An apparatus and system for stirring liquid inside a flow cell. In one implementation, the apparatus includes a rotatable disc configured to receive liquid at a top side of the disc and distribute the liquid substantially evenly around a periphery of the flow cell. The disc has a triangular cross sectional area. The apparatus may further include a set of fins attached to a bottom side of the disc, wherein the set of fins is configured to draw the liquid from the periphery of the flow cell into the center of the flow cell.