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Fluid filtration articles, including composite nonwoven fibrous webs, and methods of making and using such articles as gas or liquid filtration elements. The articles include a population of coarse microfibers having a population median diameter of at least 1 micrometer (μm) formed as a first layer,

Fluid filtration articles, including composite nonwoven fibrous webs, and methods of making and using such articles as gas or liquid filtration elements. The articles include a population of coarse microfibers having a population median diameter of at least 1 micrometer (μm) formed as a first layer, and a population of fine fibers having a population median diameter less than 10 μm formed as a second layer adjoining the first layer. At least one of the fiber populations may be oriented. In one implementation, the coarse microfibers and fine fibers are polymeric, the coarse microfibers have a population median diameter of at least 10 μm, and the fine fibers have a population median diameter less than 10 μm. In another implementation, the population of fine fibers has a population median diameter less than 1 μm. Optionally, one or both of the first and second layers may include particulates.

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1. A fluid filtration medium comprising: a population of microfibers having a population median diameter of at least 1μm formed as a first layer; anda population of sub-micrometer fibers having a population median diameter less than one micrometer (μm) formed as a second layer adjoining the first la

1. A fluid filtration medium comprising: a population of microfibers having a population median diameter of at least 1μm formed as a first layer; anda population of sub-micrometer fibers having a population median diameter less than one micrometer (μm) formed as a second layer adjoining the first layer,wherein at least one of the fiber populations is molecularly oriented. 2. The fluid filtration medium of claim 1, wherein the population of microfibers has a population median fiber diameter ranging from about 2 μm to about 100 μm, and wherein the population of sub-micrometer fibers has a population median fiber diameter ranging from about 0.2 μm to about 0.9 μm. 3. The fluid filtration medium of claim 1, wherein at least one of the population of sub-micrometer fibers and the population of microfibers comprises polymeric fibers, optionally wherein the polymeric fibers comprise polypropylene, polyethylene, polyester, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyurethane, polybutene, polylactic acid, polyvinyl alcohol, polyphenylene sulfide, polysulfone, fluid crystalline polymer, polyethylene-co-vinylacetate, polyacrylonitrile, cyclic polyolefin, polyoxymethylene, polyolefinic thermoplastic elastomers, or a combination thereof. 4. The fluid filtration medium of claim 1, wherein the first layer overlays the second layer. 5. The fluid filtration medium of claim 4, wherein the second layer is pleated. 6. The fluid filtration medium of claim 1, further comprising a porous support layer. 7. The fluid filtration medium of claim 6, wherein the porous support layer adjoins the second layer opposite the first layer. 8. The fluid filtration medium of claim 6, wherein the porous support layer adjoins the first layer opposite the second layer. 9. The fluid filtration medium of claim 6, wherein the porous support layer comprises a nonwoven fabric, a woven fabric, a knitted fabric, a foam layer, a screen, a porous film, a perforated film, an array of filaments, or a combination thereof. 10. The fluid filtration medium of claim 6, wherein the porous support layer comprises a web of bonded staple fibers, further wherein the porous support layer is bonded using thermal bonding, adhesive bonding, powdered binder, hydroentangling, needlepunching, calendering, or a combination thereof. 11. The fluid filtration medium of claim 6, wherein the porous support layer comprises microfibers, optionally wherein the microfibers forming the porous support layer are compositionally the same as the population of microfibers that forms the first layer. 12. The fluid filtration medium of claim 1, wherein at least one of the first layer and the second layer further comprises a plurality of particulates, optionally wherein the plurality of particulates is selected from the group consisting of an absorbent, an adsorbent, activated carbon, an anion exchange resin, a cation exchange resin, a molecular sieve, or a combination thereof. 13. The fluid filtration medium of claim 12, wherein at least one of the first layer and the second layer exhibits a gradient of fiber population median diameter, a gradient of particulate population mean diameter, a gradient of particulate concentration, or a combination thereof. 14. The fluid filtration medium of claim 1, wherein the first layer and the second layer are arranged into a stack on a porous support, further wherein the first layer adjoins the porous support. 15. The fluid filtration medium of claim 1, wherein the first layer and the second layer are wound into a three-dimensional geometric shape having a hollow core, wherein the second layer forms an inner layer adjacent to the hollow core, and the first layer forms an outer layer distal from the hollow core. 16. A method of using the fluid filtration medium of claim 1, comprising passing a permeating fluid through the fluid filtration medium, wherein the permeating fluid passes through the first layer before passing through the second layer. 17. A method of using the fluid filtration medium of claim 1, comprising passing a permeating fluid through the fluid filtration medium, wherein the second layer is pleated, and wherein the permeating fluid passes through the second layer before passing through the first layer. 18. A method of making a fluid filtration medium, comprising: a. forming a first layer comprising a population of microfibers having a population median fiber diameter of at least 1 μm; andb. forming a second layer impinging on the first layer, the second layer comprising a population of sub-micrometer fibers having a population median fiber diameter of less than 1 μm,wherein at least one of the fiber populations is molecularly oriented. 19. The method of claim 18, further comprising forming a region between the first layer and the second layer wherein at least a portion of the population of microfibers is intermixed with at least a portion of the population of sub-micrometer fibers. 20. The method of claim 18, further comprising adding a porous support layer adjoining the second layer opposite the first layer, optionally wherein the porous support layer comprises a web of bonded staple fibers, where the porous support layer is bonded using thermal bonding, adhesive bonding, powdered binder, hydroentangling, needlepunching, calendering, or a combination thereof. 21. The method of claim 18, wherein forming a population of sub-micrometer fibers having a median fiber diameter of less than 1 μm comprises melt blowing, melt spinning, electrospinning, plexifilament formation, gas jet fibrillation, or a combination thereof. 22. The method of claim 18, wherein forming a population of microfibers having a median fiber diameter of at least 1 μm comprises melt blowing, melt spinning, filament extrusion, plexifilament formation, or a combination thereof. 23. The method of claim 18, wherein combining the sub-micrometer and microfibers comprises mixing fiber streams, hydroentangling, wet forming, plexifilament formation, or a combination thereof. 24. A composite nonwoven fibrous article comprising: a population of coarse polymeric microfibers having a population median diameter of at least 1 μm formed as a first layer; anda population of fine polymeric microfibers having a population median diameter less than 10 μm formed as a second layer adjoining the first layer,wherein at least one of the fiber populations is molecularly oriented. 25. The article of claim 24, wherein the population of fine microfibers comprises a population of sub-micrometer fibers having a population median diameter less than 1 μm. 26. The article of claim 24, wherein the first layer further comprises a plurality of particulates, optionally wherein the plurality of particulates is selected from the group consisting of an absorbent, an adsorbent, activated carbon, an anion exchange resin, a cation exchange resin, a molecular sieve, or a combination thereof. 27. A method of making the article of claim 24, comprising: a. forming the first layer comprising the population of coarse polymeric microfibers having a population median fiber diameter of at least 1 μm;b. forming the second layer comprising the population of fine polymeric microfibers having a population median fiber diameter of less than 10 μm;c. molecularly orienting one or both of the first and second layer; andd. optionally bonding the first layer to the second layer. 28. A fluid filtration article comprising: a population of microfibers having a population median diameter of at least 1 μm formed as a first layer;a population of ultrafine microfibers having a population median diameter less than 2 μm formed as a second layer adjoining the first layer, wherein at least one of the fiber populations is molecularly oriented; anda fluid-impermeable housing surrounding the first and second layers, wherein the housing comprises at least one fluid inlet in fluid communication with the first layer, and at least one fluid outlet in fluid communication with the second layer.