The secondary fluid provides a cooler, insulating layer (or film) along the surface that acts as a heat sink, lowering the mean temperature in the boundary layer.[27] Since the secondary fluid is injected onto the surface at discrete holes on the surface, a portion of the secondary fluid is blown off the surface (especially at high momentum ratios of injected air to cross flow), decreasing the effectiveness of the film cooling process.[27] In plasma-actuated heat transfer, EFA plasma actuators are used to control the secondary fluid via a dynamic force which promotes attachment of the secondary fluid to the hot surface and improves the effectiveness of the film cooling.[2][28][29][30]