Miniaturized systems based on the principles of microfluidics are widely used in various fields, such as biochemical and medical applications. Systematic design processes are demanded for the proper use of these microfluidic devices based on mathematical simulations. In this research, passive mixing, without external forces, was simulated and analyzed with Fluent 6.1. After the simulation process, PDMS-based micro-chips were fabricated for use in experiments. In the experiments, gold nano-particles and CuSO4 solutions were used to Visualize their mixing tendencies. Because the gold nano-particles were composed of specific aggregations with metal ions, colorimetric variations could be detected by the naked eye without any optical equipment. The simulation results showed good agreement with the experimental ones. The pressure drop in the micro-mixers was small and, therefore, their structures were deemed to be suitable for integration with lab-on-a-chips or micro-total analysis systems. (C) 2009 Elsevier B.V. All rights reserved.