Langmuir 2005, 21, 7629-7632
7629
Novel Asymmetric Through-Hole Array Microfabricated on a Silicon Plate for Formulating Monodisperse Emulsions Isao Kobayashi,†,‡,§ Sukekuni Mukataka,‡ and Mitsutoshi Nakajima*,† National Food Research Institute, 2-1-12 Kannondai, Tsukuba, Ibaraki 305-8642, Japan, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8577, Japan, and Japan Society for the Promotion of Science, 6 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan Received April 6, 2005. In Final Form: June 19, 2005 We have proposed a novel microchannel (MC) structure for formulating monodisperse emulsions. The emulsification device is a silicon array of microfabricated, asymmetric through-holes with a slit and a circular channel (an asymmetric straight-through MC). The asymmetric through-holes of a uniform size stably yielded monodisperse emulsions with average droplet diameters of 35-41 µm and coefficients of variation of less than 2% by forcing the to-be-dispersed phase into the continuous phase via the throughholes. Their asymmetry enabled the stable formation of monodisperse emulsion droplets by spontaneous transformation, even using a to-be-dispersed phase with a very low viscosity below 1 mPa s. Additionally, the asymmetric straight-through MC with a high-density through-hole layout has the potential for highthroughput formulation of monodisperse emulsions.
Introduction A monodisperse emulsion is a dispersion of highly uniform droplets of one of two immiscible liquids in the continuous phase of the other. The emulsion can be kinetically stabilized against droplet coalescence for a finite period when surfactants are added prior to emulsification. Monodisperse emulsions usually have greater emulsion stability than polydisperse emulsions. Monodisperse emulsions are also useful for measuring, analyzing, and controlling many important emulsion properties.1,2 Monodisperse-emulsion-based materials (e.g., monodisperse multiple emulsions, microparticles, and microcapsules) have attracted a great deal of interest for various applications including foods,3 cosmetics,4 pharmaceuticals,5,6 chemicals,6,7 electronics,7-9 and optics.10,11 Repeated fractionation of polydisperse emulsions12 and shear-rupturing of polydisperse emulsions13 can be used to prepare quasi-monodisperse emulsions by reducing * To whom correspondence should be addressed. E-mail: mnaka@ affrc.go.jp. Tel: +81-29-838-8014. Fax: +81-29-838-7996. † National Food Research Institute. ‡ University of Tsukuba. § Japan Society for the Promotion of Science. (1) McClements, D. J. Food Emulsions: Principles, Practice and Techniques; CRC Press: Boca Raton, FL, 1999; Chapter 1. (2) Mason, T. G.; Krall, A. H.; Gang, H.; Bibette, J.; Weitz, D. A. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1996; Vol. 2, Chapter 6. (3) Katoh, R.; Asano, Y.; Furuya, A.; Sotoyama, K.; Tomita, M. J. Membr. Sci. 1996, 113, 131. (4) Ikkai, F.; Iwamoto, S.; Adachi, E.; Nakajima, M. Colloid Polym. Sci., in press, 2005. (5) Higashi, S.; Tabata, N.; Nakashima, T.; Iwata, K.; Uchiyama, F.; Tamura S.; Setoguti, T. Cancer 1995, 75, 1245. (6) Nakashima, T.; Shimizu, M.; Kukizaki, M. Adv. Drug Delivery Rev. 2000, 45, 47. (7) Omi, S. Colloids Surf., A 1996, 109, 97. (8) Sugiura S.; Nakajima, M.; Itoh, H.; Seki, M. Macromol. Rapid Commun. 2001, 22, 773. (9) Shimizu, M.; Torigoe, K.; Arazaki, I.; Nakashima, T. Proceedings of the 36th SPG Forum; Miyazaki, Japan, 2001. (10) Yi, G.-R.; Jeon, S.-J.; Thorsen, T.; Manoharan, V. N.; Quake, S. R.; Pine, D. J.; Yang, S.-Y. Synth. Met. 2003, 139, 803. (11) Nishisako, T.; Torii, T.; Higuchi, T. Chem. Eng. J. 2004, 101, 23. (12) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474. (13) Mason, T. G.; Bibette, J. Langmuir 1997, 13, 4600.
polydispersity. Membrane emulsification, which applies low external shear stress to generate emulsion droplets, enables the direct formulation of emulsions with a minimum coefficient of variation of 10% using a microporous membrane of a narrow pore size distribution.14-18 Umbanhowar et al.19 proposed a technique to formulate monodisperse emulsions via droplet breakup in a coflowing stream. Monodisperse droplets can also be generated by breaking up the to-be-dispersed phase using flow focusing in microfluidic channels20,21 or at a T-shaped junction of microfluidic channels.22,23 The droplet breakup is a sensitive process driven by the external shear stress due to the continuous-phase flow, and the resultant droplet size depends on the flow rate in each stream. Our research group previously reported microchannel (MC) emulsification in which a microfluidic channel array with a slitlike terrace24 is used to formulate monodisperse emulsions with a coefficient of variation below 5%. Droplet formation in MC emulsification does not require any external shear stress because the to-be-dispersed phase that passes through the channels transforms spontaneously into emulsion droplets because of interfacial tension effects.25 However, the major problem in MC emulsification is the low throughput of monodisperse emulsion droplets (typically
