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 SARI. (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. The enhancement effect of splitting/recombination is isolated through the comparison with a channel composed of small straight sections at right angle bends with alternate chiralities (referred as 3D-Flow), without splitting. A plain square-section channel is used as base-line reference geometry to assess the relative mixing efficiency of each configuration. Results show that the SAR technique is capable of enhancing mass transfer in creeping flows compared to non-splitting-flow mixers/exchangers, thus allowing a gain in residence time and mixer size for the same final results. Between the two SAR geometries, the superiority of Chen's configuration regarding the relative mixing efficiency is demonstrated, with 83% mixing intensification, accompanied by a cost increase of 68% in the friction coefficient. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.