B. Derby, Y. Liu
University of Manchester,
Keywords: inkjet, ink desigh, printability, Ohnesorge number, Weber number
Summary:The design of inks for inkjet delivery is a key step in developing printable materials for functional applications. The key principle is that the ink is capable of producing stable single drops over a wide range of printing conditions. Earlier work has identified the dimensionless Ohnesorge number (sometimes presented as its inverse Z = 1/Oh) as a key indicator for ink printability. Previous authors have proposed a printable range of Oh that is constrained by an upper boumd of Oh = 1, above which viscous dissipation and surface tension is too great to allow drop formation and a lower bound in the range Oh = 0.05 to Oh = 0.1 below which satellite drops form. Recently this has been shown to be too simple a picture and that satellite formation is also linked to drop velocity, characterised by the Weber number. Here we present experimental data obtained using two inkjet droplet generators, with different actuation mechanisms, to demonstrate that there is indeed a window of printability defined by a combination of the Ohnesorge number (which is independent of fluid velocity) and the Weber number. It is shown that the upper bound for Oh appears to be consistent with earlier work but that it is possible to successfully generate stable individual drops at lower values of Oh than previously proposed. However, the maximum Weber number for stable single drop generation decreases as the value of Oh decreases and because drop velocity is an important practical consideration for a usable ink higher values of Oh will be more suitable for ink design. Investigations of drop velocity also show that larger values of Oh produce a more stable drop volume as a fumction of drop velocity and hence inkjet actuation pulse voltage.