Can consecutive droplet deposition yield uniform liquid films?*

Ink-jet printing is being used to manufacture highly customizable electronic components through the successional deposition of overlapping droplets of conductive ink onto a substrate. For vanishing contact angles, the fluid drops merge together on the substrate to form a wide, uniform thread. However, in practical operating regimes involving moderate contact angles, the fluid is redistributed to form non-uniform patterns, characterized by the development of an initial wide bulge (larger than any individual drop). This ``head'' is sometimes followed by a sequence of secondary bulges connected by narrow rivulets. We conduct new experiments with high-speed imaging to examine the morphological development of the thread resulting from the deposition of a moderate number of successive micro-scale drops ($<80$) that may form up to two secondary bulges. We demonstrate that the spatial structure of the ink formation resulting from this complex deposition process can be predicted quantitatively with a simple physical model that incorporates contact angle hysteresis. The head formation occurs robustly due to an inviscid mechanism, while the secondary bulge formation arises from viscous effects.