RESEARCH PAPER Emulsification in a microfluidic flow-focusing device: effect of the viscosities of the liquids Zhihong Nie Æ MinsSeok Seo Æ Shengqing Xu Æ Patrick C. Lewis Æ Michelle Mok Æ Eugenia Kumacheva Æ George M. Whitesides Æ Piotr Garstecki Æ Howard A. Stone Received: 6 July 2007 / Accepted: 13 February 2008 / Published online: 6 March 2008 Ó Springer-Verlag 2008 Abstract We report the results of a comparative study of microfluidic emulsification of liquids with different vis- cosities. Depending on the properties of the fluids and their rates of flow, emulsification occurred in the dripping and jetting regimes. We studied the characteristic features and typical dependence of the size and of the size distribution of droplets in each regime. For each liquid, we identified a range of hydrodynamic conditions promoting generation of highly monodisperse droplets. Viscosity played an important role in emulsification: highly viscous liquids were emulsified into larger droplets with lower polydis- persity. Although it was not possible to provide a unified scaling for the volumes of the droplets, our results suggest that the break-up dynamics of the lower viscosity fluids resembles the rate-of-flow-controlled break-up, as reported earlier for the formation of bubbles in flow-focusing geometries [Garstecki P, Stone HA, Whitesides GM (2005) Phys Rev Lett 94:164501]. The results of this study can be helpful for a rationalized selection of liquids for the con- trolled formation of droplets with a predetermined size and with a narrow distribution of sizes. Keywords Microfluidic Á Hydrodynamic Á Droplets Á Flow-focusing Á Emulsification 1 Introduction Recently, several research groups have reported synthesis of polymer particles in microfluidic reactors (Cohen et al. 2001; Dendukuri et al. 2005; Jeong et al. 2005; Lewis et al. 2005; Loscertales et al. 2002; Nie et al. 2005, 2006; Nisisako et al. 2004; Seo et al. 2005a, 2005b; Takeuchi et al. 2005; Utada et al. 2005; Xu et al. 2005; Zhang et al. 2006). The syntheses included a two-step process: (1) microfluidic emulsification of monomer or polymeric flu- ids, and (2) subsequent in-situ (on chip) solidification of the droplets by means of polymerization, gelation, or solvent evaporation. Microfluidic methods allowed for the pro- duction of particles with diameters from several micrometers to hundreds of micrometers, polydispersities below 5%, and shapes and morphologies that were not achievable in the conventional synthesis of colloids (Nisisako et al. 2004; Xu et al. 2005; Nie et al. 2005, 2006; Z. Nie Á M. Seo Á S. Xu Á P. C. Lewis Á E. Kumacheva (&) Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, Canada M5S 3H6 e-mail: ekumache@alchemy.chem.utoronto.ca; ekumache@chem.utoronto.ca M. Mok Á E. Kumacheva Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College street, Toronto, ON, Canada M5S 3E5 E. Kumacheva Institute of Biomaterials and Biomedical Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, ON, Canada G. M. Whitesides Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, MA 02138-4307, USA P. Garstecki Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland H. A. Stone School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138-4307, USA 123 Microfluid Nanofluid (2008) 5:585–594 DOI 10.1007/s10404-008-0271-y