Pressure distribution and eddies at the periphery of a drop about to shed due to water shear-flow

Abstract

The physics behind the formation of eddies and their effect on an oil drop about to shed due to water shear flow are investigated. The velocities at the frontal periphery of the drop are measured after visualizing the flow and compared with those obtained numerically. A good comparison is observed. It is found that for oleophilic surfaces, two eddies are formed at the back of the drop, while no eddies are formed at the front side. One eddy at the front and three eddies at the rear are observed for drops shedding from oleophobic surfaces. The observations are the same for both experimental and numerical analyses. Eddies, velocity variation, and peripheral pressure distribution are found to be closely related. The pressure distribution along the periphery is studied. The pressure coefficient and the drag coefficient are observed to be higher for drops shedding from the oleophobic surface than from the oleophilic surface for a given volume. Therefore, less critical velocity is necessary for the drop to shed. The velocity variation along the frontal area is responsible for the drag applied. The drag coefficient is observed to increase with the volume. The formation of various eddies and the distribution of pressure along the drop periphery are responsible for the increase in drag coefficient. The pressure drag is observed to be dominant over the viscous drag for all volumes tested. A novel topology is proposed to explain the observations.