A class of designs is provided for a mixer in micro reactors where the design principle includes at least one injection zone (410) in a continuous flow path where at least two fluids achieve initial upstream contact and an effective mixing zone (i.e. adequate flow of fluids and optimal pressure drop) containing a series of mixer elements (430) in the path. Each mixer element is preferably designed with a chamber at each end in which an obstacle (450) is placed (thereby reducing the typical inner dimension of the chamber) and with optional restrictions (460) in the channel segments. The obstacles are preferably cylindrical pillars but can have any geometry within a range of dimensions and may be in series or parallel along the flow path to provide the desired flow-rate, mixing and pressure-drop. The injection zone may have two or more interfaces and may, include one or more cores to control fluids before mixing.