Lift force acting on a levitating drop over a moving wall

Abstract

We experimentally investigate a levitating drop over a moving wall. Between the drop and the wall, a thin air film exists. In this study particular attention is given to examine whether the lift force generated inside the thin air film is dominant force for the levitation. The drop is deposited onto the inner wall of a rotating hollow cylinder in our new experimental setup. The drop levitates at a stable position, where the weight of the drop balances the lift and the drag. We measure the three-dimensional shape of the air film using interferometric technique. We then calculate the lift force by two different methods. One method estimates the lift force from the mass and the stable position of the drop. The other method computes the lift by applying the lubrication theory with finite difference method on the lubrication airflow inside the air film. Note that three-dimensional shape of the air film plays a crucial role for the magnitude of this lift force. We then compare both lifts, which show quantitative agreement. It indicates that the lift force generated from the three-dimensional air film is dominant for the levitation. In addition, we investigate a levitating drop with different wall velocity of the rotating cylinder. We discuss an effect of the wall velocity on the air film shape and the lift. We then obtain two kinds of lifts by the same method. The lifts are compared and again show quantitative agreement.