Air-assisted impact of water drops on a surface

The impact of a drop when airflow is incident on the surface is studied. A polymethyl methacrylate (PMMA) substrate is chosen for the study, and a drop of water is made to fall from various heights. Airflow of various velocities is focused towards the surface when the drop impacts. It is found that generally the maximum spreading diameter increases with the Reynolds number of the airflow \(({ Re}_{\mathrm{air}})\) for a given impact Weber number \(({ We}_{\mathrm{water}})\), compared to still air. However, the maximum spreading diameter reduces suddenly and becomes almost equal to that of still air for a critical \({ We}_{\mathrm{water}}\) and \({ Re}_{\mathrm{air}}\) tested. Maximum spreading reduces further when \({ We}_{\mathrm{water}}\) is further increased while keeping \({ Re}_{\mathrm{air}}\) same. The reason is found to be the sudden increase in the rim thickness. It is known that with increasing \({ We}_{\mathrm{water}}\), the viscous force and surface tension restrict the radial flow and result in the formation of the rim at the periphery. For the present case, the restriction in the radial flow occurs due to the imposed air pressure around the drop, as well. As the rim of the drop expands outward, the pressure restricts the motion, resulting in the gain in momentum in the vertical direction, i.e. a thick finger forms at the periphery of the drop. The thicker rim flows at slower pace reducing the viscous dissipation. The rise in rebound height decreases, whereas the time of recoiling increases with \({ Re}_{\mathrm{air}}\) as well.