Flow structure in a wide microchannel with surface grooves Vishwanath Somashekar a , Michael G. Olsen a , Mark A. Stremler b, * a Department of Mechanical Engineering, Iowa State University, Ames IA 50011, United States b Department of Engineering Science and Mechanics, Virginia Tech, Mail Code 0219, Blacksburg, VA 24061, United States article info Article history: Received 20 June 2008 Received in revised form 10 July 2008 Available online 5 August 2008 Keywords: Microfluidics Secondary flow Mixing abstract We present an experimental analysis of pressure-driven flow in a high (62:1) aspect ratio microchannel having a repeated herringbone surface pattern. The velocity field is deter- mined at the groove–channel interface and at the midplane of the channel using micro- scopic particle image velocimetry. At Reynolds numbers of 0.08, 0.8 and 8, we observe secondary flow patterns consisting of counter-rotating flow cells aligned in the streamwise direction. The strength of this secondary flow is inhibited by fluid inertia when Re ¼ 8. The resulting flow structure can be viewed as splitting one wide channel into multiple smaller channels without the use of solid boundaries. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Rapid mixing is essential for the successful operation of many microfluidic devices, but the laminar flows typically pro- duced at these small scales make this difficult to achieve. The various techniques and devices used for mixing enhancement in microscale systems are too numerous to detail here; several reviews can be found in the literature (see, e.g., Ottino and Wiggins, 2004; Squires and Quake, 2005). Here, we consider the ‘passive’ approach in which mixing is produced by steady, pressure-driven flow through a microchannel; this approach enables simple fabrication and operation in support of dispos- able single-use systems. When mixing in steady, pressure-driven microchannel flow, the challenge lies in using the channel geometry to stretch and fold the fluid. Methods based on this approach include the generation of strong spatially-varying secondary flows in the channel cross-section and lamination of the fluid by physically splitting and recombining the channel. Secondary flow, i.e., flow orthogonal to the mean flow direction, can be produced by, e.g., bends in the channel geometry (Liu et al., 2000) or re- cessed grooves in the channel wall (Stroock et al., 2002a). Mixing is enhanced by varying these geometric features, and thus the secondary flow patterns, along the channel. Fluid lamination enhances mixing by physically splitting the main channel into multiple smaller channels that are later recombined (Bessoth et al., 1999; Jeon et al., 2002; Schönfeld et al., 2004). Here, we discuss a hybrid technique in which the secondary flow itself splits the fluid into parallel counter-rotating regions as it moves through a single channel. 2. Device design A schematic of the microchannel geometry considered in this study is shown in Fig. 1. This system was fabricated using standard polydimethylsiloxane (PDMS) replica molding (Jo et al., 2000; Anderson et al., 2000). The channel is 2.5 mm wide by 40 lm deep, giving an aspect ratio of approximately 62:1. The floor of the channel contains recessed grooves arranged in a 0093-6413/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechrescom.2008.07.009 * Corresponding author. Tel.: +1 11 540 231 1227; fax: +1 11 540 231 4574. E-mail address: mark.stremler@vt.edu (M.A. Stremler). Mechanics Research Communications 36 (2009) 125–129 Contents lists available at ScienceDirect Mechanics Research Communications journal homepage: www.elsevier.com/locate/mechrescom