Morphological characteristics of the drop on the travelling substrate with a sharp stepped configuration

Abstract

The shape of a drop on a partially wetting substrate strongly depends on not only its wetting velocity but also the surface characteristics of the substrate. The unexpected deformation or even breakup of the drop on the heterogeneous travelling substrate is a common phenomenon and needs to be carefully controlled in industrial processes. In this work, we designed an experiment to investigate the morphological characteristics of the drop on the substrate with a sharp stepped configuration. We experimentally studied the entire process of the drop motion on the substrate with a vertical-edged stepped structure and subdivided it into three stages: the collision stage, the inserting stage, and the stretching stage. For the collision stage, substrate velocity was determined as an important factor influencing the tail of the drop before and after the collision. For the inserting stage, it was found that the liquid is transferred to the pre-inserting zone with the reduced gap height due to the squeezing, resulting in an increase in the tail length; it was verified that the tail lengths vary with the position of the stepped frame by assuming that the drop volume is constant and the tail is simplified as a cone. For the stretching stage, three types of drop tails were observed by balancing the motion of the constraint tail in two directions: the one along the substrate movement and the one for anti-constraint of the stepped configuration; we experimentally analyzed the three types of drop tails and found that they are related to the substrate velocity, the gap height, and the height of stepped configuration.