Newsletter 2017.2 Index
Theme : "The Conference of Fluid Engineering Division"
When a droplet is deposited onto a moving wall, it can steadily levitate. It is thought that the levitation of the droplet is caused by lubrication pressure generated inside an air film between the droplet and the wall. Our purpose is to clarify the mechanism for the droplet levitation. We experimentally calculate the lubrication pressure, and compare the sum of the pressure with the droplet’s weight. On the other hand, it is said that the lubrication pressure balances with surface tension and hydrostatic pressure at the gas-liquid surface of the droplet’s bottom. We experimentally calculate and compare these pressures. As shown in Fig.1, we deposit a droplet onto an inner wall of a hollow glass cylinder with constant circumferential velocity. We measure the three-dimensional shape of the air film between the steadily levitating droplet and the moving wall by using interferometric method. Then, we calculate the lubrication pressure by applying lubrication theory. In our study, we experimentally verify a global balance of the force acting the levitating droplet and a local balance of the pressure acting the gas-liquid surface with changing wall velocity, the droplet diameter or the droplet viscosity. First, when the sum of the calculated lubrication pressure is compared with the droplet’s weight in each experiment, the both forces are agreed within measurement error. Second, we locally compare the lubrication pressure with the sum of surface tension and hydrostatic pressure. The both pressure distributions have positive pressure in a whole range and negative pressure in downstream part in each experiment. Therefore, it is considered that lubrication pressure dominantly sustains the levitating droplet, and the steady shape of the air film is kept by a local balance of pressure at gas-liquid surface. In conclusion, we clarify that the dominant factor for the droplet levitation is lubrication pressure.
Levitating droplet, Air film, Lubrication pressure
Fig. 1 A schematic view of our experimental setup.