Chemical Engineering Science 65 (2010) 405--411 Contents lists available at ScienceDirect Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces Performance comparison of micromixers L. Falk ∗ , J.-M. Commenge Laboratory of Chemical Engineering Science, CNRS—ENSIC, 1 rue Grandville, BP 20451, 54001 Nancy Cedex, France ARTICLE INFO ABSTRACT Article history: Received 15 December 2008 Received in revised form 19 May 2009 Accepted 30 May 2009 Available online 6 June 2009 Keywords: Micromixers Laminar mixing Villermaux/Dushman chemical test reaction The present paper proposes a detailed comparison of mixing efficiency of different mixers that have been characterized by the Villermaux/Dushman test reaction. Considering simple relations of mixing in laminar flow, it is shown how to obtain the theoretical mixing time and how to relate it with operating parameters as the Reynolds number of the flow and the specific power dissipation per mass unit of fluid. The comparison of the experimental and of the theoretical mixing times indicates that only a few percents of the total mechanical power transmitted to the fluid is effective for mixing. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Since more than 10 years, micromixers have demonstrated their capabilities in a large application domain ranging from lab-on-a- chip biotech devices to industrial applications in replacing batch synthesis of a chemical to continuous reaction (Hessel et al., 2004). Mixing is still a bubbling field (Wiggins and Ottino, 2004) with thousands of papers published and hundreds of patents issued each year (Kamholz, 2004, Stone et al., 2004). However, the design of micromixers is largely a trial-and-error process resulting in inef- ficiencies and suboptimal designs. Mixing issues are complicated, and sometimes counterintuitive, because the results are issued from strongly coupled processes between fluid mechanics, mass transfer and reactions. There exists a great variety of micromixers based on different mixing principles, classified in mainly two basic concepts: • active mixers that use external energy sources as mechanical stir- rers and valves, piezoelectric vibrating membranes, ultrasound, acoustic; • passive mixers that use the flow energy to create multi-lamellae structures, which are stretched and recombined to promote mix- ing by molecular diffusion. A detailed list of these mixers, their mixing principle and operat- ing conditions can be found in recent reviews proposed by Nguyen and Wu (2005), Squires and Quake (2005) and more specifically by Hessel et al. (2005). Almost every laboratory or company active in ∗ Corresponding author. E-mail address: falk@ensic.inpl-nancy.fr (L. Falk). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.05.045 this field has proposed his own, or several, mixer(s). Because of the high number of mixers and of the lack of one standard performances quantification method, it is somewhat difficult for the user to com- pare and to choose between the different micromixers according to a specific purpose. Among different characterization techniques of mixing efficiency, the so-called chemical test-reactions are attractive methods to char- acterize mixing efficiency of mixing devices. One of these methods is the Villermaux/Dushman reaction (iodide/iodate reaction) that has been retained by many authors as a standard test for microstruc- tured systems. The present paper considers published experimental charac- terization studies of different micromixers based on the Viller- maux/Dushman test reaction and proposes a detailed comparison of mixing efficiency of these mixers. Considering theoretical rela- tion of mixing in laminar flow, it will be shown how to obtain the mixing time and how to relate it with operating parameters such as the Reynolds number of the flow and the specific power dissipation per mass unit of fluid. 2. Theoretical model of mixing 2.1. Mixing by molecular diffusion in a shear flow Molecular diffusion is the ultimate and finally the only process really able to mix components of a fluid on the molecular scale. The time constant for molecular diffusion of an elementary structure or blob is the diffusion time defined as (Villermaux (1986)) t diff = A R 2 D (1) where R denotes the half-thickness of the aggregate and D the dif- fusion coefficient.