A printing system includes a liquid drop ejector, a fluid passage, and a fluid flow source. The liquid drop ejector is operable to form liquid drops having a plurality of volumes moving along a first path. The fluid passage includes a wall. A source of acoustic energy is associated with the wall. A fluid flow source is associated with the passage and is configured to provide a fluid flow through the passage. Interaction of the fluid flow and the liquid drops causes liquids drops having one of the plurality of volumes to begin moving along a second path.