Secondary Tail Formation and Breakup in Piezoacoustic Inkjet Printing: Femtoliter Droplets Captured in Flight

Abstract

The role of meniscus motion and ink viscosity in the formation of a secondary tail and its breakup are studied experimentally during the picoliter-droplet formation process of a MEMS piezoacoustic inkjet print head using laser-induced 8-ns single-flash stroboscopic imaging with a temporal resolution of 100 ns. It is found that the formation of the secondary tail is driven by meniscus motion and that the secondary tail forms reproducibly between the primary tail and the meniscus in the final microseconds before pinchoff. We demonstrate that the stability of the secondary tail can be controlled through the motion of the meniscus after the primary tail has formed. A 4 times increase in stretching rate results in a 2.2 times increase in the secondary-tail length and a 3 times higher number of femtoliter satellites. Furthermore, as expected for Rayleigh breakup, a 43% increase in ink viscosity is found to increase the secondary-tail length by 50%. Finally, it is found that, during inkjet printing, the secondary tail cascades into tertiary and quaternary tails. We show that the formation of higher-order tails is irreproducible and therefore driven by noise. The formation of thicker secondary and thinner higher-order tails results in a bimodal satellite size distribution, where the secondary satellites with a volume greater than or equal to 4 fL are located closer to the primarytail droplet, while satellites with a volume less than 4 fL are located closer to the nozzle. The main findings of the present work, that the stability of the secondary tail decreases with a decrease in stretching rate and ink viscosity, can be employed in the inkjet-printing community for waveform design to minimize internal contamination of inkjet printers.

Classifications and Applications of Inkjet Printing Technology: A Review

“…However, low-velocity satellite droplets have poor directionality and may land on an arbitrary spot on the substrate, which reduces the print quality. Multipulse waveforms are needed to prevent satellite droplets and maintain the single primary droplet [41][42][43][44][45][46]. Shin et al [41] applied a double waveform with two square pulses (unipolar M-shaped) to the actuator, which effectively ejected the single main droplet without forming satellites.…”

The retraction of jetted slender viscoelastic liquid filaments

“…A typical inkjet printhead primarily consists of an ink reservoir, a piezo-acoustic transducer and a dispensing nozzle (Wijshoff 2010), while the most simple driving waveform is a monopolar trapezoidal pulse (Castrejón-Pita et al 2008), with a pulse width equal to half the period corresponding to the resonance frequency. Inkjet printheads are usually operated in the 'pull-push' mode, where the liquid is first pulled into the nozzle during the rise time of the trapezoidal pulse and then pushed out during the fall time of the pulse (Fraters et al 2020). This results in the creation of a slender liquid jet of finite length and after pinch-off from the nozzle, a finite liquid ligament with a relatively large head droplet and a long tail.…”

“…As the ligament is travelling towards the substrate, the tail retracts into the head droplet due to surface tension. However, during such motion, the tail may also break up due to the Rayleigh-Plateau instability (Fraters et al 2020). This breakup leads to the formation of satellite droplets, which travel at a velocity lower than that of the head droplet.…”

“…The detrimental effect of the formation of satellite droplets has resulted in an increased focus on the development of techniques to suppress such satellites. These techniques are either based on changing the driving waveform (Dong, Carr & Morris 2006;Fraters et al 2020) or modifying the properties of the ink. For the latter, since the satellite droplets primarily result from a Rayleigh-Plateau instability of the retracting tail, an obvious choice is to increase the viscosity of the ink in order to stabilize the tail filament (Notz & Basaran 2004;Castrejón-Pita, Castrejón-Pita & Hutchings 2012;Driessen et al 2013;Anthony et al 2019;Planchette et al 2019;Wang et al 2019).…”

A multi parametric measurement and control system, implemented on flexible substrates with printing technologies

“…By increasing voltage amplitude (90 V for 15 us), as discussed above droplet speed increases; this leads to the tail not being able to catch up with the main droplet mid-air. The tail then breaks up into a number of satellite droplets because of Rayleigh -Plateau instability [76] (Figure 3.17b), their number and volume being heavily influenced by ink physical properties and driving pulse morphology. These satellite droplets are considered unwanted phenomena when developing a welldefined geometry; they follow a completely different flight path, leading to printed geometries that do not correspond to the initial design.…”