Abstract: A static mixer or a flow reactor comprising a static mixer for mixing streams of fluid transported through or circulated in a flow reactor is provided. A static mixer element reduces the size of gas bubbles in a liquid for installation in a flow channel, and comprises, a first mixer unit and a second and adjacent mixer unit, which mixer units are displaced relative to each other in the flow direction, such that a through opening in the first mixer unit at least partly faces a surface of the second mixer unit thereby forcing the fluid flow to change direction. Adjacent mixer units have substantially the same height or thickness, and are substantially parallel having a distance between them that may be near zero or up to 10 times the thickness of a mixer unit.