A valve assembly is disclosed with a housing having generally collinear inlet and outlet ports in the side walls thereof and an interior annular face about each of the inlet and outlet ports. The door included a number of rectangular polycarbonate slats in frame members. The number and distribution of sources is varied to provide the size and uniformity of the plasma field required to treat the substrate. The invention also relates to a method for preparing and using same as a modifier of the rheology of the oil phase of a topical cosmetic, dermopharmaceutical, or pharmaceutical composition.