A tray for use in a mass transfer column to facilitate interaction between fluids when they are flowing within the mass transfer column, said tray comprising: a tray deck having an upper surface; an inlet area on the tray deck for receiving a downward flow of liquid onto the upper surface of the tray deck; a plurality of apertures distributed across an area of the tray deck and extending through the tray deck to permit upward passage of fluid through the tray deck for interaction with the liquid after it leaves the inlet area and flows across and above the area of the tray deck on which the apertures are distributed; an outlet positioned remotely from the inlet area on the tray deck for allowing removal of liquid from the upper surface after it has flowed from the inlet area and interacted with the fluid passing upwardly through the apertures in said area; a downcomer extending downwardly from the outlet to receive liquid when it enters the outlet and then convey it downwardly; and a plurality of barriers positioned between the inlet area and the outlet and extending upwardly from the upper surface of the tray deck and across a flow path of the liquid when it flows from the inlet area to the outlet, wherein said barriers each comprise pickets for impeding the flow of the liquid as it flows from the inlet area to the outlet, wherein said pickets are rectilinear and are uniformly sized, wherein adjacent ones of the pickets are spaced apart along some of their height to form openings in the spacing between the adjacent ones of the pickets for allowing passage of the liquid successively through the barriers as it flows from the inlet area to the outlet, wherein the tray is a single-pass tray and the inlet area and the outlet are located at opposite ends of the tray deck, and wherein said pickets in adjacent ones of the barriers are offset from each other. a tray deck having an upper surface; an inlet area on the tray deck for receiving a downward flow of liquid onto the upper surface of the tray deck; a plurality of apertures distributed across an area of the tray deck and extending through the tray deck to permit upward passage of fluid through the tray deck for interaction with the liquid after it leaves the inlet area and flows across and above the area of the tray deck on which the apertures are distributed; an outlet positioned remotely from the inlet area on the tray deck for allowing removal of liquid from the upper surface after it has flowed from the inlet area and interacted with the fluid passing upwardly through the apertures in said area; a downcomer extending downwardly from the outlet to receive liquid when it enters the outlet and then convey it downwardly; and a plurality of barriers positioned between the inlet area and the outlet and extending upwardly from the upper surface of the tray deck and across a flow path of the liquid when it flows from the inlet area to the outlet, wherein said barriers each comprise pickets for impeding the flow of the liquid as it flows from the inlet area to the outlet, wherein said pickets are rectilinear and are uniformly sized, wherein adjacent ones of the pickets are spaced apart along some of their height to form openings in the spacing between the adjacent ones of the pickets for allowing passage of the liquid successively through the barriers as it flows from the inlet area to the outlet, wherein the tray is a single-pass tray and the inlet area and the outlet are located at opposite ends of the tray deck, and wherein said pickets in adjacent ones of the barriers are offset from each other.