a diffuser comprises a conduit having a cross - sectional area that increases in a direction fluid flow . in one embodiment , the diffuser is used to reduce the incidence and severity of flow fluctuations that occur in an electrostatic deposition apparatus . in some embodiments , the diffuser includes one or more flow control features . a first flow - control feature comprises one or more appropriately - shaped annular slits through which fluid having a greater momentum than a primary fluid moving through the diffuser is injected into the “ boundary layer ” near the wall of the diffuser . a second flow control feature comprises one or more annular slits or , alternatively , slots or holes that are disposed at appropriate locations around the circumference of the diffuser through which a portion of fluid flowing in the boundary layer is removed . boundary - layer flow removal is effected , in one embodiment , by creating a pressure differential across such annular slit or slots . among other benefits , such flow control features reduce any tendencies for flow separation of the primary fluid in the diffuser .