In one embodiment, a 3D printing system includes: a stage on which a substrate is disposed; first and second syringe pumps; first and second syringes; a hydrodynamic flow focusing nozzle having a central channel coupled to the first syringe to receive a printing ink and two side channels on two sides of the central channel and coupled to the second syringe to receive a sheath fluid to pinch the central channel; and a pulse generator to apply an electric potential between the hydrodynamic flow focusing nozzle and the substrate to deposit the printing ink on the substrate on-demand and control ejection frequency of the printing ink. The first syringe pump is controllable to adjust a printing ink flow rate of the printing ink to deposit the printing ink onto the substrate. The second syringe pump is controllable to adjust a sheath fluid flow rate of the sheath fluid.