연성 미세유체 소자를 이용한 균일한 콜로이드 집합체 제조

화학공학의 이론과 응용 제8권 제1호 2002년

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

이기라*, Todd Thorsen**, Vinothan N. Manoharan***, David J. Pine****, Stephan R. Quake*****, 양승만*

한국과학기술원 생명화학공학과*, Division of Chemistry and Chemical enginerring, California Institute of technology**, Department of Chemical Engneering, Uinversity of California, Santa Barbara***, Department of Chemical Engneering and Materials, Uinversity of California, Santa Barbara****, Department of Applied Physics, California Institute of Technology*****

Generation of Uniform Colloidal Assemblies in Soft Microfluidic Devices

Gi-Ra Yi*, Todd Thorsen**, Vinothan N. Manoharan***, David J. Pine****, Stephan R. Quake*****, and Seung-Man Yang*

Department of Chemical and Biomolecular Engineering, KAIST*, Division of Chemistry and Chemical enginerring, California Institute of technology**, Department of Chemical Engneering, Uinversity of California, Santa Barbara***, Department of Chemical Engneering and Materials, Uinversity of California, Santa Barbara****, Department of Applied Physics, California Institute of Technology*****

서론

Monodisperse spherical colloids such as polymer latexes and silica suspensions serve as model systems for practical colloidal materials or biological systems because of their uniform response to external electromagnetic and flow fields.[1] In addition, their self-organization with three-dimensional (3D) periodicity on the scale of optical wavelengths has received much attention as a promising route for 3D photonic band gap (PBG) materials.[2,3] Recently, various organic or inorganic monodisperse colloidal dispersions have been prepared with non-spherical shapes such as ellipsoids, [4] dumbbells, [5] and hollow spheres. [6,7] Their complexity may make it possible to fabricate novel photonic band gap materials as well as to model real colloids with more irregular structures. Another approach to preparing such structured particles is to use assemblies of spherical colloids. For example, two-dimensional (2D) colloidal aggregates with well-defined size, shape and structure can be obtained under 2D confinement in a patterned photoresist film.[8] Simple heterogeneous colloidal aggregates can be prepared using specific chemical or biochemical interaction between two kinds of monodisperse colloids.[9-11] Recently, 3D homogeneous colloidal assemblies with well-defined structure and shape have been reported.[12-14] However, it is still challenging to fabricate uniform 3D colloidal assemblies at micrometer length scales.

본론

In this presentation, we report on soft-lithography-based microfluidic approaches for the fabrication of micron-sized 3D colloidal assemblies with narrow size distribution and internal ordered structure of monodisperse latex. Our strategy is to generate a monodisperse particle-containing water-in-oil emulsions using emulsion-generating soft microfluidic devices of Thorsen et al. (see Figure 1a) and then produce uniform spherical colloidal aggregates by slowly removing the water from the droplets. While standard cross-flow techniques have generated monodisperse emulsion by forcing the discontinuous phase into an open continuous phase through narrow pores[15], Microfluidic devices are more versatile tools for generation of the monodisperse emulsions or vesicles because droplet generation process is easy to be visualized and changed by designing the channel[16]. Uniform emulsion have been generated at regular intervals in soft microfluidic devices by droplet break-off at the junction of two microfluidic channels as shown in Figure 1(b). Balance of surface tension and high shear forces at the leading edge of the water perpendicular to the oil flow determine the diameter picoliter-scale droplets and generation frequency. Because the system remains at low Reynolds number, generated droplets are moving in pattern as they are formed. As in previous work of Thorsen et al., in which various dynamic pattern formation of emulsions was investigated by modifying the relative pressure of water and oil, separated droplet pattern was generated under conditions where the water pressure is lower than the oil pressure (B, C, and H pattern in Fig. 3 of Ref. 15). Generated droplets suspended in oil have kept moving along the microchannel and was shrunk down slowly as water was dissolved in oil phase (Figure 1c). Finally, consolidated spherical colloidal aggregates were obtained at end of microchannel (Figure 1d). It is noteworthy that no stabilizer was necessary to be added in oil phase because generated droplets didn't run into each other in our linear laminar flow and moreover emulsion system without surfactant is to facilitate water diffusion from emulsion droplets. As well, consolidated particle aggregates in final step were stable against coalescence by collision.

The microfluidic devices in our experimentals are fabricated by pouring silicone prepolymer(polydimethylsiloxane; Sylgard184, DowCorning) on a silicon wafer mold containg positive-relief channels patterned in thick photoresist(AZ9260, Clariant), which is then cured at 80oC, for 40 min. Then, the replicated channels in PDMS are completely enclosed with a coverslip coated with precured thin layer of PDMS through an additional curing at same temperature, for 1.5 hours. The measured channel dimensions are approximately 60 mm wide × 7 mm high, tapering to 15 mm × 6 mm in the region where the water and oil meet at the crossflow intersection (Figure 1). The fluids are introduced into the PDMS microfluidic devices through pressurized reservoirs containing suspension and oil. Pressure was applied to the reservoirs with compressed nitrogen, and the device output channel was allowed to vent to the atmosphere. All reported pressures are relative to atmospheric pressure (psig).

PDMS is not compatible with all kinds of oils. Specifically, low molecular hydrocarbon oil is lethal to PDMS microfluidic devices. According to our test, Silicone and Fluorine-modified oil were suitable oil appropriate for our PDMS, which were purchased from Gelest and ShinEtsu, respectively. Additional conditions for our experimentation are that they should dissolve water slightly for obtaining the particle aggregates fianlly and colloidal latex particles should be stay in droplets not escape from droplet into such oil phase. In our previous work, we found that they satisfied these requirement for our study. While other system like mixture of mineral oil and surfactant cause the particles in aqueous phase to escape oil phase and then encapsulated droplets were destroyed spontaneously, which means that colloidal encapsulation process in such emulsion system was impossible.

For facilitating the shrinking rate of droplet diameter, which was proportional to retention time since droplet generation, initial droplet diameter, and particle concentration inside droplet, any kind of surfactant in oil phase have not used in our system. However, water soluble surfactant as stabilizer of latex particles inside was added for preventing particles from adsorbing to PDMS channel wall because there was no repulsion force between PDMS channel wall and particle, which was prepared by emulsifier-free emulsion polymerization. We have tested several polymeric surfactant, including Tween 20 (Aldrich), hydrophilic methyl methacrylate graft copolymer (Atlox 4913; UniQema), and Pluronic P123 (BASF).

결론

화학공학의 이론과 응용 제8권 제1호 2002년

For large number, there is negligible number fluctuation between a few percentages. However, as you can expect and see in Figure 1d, they have statistical variation of particle number inside droplets for small colloidal aggregates, which is approximately calculated the number fluctuation from particle number in two dimensional planes of video movie file. Particle number density is very important parameter for developing novel photonic crystals out of our final colloidal aggregates in the future. Therefore, this number fluctuation of colloidal aggregate with small number of particle is so critical that we couldn't tell that those generated colloidal aggregates were uniform enough for photonic crytals application. However, we believe that our approach would be extended to uniform colloidal aggregates of a few particles by adding other controlling microfluidic parts.

참고문헌

1.	W. B. Russel, D. A. Saville, W. R. Schowalter, Colloidal Dispersions, Cambridge University Press, New York 1989.

2.	A. D. Dinsmore, J. C. Crocker, A. G. Yodh, Curr. Opin. Coll. Interf. Sci. 1998, 3, 5.

3.	O. D. Velev, E. W. Kaler, Adv. Mater. 2000, 12, 531.

4.	E. Snoeks, A. van Blaaderen, T. van Dillen, C. M. van Kata, M. L. Bronersma, A. Polman, Adv. Mater. 2000, 12, 1511.

5.	Y. Lu, Y. Yin, Y. Xia, Adv. Mater. 2000, 13, 415.

6.	P. Jiang, J. F. Bertone, V. K. Colvin, Science 2001, 291, 453.

7.	Z. Zhong, Y. Yin, B. Gates, Y. Xia, Adv. Mater. 2000, 12, 206.

8.	Y. Yin, Y. Xia, Adv. Mater. 2001, 13, 267.

9.	M. S. Fleming, T. K. Mandal, D. R. Walt, Chem. Mater. 2001, 13, 2210.

10.	A. L. Hiddessen, S. D. Rodgers, D. A. Weitz, D. A. Hammer, Langmuir 2000, 16,9744.

11.	K. Furusawa, O. D. Velev, Coll. Surf. A 1999, 159, 359.

12.	O. D. Velev, A. M. Lenhoff., E. W. Kaler, Science 2000, 287, 2240.

13.	G.-R. Yi, J. H. Moon, S.-M. Yang, Adv. Mater. 2001, 13, 1185

14.	O. D. Velev, K. Nagayama, Langmuir 1997, 13, 1856.

15.	T. Thorsen, R. W. Roberts, F. H. Arnold, S. R. Quake, Phys. Rev. Lett. 2001, 86, 4163.

16.	P. B. Umbanhowar, V. Prasad, D. A. Weitz, Langmuir 2000, 16, 347.

Figure 1.  Schematics of microchannel for monodisperse emulsion generation

(a)	(b)

(c)	(d)

Figure 1.  Emulsion generation by droplet breakup at the junction point of water and oil (a and b), droplet shrinkage during the traveling of suspension droplet along the microchannel (c) and consolidated colloidal assemblies (d) (inset shows the tetrahedra colloidal assembly)