The invention provides a mixing system comprising the following: A) at least one extensional flow mixer comprising: a generally open and hollow body having a contoured outer surface and having: a single entrance port and a single exit port; a design for compressing a bulk stream, and a design for broadening the bulk stream and the at least one injected additive stream; B) a flow conductor; and C) a primary additive stream injector, as described herein; and wherein the extensional flow mixer is followed by D) a first helical static mixing element that is at least one half “flow conductor diameter (D1)” downstream of the exit port of the extensional flow mixer; and wherein the mixing system comprises at least four helical static mixing elements, placed such that the leading edge of the first helical static mixing element is located perpendicular to the main axis (major axis) of the exit port of the extensional flow mixer.