Much research has been conducted into the wringing force phenomenon. As a result it is believed to be primarily due to intermolecular attraction between the material at the surface of the blocks themselves and with any liquid (often oil) in the wringing film. Surface tension of the oil and differential air pressure between the exposed surfaces and the wrung surfaces are thought to be secondary causes. It is also known that the force varies depending on the flatness, surface roughness and type of liquid used on the wrung faces. It has been established that, if wringing is properly performed, the thickness of the wringing film is around 0.01μm (10 nm) or less.