Colloids and Surfaces A: Physicochem. Eng. Aspects 456 (2014) 296–306 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects j ourna l h om epa ge: www.elsevier.com/locate/colsurfa Rheology effects on cross-stream diffusion in a Y-shaped micromixer Alireza Ahmadian Yazdi, Arman Sadeghi, Mohammad Hassan Saidi ∗ Center of Excellence in Energy Conversion (CEEC), School of Mechanical Engineering, Sharif University of Technology, P.O. Box: 11155-9567, Tehran, Iran h i g h l i g h t s • Rheology effects on cross-stream dif- fusion in a Y-shaped micromixer are investigated. • Non-linear rheology may result in significant alteration of species con- centration field. • Non-linear rheology effects are pro- nounced in the presence of thick EDLs. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 25 February 2014 Received in revised form 28 April 2014 Accepted 6 May 2014 Available online 22 May 2014 Keywords: Electroosmotic flow Micromixer Non-Newtonian fluid Numerical modeling a b s t r a c t Micromixers are one of the essential components of modern bio-microfluidic devices. Since most bio- fluids are complex and their rheological behavior usually cannot be described by the Newton’s law of viscosity, it is vital to take into account the non-linear behavior of the fluids being manipulated in these devices in the pertinent simulations. In this paper, the non-Newtonian rheology effects on mass transport in an electrokinetically driven Y-shaped micromixer of rectangular cross section are being investigated. The fluid rheological behavior is assumed to be efficiently described by the power-law viscosity model. The governing equations are solved in dimensionless form through a finite difference based numerical procedure for non-uniform grid. The results show that the deviations of the fluid rheological behavior from the predictions of the Newton’s law of viscosity may result in significant alteration of the species concentration field, especially for thick EDLs. In this respect, a higher value of the flow behavior index gives rise to a thicker diffusion layer in the presence of a purely electroosmotic flow. Whereas the same is observed for a pressure assisted flow, the opposite is right in the presence of an adverse pressure gradient. Moreover, the diffusion layer extent is an increasing function of EDL thickness. The relevant functionality is pronounced by increasing the flow behavior index. In addition, the effect of decreasing both Péclet number and the rectangular geometry aspect ratio is to enhance the mixing efficiency. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Major advancements in microfabrication technology in the late 20th century led to the development of various microfluidic devices including lab-on-a-chip (LOC) systems. The lab-on-a-chip devices are microscale laboratories on a chip that can perform biochemical ∗ Corresponding author. Tel.: +98 21 66165522. E-mail addresses: ar ahmadianyazdi@mech.sharif.edu (A. Ahmadian Yazdi), armansadeghi@mech.sharif.edu (A. Sadeghi), saman@sharif.edu (M.H. Saidi). diagnoses. The main advantages of these devices are ease of use, speed of analysis, low sample and reagent consumption, and high reproducibility due to standardization and automation [1]. With the advent of LOCs, electroosmosis has featured as a predominant mechanism for flow actuation. Electroosmotic micropumps have many advantages over the other types of micropumps. For example, unlike the classical pressure-driven micropumps involving moving components, the electroosmotic pumps have no moving parts and are simpler to be designed and fabricated [2,3]. Moreover, these pumps are bidirectional and http://dx.doi.org/10.1016/j.colsurfa.2014.05.021 0927-7757/© 2014 Elsevier B.V. All rights reserved.