The Reynolds squeeze film approach assumes a continuous fluid flow regime. To ensure a continuous flow regime, the characteristic length (that is, gap thickness) must be more than one hundred times larger than the mean free path of the fluid particles.
Lm (po) is the mean free path of the fluid at the operating pressure P0, and L0 is the mean path at reference pressure P0. If the fluid is air at atmospheric pressure (1.01325*105), then by definition L0 is approximately 64*10-9 meters (64 nm). So, for continuum theory to be directly applicable (without modification) to air at atmospheric pressure, the gap should be greater than 6.4 μm (64*10-9 meters * 100).
The applicability of the continuum theory is generally assessed using the Knudsen number, which is equal to the mean free fluid path divided by the gap. For continuum theory to be valid, the Knudsen number should be less than 0.01.
For high Knudsen numbers, the continuum theory is not valid. However, the dynamic viscosity can be adjusted to simulate the high Knudsen number flow regime. The default flow regime for FLUID136 and FLUID138 is continuum theory (KEYOPT(1) = 0). Set KEYOPT (1) = 1 to specify the high Knudsen number flow regime. For FLUID136, set KEYOPT(1) = 2 to specify the high Knudsen number flow regime with surface accommodation factors.
The type of reflection of the gas molecules at the wall interface is specified using accommodation factors. Squeeze film models assume diffuse reflection of the gas molecules at the wall interface (accommodation factor = 1). This assumption is valid for most metals, but is less accurate for micromachined surfaces, particularly those fabricated from silicon. Materials such as silicon cause specular reflection. Typical accommodation factors for silicon are between 0.80 and 0.90. For more information on accommodation factors, see Flow Between Flat Surfaces in the Mechanical APDL Theory Reference.