Very low Reynolds number laminar flow in Split-And-Recombine (SAR) static mixers is numerically investigated by using a finite volume method. In these configurations, advective chaotic flow structures are created by the passive control of the flow, mimicking the baker's transformation, through a series of flow splitting, rotations, and re-combinations that generate intertwined lamellar structures. This process leads to extra surface creation, which ultimately intensifies mass transfer. The main aim of this study is firstly to demonstrate the interest of this technique for enhancing mixing, while keeping pressure drops moderate, and secondly to optimize this type of geometry. For the latter stake, two different SARs, namely SAR1 (Gray's configuration) and SAR2 (Chen's configuration), are compared in terms of distributive and dispersive mixing and energy expenditures evaluated by the pressure drop.