A nozzle, die, apparatus, system and method for forming a fiber population having a median diameter less than one micrometer, and nonwoven fibrous webs including a population of such sub-micrometer fibers. The nozzle includes a first conduit having a first terminal end, a second conduit positioned coaxially around the first conduit and having a second terminal end proximate the first terminal end, wherein the first and second conduit form an annular channel between the first and second conduit, and additionally wherein the first terminal end extends axially outwardly beyond the second terminal end. The die includes at least one such nozzle, and the apparatus and system include at least one such die. Methods of making nonwoven fibrous webs including a population of sub-micrometer fibers, and articles including such nonwoven fibrous webs, are also disclosed.

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