Experimental study on pressure balance of a droplet levitating over a moving wall

We experimentally investigate droplet levitation over a moving glass wall in a steady state. The droplet is supported by the lubrication pressure in an air film of µm thickness between the moving wall and the droplet. The previous theoretical consideration indicated that the lubrication pressure and the bottom shape of the deformable droplet are mutually determined. However, there is little experimental validation of the pressure balance on the droplet surface, which is essential in levitating droplet problems. To verify pressure balance experimentally, this study measures three-dimensional shape of the air film, including mean curvature distribution of the surface, and calculate pressure distribution of the levitating droplet. Remarkably the lubrication pressure is found to balance with the surface tension and hydrostatic pressure. We also verify that the lift force, integral of the lubrication pressure, balances with the gravity force acting on the droplet. In addition, we provide new information based on the investigation of several parameters, especially the droplet viscosity varied over two orders magnitude.