Propagation and elongation of aerodynamically driven Newtonian drops on rough surfaces

When a sessile liquid drop is exposed to airflow parallel to the solid substrate, it can begin to deform and move along the substrate surface. The drop dynamics are governed by aerodynamic forces and capillary forces associated with contact angle hysteresis. In this experimental study, the behavior of drops on surfaces with varying roughness in a wind tunnel is investigated using a high-speed video system. Image processing is applied to determine the evolution of the drop’s position, height, and length over time. These values are obtained for drops of various initial volumes, viscosities, and surface tensions. We observed that, above a certain critical airflow velocity, the drop transforms into a lengthening rivulet. The corresponding threshold conditions scale well with the critical aerodynamic Weber number, based on air density and attack velocity. Across a wide range of experimental parameters, the critical Weber number correlates exclusively with the aerodynamic capillary number. This database is used in the manuscript submitted in Physics in Fluids.