Fluid substances, particularly substances having differing viscosities, are combined and mixed by first segmenting a flow of a first fluid material to form a flowing segmented substance matrix containing a network of voids, such as by passing the flow through a plate having a plurality of passages therethrough and a second fluid substance is discharged into the segmented flow at a position which is substantially centrally disposed with respect to a cross-section of the segmented flow to combine the substance and additive, and then, the combined flow is mixed. The combined flow may be compressed and then mixed or may be compressed and then expanded prior to mixing. Additionally, a flow of a substance from a source such as an extruder may be partitioned into a plurality of streams and then, at least one of the streams is treated as specified above.