A droplet forming flow cytometer system (1) allows high speed processing without the need for high oscillator drive powers through the inclusion of an oscillator or piezoelectric crystal (10) within the nozzle volume (3) and directly coupled to the sheath fluid. The nozzle container (27) continuously converges so as to amplify unidirectional oscillations (11) which are transmitted as pressure waves through the nozzle volume (3) to the nozzle exit so as to form droplets from the fluid jet. A variation in substance concentration is achieved through a movable substance introduction port (9) which is positioned within a convergence zone (32) to vary the relative concentration of substance to sheath fluid while still maintaining optimal laminar flow conditions. This variation may be automatically controlled through a sensor and controller configuration.