Previous studies in the literature about the micromixer are mainly focused on mixing index and pressure drop. This is not enough to completely understand and explain the mass transfer mechanism in the mixing process. It is heuristic to understand the mass transfer mechanism by field synergy principle. Fig.8 indicates the local distributions of the synergy angle α of the microchannels. It is shown that the small α is more concentrated next to the gaps and baffles. According to Eqs (11) and (12), small α indicates high flow resistance. In comparison to three other types of cases, smaller α is achieved by the combination of gaps and baffles in micromixer G-B-G, which is in accordance with the pressure drop results in Fig 7(b).