Role of surfactants in droplet formation in piezoacoustic inkjet printing across microsecond-to-second timescales

In piezoacoustic drop-on-demand inkjet printing, a single droplet is produced for each piezoelectric actuator driving pulse. This droplet is typically multicomponent, including surfactants to control the spreading and drying of the droplet on the substrate. However, the role of these surfactants in the droplet formation process remains rather elusive. Surfactant concentration gradients may manifest themselves across microsecond-to-second timescales, spanning both the rapid ejection of ink from the nozzle exit and the comparatively slower idling timescale governing the firing of successive droplets. In the present work, we study the influence of surfactants on droplet formation across 6 orders of magnitude in time. To this end, we visualize the microsecond droplet formation process using stroboscopic 8 ns laser-induced fluorescence microscopy while we vary the nozzle idle time. Our results show that increasing the idle time up to $\mathcal{O}$(1) s affects only the breakup dynamics of the inkjet but not its velocity. By contrast, for idle times greater than $\mathcal{O}$(1) s, both the breakup dynamics are altered and the velocity of the inkjet increases. We show that the increased velocity results from a decreased surface tension of the ejected droplet, which we observed from the shape oscillations of the jetted droplets in flight. The measured decrease in surface tension is surprising as the microsecond timescale of droplet formation is much faster than the typical millisecond-to-second timescale of surfactant adsorption. By varying the bulk surfactant concentration, we show that the fast decrease in surface tension results from a local surfactant concentration increase to more than 200 times the critical micelle concentration. Numerical simulations then show that the evaporation-driven increased surfactant concentration present at the nozzle exit results in the surface of the droplet being fully coated during its ejection. Altogether, our results suggest that a local high concentration of surfactant allows surfactant adsorption to the interface of an inkjet on the microsecond-to-millisecond timescale, which is much faster than the typical millisecond-to-second timescale associated with surfactant adsorption.