Cavity generation by accelerated relative motions between solid walls contacting in liquid

A contact motion between two solids in a liquid can give rise to a cavity. To investigate this 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. The results are as follows: generation and subsequent expansion of a cavity are influenced by factors such as the initial clearance between the walls, the acceleration of motion, the initial pressure, the liquid viscosity, and the contact area. In contrast, air solubility in off does not affect generation, but the size of a bubble remaining in the liquid. The pressure in a liquid film between two parallel walls normally moving away with a constant acceleration has also been theoretically analysed, providing the basis for the generation of tensile stresses in liquid films. Finally, the following idea of mechanism has been proposed: a tensile stress occurring in a liquid film breaks the liquid-solid interface, producing a rift, which develops into a visible cavity. Considering that liquid machines have many parts of contacting solids making relative motions, it is presumed that cavitation in them can start from this phenomenon.