disks Minseok Seo, Zhihong Nie, Shengqing Xu, Patrick C. Lewis, and Eugenia Kumacheva* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Received January 9, 2005 We used coupling of flow and geometric confinement to assemble emulsion droplets in two-dimensional gliding lattices with a high degree of order and symmetry. Highly monodisperse discoid droplets with circular shapes were generated in a microfluidic flow-focusing device. Originally, close-packed lattices formed from these circular discoid droplets. Progressive confinement led to the gradual deformation of the circular disks: first, they elongated in the direction parallel to the direction of flow and then transformed into hexagons. Assembly driven by the combination of flow and confinement also allowed for the formation of lattices from droplets with a bimodal size distribution. We used photopolymerization of the monomer droplets to trap the lattice structure in the solid state and produce highly periodic arrays of solid polymer disks.
Periodic arrays of colloid particles have potential applications in the fabrication of photonic crystals, gratings, microlens arrays, and sensors. Assembly of colloids by coupling of geometric confinement and an external force (e.g., capillary force, flow, or electric field) is a promising route to the production of lattices with a high degree of order and symmetry.1-9 Recently, microfluidic devices * To whom correspondence may be addressed. E-mail:
[email protected]. (1) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718. (b) Ozin, G. A.; Yang, S. M. Adv. Funct. Mater. 2001, 11, 95. (2) Weitz, D. A. Science, 2004, 303, 968. (3) Garstecki, P.; Gitlin, I.; DiLuzio, W.; Whitesides, G. M.; Kumacheva, E.; Stone, H. A. Appl. Phys. Lett. 2004, 85, 2649. (4) Shiu, J. Y.; Kuo, C. W.; Chen, P. J. Am. Chem. Soc. 2004, 126, 8096. (5) Kumacheva, E.; Golding, R. K.; Allard, M.; Sargent, E. H. Adv. Mater. 2002, 14, 221. (b) Golding, R. K.; Lewis, P. C.; Kumacheva, E.; Allard, M.; Sargent, E. H. Langmuir 2004, 20, 1414. (6) Kumacheva, E.; Garstecki, P.; Wu, H.; Whitesides, G. M. Phys. Rev. Lett. 2003, 91, 128301. (7) Thorsen, T.; Roberts, R. W.; Arnold, F. H.; Quake, S. R. Phys. Rev. Lett. 2001, 86, 4163. (8) (a) Link, D. R.; Anna, S. L.; Weitz, D. A.; Stone, H. A. Phys. Rev. Lett. 2004, 92, 054503. (b) Sugiura, S.; Nakajima, M.; Seki, M. Langmuir 2002, 18, 3854.
have been successfully used to achieve coupling of flow and confinement: two-dimensional (2D) colloid crystals of bubbles, droplets, and solid microbeads were obtained in the microfluidic channels.5-9 We used a microfluidic flow-focusing device (MFFD) described elsewhere10 to generate highly monodisperse oil or monomer droplets. Under particular operating conditions, the droplets assembled into gliding 2D lattices. Following increase in total volume of droplets produced per unit time, the lattices underwent a transition from a close-packed array of circular disks to the array of hexagonal disks. We also obtained highly ordered dynamic lattices from the droplets with bimodal size distribution. We in situ photopolymerized the monomer droplets assembled in 2D lattices and achieved good control over the periodicity of arrays of solidified polymer disks. The masters were prepared using SU-8 photoresist (MicroChem) bas-relief structure on silicon wafers. The height of the channels of the MFFD was 87 ( 1.0 µm. The (9) Zheng, B.; Roach, L. S.; Ismagilov, R. F. J. Am. Chem. Soc. 2003, 125, 11170. (10) Anna, S. L.; Bontoux, N.; Stone, H. A. Appl. Phys. Lett. 2003, 82, 364.
10.1021/la050070p CCC: $30.25 © 2005 American Chemical Society Published on Web 04/16/2005
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Figure 1. (a) Schematic of a discoid droplet formed in MFFD. The width of the orifice and the height (h) and width (w) of the channel were 30, 87 ( 1.0, and 1000 µm, respectively. (b) Variation in volume of oil droplets plotted vs capillary number. Flow rate of an aqueous phase varied from 0.010 to 0.170 mL/h; flow rate of oil (µ ) 0.65 cP) was 0.020 mL/h. The line is given for eye guidance. (c) Size distribution for discoid droplets (d ) 193 µm) produced at oil (µ ) 0.65 cP) and water flow rates of 0.020 and 0.080 mL/h, respectively. Experimental size distribution was fitted with a Gaussian distribution. Coefficient of variation in size for these droplets is 1.68%.
MFFD was fabricated in polyurethane, PU, elastomer using standard soft-lithography.11 The use of PU polymer allowed for the production of direct oil-in-water emulsions. Dimethacrylate oxypropyldimethylsiloxane (DMOS) and silicone oil used in the present work had a lower interfacial energy with poly(dimethylsiloxane) (PDMS) than with the aqueous phase; therefore in the PDMS molds inverse water-in-oil emulsions were formed. A similar effect was reported by Weitz et al.12 Two immiscible liquids: oil or monomer (a droplet phase) and an aqueous 2 wt % sodium dodecyl sulfate (SDS) solution (a continuous phase) were forced into a narrow orifice (Figure 1a). The oil or monomer thread broke up in a periodic manner to release highly monodisperse droplets into the outlet channel. These droplets did not adhere to the microchannel surface and readily moved down the outlet channel. Oil and aqueous phases were supplied via polyethylene (Intramedic) tubing attached to syringes operated by digitally controlled syringe pumps (Harvard Apparatus PHD 2000). The flow of the fluids was controlled using two syringe pumps. When the flow rates of the liquids were changed, the system was equilibrated for at least 10 min. An Olympus BX51 microscope (Olympus) and a high-speed camera (Photometrics CoolSNAP ES) were used to acquire images. We used Image Pro (Media Cybernetics) software to measure the diameter of the droplets and analyze their size distribution. We generated droplets by emulsifying silicone oil with viscosity 0.65, 5.0, 10, 20, and 50 cP or DMOS (viscosity 20 cP) in an aqueous SDS solution. DMOS was mixed with 3.5 ( 0.5 wt % of a photoinitiator 1-hydroxycyclohexyl phenyl ketone, HCPK. In the MFFD used in the current work, the diameter of the undeformed droplets was larger than the height, h, of the microfluidic channel, thus causing the droplets to assume a discoid shape (Figure 1a). The disks had circular interfaces with the top and bottom of the outlet channel. The diameter, d, of the disks was determined by the height of the channel and volume of the droplet. We calculated the volume, VD, of disks as V ) Ah, where A is the area of the large facet of the disk and h is the height of the outlet channel of MFFD.13 In the present work the aspect ratio, d/h, of oil or monomer disks varied from 10.1 to 1.0. The emulsification process was governed by the shear stress imposed on the droplet phase. Figure 1b shows the (11) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (12) Pautot, S.; Frisken, B, J.; Cheng, J.; Xie, X., S.; Weitz, D. A. Langmuir 2003, 19, 10281. (13) We ignored the curvature of disk interfaces with the bottom and top walls of the outlet channel.
Figure 2. (a-d) Optical microscopy images (top view) of the typical transitions in lattice structure following decrease of Ca from 8.5 × 10-4 (a) to 1.59 × 10-4 (d). Flow rate of silicone oil was 0.02 mL/h, viscosity of silicon oil was µ )50.0 cP. Scale bar is 200 µm. (e) Variation in volume fraction of droplets in the microchannel vs capillary number.
Figure 3. Optical microscopy images of hexagonal lattices with a different number of columns (a-g). Volume of droplets ×10-6, mL: (a) 54.20; (b) 24.33 (c) 10.8 (d) 6.08, (e) 3.93, (f) 2.73, (g) 2.01. (h, i) Hexagonal lattices formed by droplets with bimodal volume distribution. Volumes of droplets ×10-6 mL: 20.99 and 3.42 (h), 9.82 and 3.89 (i). Scale bar is 200 µm.
decrease in droplet volume with increasing capillary number, Ca ) µv/γ, where µ is the dynamic viscosity of the aqueous phase, v is a characteristic velocity of the aqueous phase, and γ is the value of interfacial tension between the oil and aqueous fluids, γ ≈ 2.71 mN/m.14 The discoid droplets with volume below 10.6 × 10-6 mL had a very narrow size distribution (Figure 1c). Under typical operating conditions the coefficient of variation (defined as standard deviation in droplet diameter d divided by mean diameter) was below 3.0% for the droplets obtained from silicone oil with different viscosity. Thus by varying the value of Ca, we produced highly monodisperse oil droplets with a controlled volume. The velocity of droplets in the downstream channel of MFFD was slower than that of the continuous phase.6 Below Ca ) 1.6 × 10-4 the discoid droplets assembled into 2D close-packed lattices filling the entire volume micro(14) We measured this value using a drop shape method called ADSA (axisymmetric drop shape analysis).
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Figure 4. Optical microscopy images of a lattice of DMOS disks before (a) and after (b) in situ photopolymerization. Flow rates are 0.0030 and 0.1000 mL/h for the monomer and aqueous phases, respectively. (c) Typical SEM image of the solidified disks. Scale bar is 200 µm.
channel. The smallest diameter of circular disks organized in ordered arrays was ca. 87 µm. We note that this size was valid for the width and height of the downstream channel of 1000 and 87 µm, respectively. Figure 2a-d shows typical optical microscopy images of the gliding lattices with six columns of oil droplets aligned parallel to the channel wall. Initially, the lattices formed from the discoid droplets with circular shapes (Figure 2a, top view). The trend to accommodate larger droplets in the constrained geometry of microchannel led to the gradual deformation of circular oil disks. First, the circular disks elongated in the direction parallel to the direction of flow (Figure 2b). Then, larger droplets transformed into the hexagonal disks, with pentagons adjacent to the microchannel walls (Figure 2c,d). In the latter lattice, the facets met at an angle of 120°, similar to Plateau borders in dry foams.15 The transformation of circular disks to hexagonal disks occurred at a distance from the orifice smaller than 800 µm. We characterized the structural transitions in the gliding lattices by calculating the total volume fraction, φ, occupied by oil droplets. Figure 2e shows the variation in the value of φ for the six-column lattice. For the honeycomb arrays with 10 µm thick walls between the droplets, the maximum volume fraction of droplets was as high as 99.5%. We varied the value of Ca by changing the flow rates of the oil and aqueous phases and viscosities of oil, to obtain the flowing lattices with the number of columns, n, varying from 1 to 13. For n > 8 the circular disks did not undergo transformation into hexagonal disks. Figure 3a-g shows typical optical microscopy images of dynamic hexagonal lattices obtained from the monodisperse oil droplets. The number of columns increased with progressively decreasing droplet volume. The transition between the lattices with a different number of columns occurred through the assembly of circular and ellipsoidal disks and/ or via packing of droplets with pentagonal and hexagonal shapes. Assembly driven by coupling of flow and geometric confinement allowed the formation of gliding lattices from droplets with a bimodal size distribution. Parts h and i of Figure 3 show two representative lattices formed by the oil droplets with different volumes (still with coefficient of variation below 2% for each population). The ratio between the volumes of large and small droplets was 6.5/1 and 2.6/1 in Figure 3h and Figure 3i, respectively. Yet, in each case they assembled in a perfectly ordered lattice. (15) Weaire, D.; Hutzler, S. Physics of Foams; Oxford University Press: Oxford, 1999.
The smaller droplets were always adjacent to the microchannel walls. We conducted UV-initiated free-radical photopolymerization of DMOS to trap the hexagonal lattice formed by monomer droplets in the solid state. Both the stationary and the gliding lattices could be exposed to UV irradiation, leading to the arrays of disks or the individual polymer disks, respectively. Here we show the results obtained for the stationary lattices (the production of polymer disks in MFFD is reported elsewhere).16 The lattice of the DMOS droplets mixed with HCPK was exposed for 30-180 s to the UV light (UV lamp, UVAPRINT 40C/CE, Dr. K. Ho¨nle GmbH UV-Technologie with an output of 400 W at a wavelength of 330-380 nm). Parts a and b of Figure 4 show an optical microscopy image of the array of droplets of DMOS before and after photopolymerization, respectively. After solidification the droplets shrank by ca. 7% and acquired the shape shown in Figure 4b. The volume fraction of the disks reduced from 99.5 to 92.4%. (For the polymerized gliding lattices the shrinkage of disks led to the reduction of their friction with the MFFD walls due to presence of a thin aqueous layer between the disks and the walls of the channel; this layer helped to avoid clogging of the microchannel.) Figure 4c shows a typical scanning electron microscopy image of polyDMOS disks with aspect ratio 3.50. A highly periodic structure of the 2D lattice of droplets was preserved in the solid state. In summary, we have demonstrated the assembly of liquid disks in 2D gliding lattices which possess a high degree of order and symmetry. The lattices were obtained from monodisperse droplets and droplets with a bimodal size distribution. Following increase in droplet volume, the lattices underwent a gradual transition from a closepacked array of circular disks to the array of hexagonal disks. We photopolymerized monomer droplets assembled in a 2D lattice and in this manner trapped the structure of the gliding lattices in the solid state. The fluid and solid arrays assembled in the microfluidic device may find applications in the fabrication of gratings and microlens arrays and in chemical and biological analysis and synthesis. Acknowledgment. E.K. acknowledges the Canada Research Chair support. LA050070P (16) Xu, S.; Nie, Z;. Seo, M.; Lewis, P. C.; Kumacheva, E.; Stone, H. A.; Garstecki P.; Weibel, D. B.;Gitlin, I.; Whitesides, G. M. Angew. Chem., Int. Ed. 2005, 44, 724-728.
