This paper presents the performance of a passive planar rhombic micromixer with diamond-shaped obstacles and a rectangular contraction between the rhombi. The device was experimentally optimized using water for high mixing efficiency and a low pressure drop over a wide range of Reynolds numbers (Re = 0.1-117.6) by varying geometrical parameters such as the number of rhombi, the distance between obstacles and the contraction width. Due to the large amount of data generated, statistical methods were used to facilitate and improve the results of the analysis. The results revealed a rank of factors influencing mixing efficiency: Reynolds number > number of rhombi > contraction width > interobstacles distance. The pressure drop measured after three rhombi depends mainly on Re and interobstacle distance. The resulting optimum geometry for the low Re regime has a contraction width of 101 mu m and inter-obstacles distance of 93 mu m, while for the high Re regime a contraction width of 400 v and inter-obstacle distance of 121 mu m are more appropriate. These mixers enabled 80% mixing efficiency creating a pressure drop of 6.0 Pa at Re = 0.1 and 5.1 x 10(4) Pa at Re = 117.6, with a mixer length of 2.5 mu m. To the authors' knowledge, the developed mixer is one of the shortest planar passive micromixers reported to date.