A study of ejection modes for pulsed-DC electrohydrodynamic inkjet printing

For electrohydrodynamic-driven drop-on-demand printing techniques, either continuous- or pulsed-DC voltages can generate drops. To generate uniform micro-drops for high-resolution printing, the pulsed-DC voltage method is superior to continuous-DC voltage methods because of its controllability. Voltage amplitude and duration (or duty cycle or relaxation time, τ) are the primary parameters affecting the performance of drop-generation or ejection. When charge accumulates on the fluid meniscus at the nozzle, a drop is ejected. Charge density is the product of voltage (amplitude) and duration. In theory, charge densities from low-amplitude, long-duration voltages are equivalent to those of large amplitude and short duration. However, we demonstrate that drop-ejection mode differs significantly, despite equivalent products when voltage amplitude and duration change. At various voltage amplitudes and durations, four ejection main modes are identified: microdripping, spindle, string-jet, and spray modes. Longer voltage durations yield excessively large, spindle, string-jet, and spray modes. Conversely, no ejection is observed for short voltage durations. The microdripping mode, most desirable for uniform and high-resolution printing, appears for the narrowed range of duration under given pulsed-voltage. The identification map has been constructed for these modes; this map can be used as a guideline to yield a stable microdripping mode for high quality printing.