Gas flow near a smooth plate.

We examine gas flow adjacent to a molecularly smooth, solid, muscovite mica. The fluctuations in force acting on a glass sphere as a function of proximity to a mica plate were measured in air and were used to obtain the damping. The damping was interpreted as a lubrication force. The measured damping as a function of separation in the slip-flow regime corresponds to a slip length of 480 ± 70 nm, which is equivalent to highly specular gas molecule collisions. A slip-flow model fits the data for separations as small as one mean free path.