Inkjet metrology II: resolved effects of ejection frequency, fluidic pressure, and droplet number on reproducible drop-on-demand dispensing.

We report highly reproducible gravimetric and optical measurements of microdroplets that lend insights into the fundamentals of drop-on-demand (DOD) printing. Baseline fluidic pressure within the DOD dispenser was controlled to within 0.02 hPa, enabling long-term stability in dispensed droplet mass with observed variations near 1% (RSD) for isobutanol. The gravimetric measurements were sensitive enough to detect and avoid unwanted effects from air bubbles within the dispenser. The gravimetric and optical velocity measurements enabled consistent determination of droplet kinetic energy that governed baseline behavior across the operational variables. Mass and velocity were influenced in a nonlinear manner by the frequency of droplet ejection, the fluidic pressure within the dispensing device, and the number of droplets dispensed in a burst. Resolved effects were attributable to several possible mechanisms including acoustic resonances, energy partitioning from systematic orifice refill dynamics, pressure wavelets created within the dispenser cavity during "first-drop" formation, and residual ring-down after last-drop emergence.