Why consecutive droplet deposition cannot yield liquid lines of uniform depth

Microdroplet deposition is a promising technology spanning applications from tissue engineering to microelectronics. New high-speed imaging measurements reveal that sequential linear deposition of overlapping droplets leads to striking non-uniform morphologies for moderate contact angles. A simple physical model confirms that surface tension causes the droplets to merge, contact-angle hysteresis underlies the initial non-uniformity, but viscous effects cause subsequent periodic bulges. We find excellent agreement between the experiments and model, which captures the dynamics drop by drop.