The present invention provides a method of controlling a chemical process, the method comprising the steps of: (i) providing a laminar flow of a first fluid, the first fluid providing a first reagent or one or more precursor thereof), a laminar flow of a second fluid, the second fluid providing a second reagent (or one or more precursor thereof) and a laminar flow of barrier fluid; and (ii) causing the first and barrier fluids to contact one another so that the barrier fluid forms a barrier between the first reagent (or one or more precursor thereof) and the second reagent (or one or more precursor thereof) wherein step (ii) comprises forming segments of first fluid encased or sandwiched by barrier fluid, the segments being surrounded by the second fluid, and the barrier fluid is permeable to one or both of the first and second reagents. Devices for performing the method of the present invention are also provided.