A contact motion between two solids in liquid can give birth to a cavity. To investigate the phenomenon, processes of cavity generation in a liquid between two walls making accelerated relative motions in their normal and tangential directions have been observed in detail, using microscopes and high-speed video cameras. Generation and subsequent expansion of a cavity are dominated by such various factors as the thickness of a liquid film, the acceleration of motion, the initial pressure, the liquid viscosity and the contact area. On the other hand the air solubility in oil influences, not the generation but the size of a bubble remaining after the process is over. The pressure in a liquid film between two parallel walls normally separating with a constant acceleration has also been theoretically analyzed, providing the basis for occurrence of tensile stresses in liquid films. Finally, the following idea has been proposed as a mechanism of the phenomenon; a tensile stress occurring in a liquid film breaks the liquid-solid interface, producing a rift which can develop to a visible cavity. Considering liquid machines have many parts of solid-solid contact in liquid, it should be pointed out that some of the cavitations occurring in them can start from these cavities.
