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In the last decades, the usage of inkjet printing has extended far beyond graphic arts covering a wide range of fields such as printed electronics or printed medicines. The latter application consists in manufacturing dosage forms by printing ink(s) which contains the active ingredient(s) onto appropriate substrates (paper, placebo pills...) which are then administrated to the patient. While such an approach opens routes to personalized medicines where the doses and drug combination are adjusted to each patient, it requires a very strict control of the printing process. Indeed, not only the position of the drops must be controlled but also their volume which is strongly modified by the presence of satellite drops.
While empirical knowledge has been accumulated about ink jettability for given devices only a few studies have focused on the formation of satellite drops providing no consensus and practical criterion to suppress such satellites [Hoath et al. PoF 2013]. Our work aims to fill this gap.
Practically, we used two piezo-based print-heads of the same geometry and printed 16 inks consisting of aqueous solutions of sugar and ethanol in different proportions. For each printable ink, we have varied the pulse length and voltage of the piezo signal and recorded drop formation processes with and without satellites. Initially the ejected liquid has always the shape of a main drop connected to the nozzle by a liquid ligament which quickly breaks off leaving the drop and its cylindrical tail freely falling. Two competing processes are thus simultaneously taking place: the recoil of the tail into the main drop due to capillary pressure gradient and the ligament pinch-off close to the drop. Our analysis suggests that the formation of satellite drops is kinetically driven and corresponds to cases where pinch-off is faster than recoil. To go further, we derive the recoiling time based on a modified Taylor velocity that accounts for viscous effects. For pinch-off, we demonstrate the existence of two regimes. While short tails pinch-off time increases with the tail length an opposite trend originating from the recoil flux is observed for long tails. This finding is of the highest practical importance since it enables the printing parameters to be rationally tuned for both short and long tails. Finally, we show that the comparison of the newly established theoretical recoil and pinch-off times successfully predict the formation of satellite drops.
