A composite
filtration medium web of fibers containing a controlled dispersion of a mixture of sub-micron and greater than sub-micron
diameter polymeric fibers is described. The
filtration medium is made by a two dimensional array of cells, each of which produces a single
high velocity two-phase solids-gas jet of discontinuous fibers entrained in air. The cells are arranged so that the individual jets are induced to collide in flight with neighboring jets in their region of
fiber formation, to cause the individual nascent fibers of adjacent jets to deform and become entangled with and partially
wrap around each other at
high velocity and in a localized fine scale manner before they have had an opportunity to cool to a relatively rigid state. The cells are individually adjusted to control the mean diameters, lengths and trajectories of the fibers they produce. Certain cells are adjusted to generate a significant percentage of fibers having diameters less than one micron
diameter, and which are relatively shorter in length and certain other cells are adjusted to generate a significant percentage of structure-forming reinforcing fibers having diameters greater than one micron
diameter which are relatively longer in length. By employing appropriate close positioning and orientation of the cells in the array, the sub-micron fibers are caused to promptly entangle with and partially
wrap around the larger reinforcing fibers. The larger fibers thereby trap and immobilize the sub-micron diameter fibers in the region of formation, to minimize the tendency of sub-micron diameter fibers to clump,
agglomerate, or
rope together in flight. Also, the larger fibers in flight are made to form a protective curtain to prevent the sub-micron fibers from being carried off by stray air currents.