SHORT COMMUNICATION Gas–liquid flow stability and bubble formation in non-Newtonian fluids in microfluidic flow-focusing devices Taotao Fu • Youguang Ma • Denis Funfschilling • Huai Z. Li Received: 12 July 2010 / Accepted: 20 October 2010 / Published online: 8 December 2010 Ó Springer-Verlag 2010 Abstract This communication describes the gas–liquid two-phase flow patterns and the formation of bubbles in non- Newtonian fluids in microfluidic flow-focusing devices. Experiments were conducted in two different polymethyl methacrylate (PMMA) square microchannels of, respectively, 600 9 600 and 400 9 400 lm. N 2 bubbles were generated in non-Newtonian polyacrylamide (PAAm) solutions of differ- ent concentrations. Slug bubble, missile bubble, annular and intermittent flow patterns were observed at the cross-junction by varying gas and liquid flow rates. Gas and liquid flow rates, concentration of PAAm solutions, and channel size were varied to investigate their effect on the mechanism of bubble formation. The bubble size was proportional to the ratio of gas/ liquid flow rate for slug bubbles and could be scaled with the ratio of gas/liquid flow rate as a power–law relationship for missile bubbles under wide experimental conditions. Keywords Microfluidics  Gas–liquid flow  Bubble  Flow-focusing device  Micro-PIV 1 Introduction This article describes a microfluidic approach for on-chip production of the gas–liquid two-phase flow patterns and the bubble formation in non-Newtonian fluids by hydro- dynamic flow-focusing technique. Over the last decades, microfluidics has gained increasing importance in a wide range of academic investigations and industrial applica- tions (Anna et al. 2003; Whitesides 2006). Bubbles and droplets are frequently encountered in the application of microfluidics and have been received much attention in recent years. They are usually generated by either a flow- focusing device or a T-junction based on cross-flowing rupture technique (Dollet et al. 2008; Gan ˜a ´n-Calvo and Gordillo 2001; Garstecki et al. 2005). Highly uniform bubbles and droplets can be generated within these devices. And the bubble size could be controlled by changing var- ious parameters, such as flow rates, channel size, viscosity, and surface tension of the liquid phase (Gan ˜a ´n-Calvo and Gordillo 2001; Garstecki et al. 2004; de Menech et al. 2008). Most of previous works concern the bubble formation in Newtonian fluids, while many fluids for both academic research and industrial applications are likely to exhibit complex non-Newtonian behaviors (Arratia et al. 2008; Groisman et al. 2003; Li 1999; Qiu et al. 2010; Sang et al. 2009; Skurtys et al. 2008). Some authors have investigated the effect of fluids’ rheological properties on bubble or droplet formation in microfluidic devices (Arratia et al. 2008; Husny and Cooper-White 2006; Qiu et al. 2010; Skurtys et al. 2008). However, in comparison to the pro- duction of bubbles in Newtonian fluids in microfluidic devices, the generation of bubbles in non-Newtonian fluids in such devices is still in an elementary stage. In the present work, we realized the ordered gas–liquid two-phase flows and different kinds of bubbles in non- Newtonian fluids in microfluidic flow-focusing devices. The bubble formation mechanism was investigated under various operating conditions. Besides the two-phase flow T. Fu  Y. Ma State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China T. Fu  D. Funfschilling  H. Z. Li (&) Laboratory of Reactions and Process Engineering, Nancy-Universite ´, CNRS, 1, rue Grandville, BP 20451, 54001 Nancy Cedex, France e-mail: Huai-Zhi.Li@ensic.inpl-nancy.fr 123 Microfluid Nanofluid (2011) 10:1135–1140 DOI 10.1007/s10404-010-0741-x