Droplet levitating over a moving wall: stability of a lubricating air film

A drop of viscous liquid may levitate when placed over a rapidly-moving inclined solid surface. Here, the droplet's weight is supported by a micrometer-scale air film that develops between the drop and the underlying solid (Sawaguchi et al., JFM 2019). This lubricating air film may develop instabilities, with capillary waves growing and propagating towards the back side of the droplet. We capture this instability experimentally and rationalize its dynamics theoretically.