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Systems and methods for forming fiber webs, including those suitable for use as filter media and battery separators, are provided. In some embodiments, the systems and methods may involve the use of one or more fiber mixtures to form a fiber web. The fiber mixtures may flow in different portions of

Systems and methods for forming fiber webs, including those suitable for use as filter media and battery separators, are provided. In some embodiments, the systems and methods may involve the use of one or more fiber mixtures to form a fiber web. The fiber mixtures may flow in different portions of a system for forming a fiber web that may be separated by a lamella, and may join at a fiber web forming zone to produce a fiber web having multiple layers. The amount of mixing of the fiber mixtures at or near the fiber web forming zone may be controlled to produce fiber webs having different structural and/or performance characteristics. In some embodiments, the systems and methods described herein can be used to form fiber webs having a gradient in a property across a portion of, or the entire, thickness of the fiber web.

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1. A method of forming a fiber web, comprising: introducing a first fiber mixture and a second fiber mixture into a flow zone of a system for forming a fiber web;flowing the first fiber mixture in a lower portion of the flow zone;flowing the second fiber mixture in an upper portion of the flow zone,

1. A method of forming a fiber web, comprising: introducing a first fiber mixture and a second fiber mixture into a flow zone of a system for forming a fiber web;flowing the first fiber mixture in a lower portion of the flow zone;flowing the second fiber mixture in an upper portion of the flow zone, wherein the lower and upper portions of the flow zone are separated by a lamella;disrupting laminar flow of a fiber mixture in the lower portion or upper portion of the flow zone using a flow impediment positioned in the lower portion or upper portion of the flow zone, respectively;collecting fibers from the first and second fiber mixtures in a fiber web forming zone; andforming a fiber web comprising fibers from the first and second fiber mixtures, wherein the fiber web comprises a gradient in at least one of fiber diameter, fiber type, fiber composition, fiber length, pore size, material density, basis weight, solidity, a proportion of a component, hydrophilicity/hydrophobicity, and conductivity across a portion, or all, of a thickness of the fiber web. 2. The method of claim 1, wherein the lamella positioned in the flow zone and separating the flow zone into a lower portion and an upper portion is a primary lamella, wherein the flow impediment is a secondary lamella, and wherein the secondary lamella is positioned in one of the lower and upper portions of the flow zone and positioned to divide portions of the first fiber mixture, or portions of the second fiber mixture, in the flow zone. 3. The method of claim 2, wherein the primary lamella and/or secondary lamella is connected to a control system to control the height of the primary and/or secondary lamella in the flow zone. 4. The method of claim 3, wherein the control system is an electromechanical control system. 5. The method of claim 2, comprising changing the height of the primary and/or secondary lamella within the flow zone. 6. The method of claim 2, comprising changing the height of the primary and/or secondary lamella within the flow zone while the first and second fiber mixtures are flowing in the flow zone. 7. The method of claim 2, comprising introducing a third fiber mixture into the flow zone, the system comprising a third lamella that separates the flow zone into three portions, the third lamella positioned to divide the third fiber mixture from the first and/or second fiber mixtures in the flow zone. 8. The method of claim 1, wherein the flow impediment is a disruptive member. 9. The method of claim 8, wherein the disruptive member comprises one or more openings. 10. The method of claim 8, wherein the disruptive member is a circular roll or a wheel. 11. The method of claim 10, comprising a motor connected to the disruptive member for controlling a rotational rate of the disruptive member. 12. The method of claim 8, comprising flowing the first or second fiber mixtures past the disruptive member and causing the disruptive member to rotate at least in part by the flow of the fiber mixtures. 13. The method of claim 8, comprising controlling a rotational rate of the disruptive member at least in part using a motor. 14. The method of claim 8, comprising changing the position and/or configuration of the disruptive member within the flow zone while the first and second fiber mixtures are flowing in the flow zone. 15. The method of claim 1, wherein the lamella positioned in the flow zone and separating the flow zone into a lower portion and an upper portion is a primary lamella, and wherein the flow impediment comprises a textured surface portion comprising a plurality of surface features associated with the primary lamella. 16. The method of claim 15, wherein the textured surface portion comprises a plurality of protrusions. 17. The method of claim 15, wherein the textured surface portion comprises a plurality of indentations. 18. The method of claim 15, wherein the plurality of features have a height or a depth of at least 5 mm. 19. The method of claim 15, wherein the plurality of features have a lateral dimension of at least 5 mm. 20. The method of claim 15, wherein both of the top and bottom surfaces of the lamella comprises a textured surface portion. 21. The method of claim 15, wherein at least 20% of the area of the top or bottom surface of the lamella comprises a textured surface portion. 22. The method of claim 1, wherein the lamella positioned in the flow zone and separating the flow zone into a lower portion and an upper portion is a primary lamella, and wherein the flow impediment comprises a variable volume member associated with the primary lamella. 23. The method of claim 22, wherein the variable volume member contains a liquid. 24. The method of claim 22, wherein the variable volume member contains a gas. 25. The method of claim 22, comprising changing the volume of the variable volume member by at least 2 times upon expansion or contraction. 26. The method of claim 22, comprising changing the volume of the variable volume member while the first and second fiber mixtures are flowing in the flow zone. 27. The method of claim 22, comprising changing the volume of the variable volume member at a frequency of at least 10 cycles/min. 28. The method of claim 22, wherein the variable volume member is expanded in the upper portion of the flow zone. 29. The method of claim 22, wherein the variable volume member is expanded in the lower portion of the flow zone. 30. The method of claim 22, wherein the variable volume member is expanded to have a volume of greater than about 10 cm3. 31. The method of claim 22, wherein the flow impediment creates intermixing of the first and second fiber mixtures at a fiber web forming zone. 32. The method of claim 1, wherein the first fiber mixture comprises a plurality of first fibers and the second fiber mixture comprises a plurality of second fibers, and wherein the first and second fibers are the same. 33. The method of claim 1, wherein the first fiber mixture comprises a plurality of first fibers and the second fiber mixture comprises a plurality of second fibers, and wherein the first and second fibers are different. 34. The method of claim 1, comprising forming a fiber web comprising a gradient in at least one of efficiency, dust holding capacity, pressure drop, air permeability, and porosity across a portion, or all, of a thickness of the fiber web. 35. The method of claim 1, wherein disrupting laminar flow creates intermixing of the first and second fiber mixtures at a fiber web forming zone. 36. A method as in claim 1, wherein the fiber web comprises a gradient in at least one of fiber diameter, fiber type, fiber composition, fiber length, pore size, and solidity across a portion, or all, of a thickness of the fiber web. 37. A method of forming a fiber web, comprising: introducing a first fiber mixture and a second fiber mixture into a flow zone of a system for forming a fiber web;flowing the first fiber mixture in a lower portion of the flow zone;flowing the second fiber mixture in an upper portion of the flow zone, wherein the lower and upper portions of the flow zone are separated by a primary lamella;disrupting laminar flow of a fiber mixture in the lower portion or upper portion of the flow zone using a flow impediment positioned in the lower portion or upper portion of the flow zone, respectively, wherein the flow impediment is a secondary lamella, and wherein the secondary lamella is positioned to divide portions of the first fiber mixture, or portions of the second fiber mixture, respectively, in the flow zone;collecting fibers from the first and second fiber mixtures in a fiber web forming zone; andforming a fiber web comprising fibers from the first and second fiber mixtures. 38. A method of forming a fiber web, comprising: introducing a first fiber mixture and a second fiber mixture into a flow zone of a system for forming a fiber web;flowing the first fiber mixture in a lower portion of the flow zone;flowing the second fiber mixture in an upper portion of the flow zone, wherein the lower and upper portions of the flow zone are separated by a lamella;disrupting laminar flow of a fiber mixture in the lower portion or upper portion of the flow zone using a flow impediment positioned in the lower portion or upper portion of the flow zone, respectively, wherein the flow impediment comprises a variable volume member associated with the lamella;collecting fibers from the first and second fiber mixtures in a fiber web forming zone; and forming a fiber web comprising fibers from the first and second fiber mixtures. 39. A method of forming a fiber web, comprising: introducing a first fiber mixture and a second fiber mixture into a flow zone of a system for forming a fiber web;flowing the first fiber mixture in a lower portion of the flow zone;flowing the second fiber mixture in an upper portion of the flow zone, wherein the lower and upper portions of the flow zone are separated by a lamella;disrupting laminar flow of a fiber mixture in the lower portion or upper portion of the flow zone using a flow impediment positioned in the lower portion or upper portion of the flow zone, respectively;collecting fibers from the first and second fiber mixtures in a fiber web forming zone; and forming a fiber web comprising fibers from the first and second fiber mixtures, wherein the fiber web comprising a gradient in at least one of efficiency, dust holding capacity, pressure drop, air permeability, and porosity across a portion, or all, of a thickness of the fiber web. 40. A method of forming a fiber web, comprising: introducing a first fiber mixture and a second fiber mixture into a flow zone of a system for forming a fiber web;flowing the first fiber mixture in a lower portion of the flow zone;flowing the second fiber mixture in an upper portion of the flow zone, wherein the lower and upper portions of the flow zone are separated by a lamella;disrupting laminar flow of a fiber mixture in the lower portion or upper portion of the flow zone using a flow impediment positioned in the lower portion or upper portion of the flow zone, respectively, wherein the flow impediment is a disruptive member, and wherein the disruptive member comprises one or more openings;collecting fibers from the first and second fiber mixtures in a fiber web forming zone; and forming a fiber web comprising fibers from the first and second fiber mixtures.