Ink-jet delivery of particle suspensions by piezoelectric droplet ejectors

The volume and velocity of droplets ejected from a piezoelectric droplet generator, as used in ink-jet printing, have been studied for a range of concentrated suspensions of submicron alumina particles as a function of driving signal voltage, frequency, and peak shape. Drop velocity and volume are found to show a linear relation with driving voltage, but show a more complicated and periodic behavior with changing frequency and peak width. This periodic dependence is shown to be a function of the acoustic properties of the fluid-filled chamber in the droplet generator. However, a simple model considering propagation of pressure waves along the tubular actuator is not consistent with experimental data if the ends are modeled as step changes in acoustic impedance. By comparing the data with that in the literature, it is proposed that the acoustic boundary condition (impedance change) at the orifice where drops are ejected is a function of orifice size and extrapolates to a closed boundary condition, as the orifice diameter tends to zero. At equivalent driving signal frequencies, the drop volume ejected, normalized by actuator volume displacement, is shown to be a function of the Ohnesorge number of the orifice through which the drops are ejected. (C) 2005 American Institute of Physics.