In many MEMS devices a released structure moves relative to the substrate, compressing or shearing a thin layer of gas between the two structures. The fluid film can be the dominant form of damping for the moving microstructure. If the film is compressed by a structure moving perpendicular to its plane, this type of damping is usually referred to as squeeze-film damping. Damping induced by motion of the structure parallel to the plane in which the fluid flows is called slide-film damping.

It is often convenient to model the film using the Reynolds equation — enabling the 3D gas layer to be modeled on a 2D surface. The Reynolds equation applies for isothermal flows of a thin film in which the film height, h, is much less than a typical lateral dimension of the device, L. It must also be possible to neglect inertial flow effects in comparison to the viscous effects (for MEMS devices, this assumption is reasonable below MHz frequencies). Additionally, the curvature of the reference surface is neglected.

For gas damping, especially in vacuum packaged devices, the Knudsen number for the thin film gap is an important parameter to consider. This is the ratio of the gas mean free path to the gap height. Below Knudsen numbers of 0.01 it is necessary to use a slip boundary condition for the fluid by specifying a characteristic slip length. As the Knudsen number decreases further additional rarefaction effects need to be accounted for. These can be included empirically in the Reynolds equation using models that require the mean free path and the tangential momentum accommodation coefficient (TMAC) to be specified. The TMAC is a measure of the mean fractional loss of tangential momentum of the gas molecules on collision with the walls — typical values for various gas surface combinations are available in Ref. 1 (see Table 6 in this reference).