Mixing of fluids

A micro-reactor system for contacting fluids is described. The system comprises a first microfluidic channel structure for guiding a first fluid to at least one output nozzle thus generating a first sub-flow, a second microfluidic channel structure for guiding a second fluid to at least a second output nozzle thus generating a second sub-flow, said first output nozzle being aligned with said second output nozzle and arranged for contacting the first sub-flow and the second sub-flow. The micro-reactor comprises at least a third microfluidic channel structure for at least a third,inert,fluid generating at least a third sub-flow arranged to be positioned adjacent at least the first and/or the second sub- flows so as to act as a wall between said first and/or second sub-flows.