A Sessile Drop Facing a Shear Flow: Surrounding Flow Dynamics During the Deformation of the Drop

When oil drops are recovered from surfaces by providing waterflow, the drop initially exhibits deformation. How the waterflow behaves during the deformation is examined experimentally and numerically. The flow observed from both approaches compares well. The drop exhibits deformation as the water shear flow velocity increases. The ranges of a deformation parameter, Di, and the Reynolds number (Re) varies from 0.02 to 0.28 and 1 to 97, respectively, for the oleophobic surface. Di and Re depend on each other. For oleophilic surfaces, Di and Re are found to be between 0.007 to 0.59 and 1 to 319, respectively. It is observed that when a drop continues to deform on an oleophobic surface due to the flow, four eddies are formed in the surrounding-however, the position of the eddies changes as the flow increases. For oleophilic surfaces, the number of eddies varies. The strength of the rear eddies increases, and the frontal eddies decrease with the Re. The reason for the initiation of early deformation on the oleophobic surface is found to be due to a higher pressure variation before and after the drop. A lower flow separation angle and higher strength of the eddies are the reason for the higher pressure difference. The pressure, viscous, and total drag coefficients on both surfaces, are observed to decrease with the flow. The pressure drag dominates over the viscous one for all Re on the oleophobic surface, while the viscous drag dominates at lower Re for drops on the oleophilic surface.