Passive mixers rely on the channel geometry to mix fluids and mixing depends primarily on diffusion. However, many previously reported designs either work efficiently only at moderate to high Reynolds numbers (Re) or require a complex 3D channel geometry that is often difficult to fabricate. In this paper, we report the design, simulation, and characterization of a planar O passive microfluidic mixer with two types of obstacles to enhance mixing performance. Numerical investigation on mixing

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... flow structures in microchannels is carried out using the computational fluid dynamics (CFD) software ANSYS 15 for a wide range of Reynolds numbers from 1 to 200. The results show that the O mixer with obstacles has far better mixing performance than the O mixer without obstacles. The reason is that fluid path length becomes longer due to the presence of obstacles which gives fluids more time to diffuse. For all cases, the O mixer with circular & fin obstacles have 3 times more efficient compared to the O mixer without obstacles. It is also clear that efficiency increase with axial length as expected. Efficiency can be simply improved by adding extra mixing units to provide adequate mixing. The value of the pressure drop is the lowest for the O mixer because there is no obstacle inside the channel. However, the O mixer with circular & fin obstacles has the lowest mixing cost, an important characteristic for integration into complex, cascading microfluidic systems, which makes it the most cost-effective mixer. Due to the simple planar structure and low mixing cost, it can be easily realized and integrated into devices for various macromixing applications.