The flow structure inside a microfabricated inkjet printhead

A micrometer resolution particle image velocimetry system has been adapted to measure instantaneous velocity fields in an inkjet printhead, The technique uses 700-nm-diameter fluorescent now tracing particles, a pulsed Nd:YAG laser, an epi-fluorescent microscope, and a cooled interline transfer charge-coupled device camera to record images of flow tracing particles at two known instances in time. Instantaneous velocity vector fields are obtained with spatial resolutions of 5-10 mu m and temporal resolutions of 2-5 mu s. The relationship between instantaneous velocity fields is compared to instantaneous shapes of the meniscus, The flow in the nozzle is highly unsteady and characterized by a maximum velocity of 8 ms(-1), Reynolds numbers of Re = 500, and accelerations of up to 70 000 times gravity (i.e., 70 000 g), Since the flow field is periodic for each ejection cycle, the instantaneous measurements can be phased averaged to determine the evolution of the average flow field. The ejection cycle period is 500 mu s, and consists of four primary phases: infusion, inversion, ejection, and relaxation. During infusion, the actuator plate is deflected downward creating a low pressure that draws fluid into the inkjet cavity through the orifice and pulls the meniscus into the cavity through the nozzle, The meniscus grows, begins to decrease in size, and then deforms in shape, becoming inverted for approximately 6 mu s. The meniscus exits the cavity through the nozzle during droplet ejection. During relaxation, the flow undergoes viscously-damped oscillations, and reaches equilibrium before the next ejection cycle begins. [453].