Anisotropic wetting and final shape of droplets impacting on micropillars with non-vertical lateral walls

The control of the droplet shape during the impact on a solid surface is of relevance for several practical applications, such as inkjet printing technologies. Although several studies have reported factors affecting the final droplet shape, understanding of the liquid spreading process is still lacking. In this work, droplets of different velocities are deposited on surfaces patterned with conical and cylindrical pillars. It is shown that for the structures of the same height and pitch, the shape of the lateral wall of the micro-structures affects the droplet shape. In addition, at higher deposition velocity, the initial polygonal/square shape of the droplets evolves into a more circular shape. The change in the shape due to the lateral wall of the structures is the result of the solid–liquid contact both above and below the structures and the way in which the liquid is able to move in between the structures for different passage areas.