Measurement of the oscillatory flow field inside tapered cylindrical inkjet nozzles using micro-particle image velocimetry

The flow field within a tapered, cylindrical, piezoelectrically actuated glass inkjet nozzle is captured using fluorescence micro-PIV (mu PIV) assisted by a novel, custom-designed PDMS micro-fabricated nozzle holder and a microsecond-resolution cyclic triggering system. The presented work overcomes key imaging challenges such as distortion from the refractive index mismatched curved glass-air interface and the typically large depth of field/correlation found in inkjet imaging set-ups. The PDMS holder permits fluorescence imaging of the seeded flow tracing particles with minimal distortion as the holder is refractive-index-matched with the glass nozzle. The cyclic triggering system allows visualization of the transient phases of a periodic droplet ejection event. The system utilizes an inverted microscope with an objective lens capable of producing a low depth of correlation of 12.25 A mu m. Double-frame images for A mu PIV were acquired beginning from the onset of droplet formation to study the flow field evolution during droplet formation and after droplet break-off. An oscillatory flow field was observed within the nozzle during the droplet ejection process which closely correlates with modelling results.