To form a smooth gradient and a preparatory liquid chromatographic system having a plurality of syringe type pumps, solvents are alternately drawn and inserted into each cylinder. The solvents are alternately drawn into and through a long narrow path formed of tubing to create axial mixing and spreading out of the interface and then are drawn into the cylinders during the refill stroke of the piston at a sufficiently high velocity to create turbulence. The drawing of the solvents into one side of the cylinder a radial distance from the center creates circular eddy currents.