A thin film is a small gap of fluid between moving surfaces. This thin layer of fluid can alter the structural response of the structure by adding stiffness and/or damping to the system. Movement normal to the gap produces a squeeze film effect. Movement tangential to the gap produces a slide film effect.
Thin film effects are important in macrostructures, in hydrodynamic bearings, for example, as well as in microstructures where the damping and stiffening effects of thin layers of air can significantly affect the behavior of devices used in micro-electromechanical systems (MEMS). Squeeze film effects are important in devices such as accelerometers and micromirrors. Slide film effects are important in devices such as comb drives.
Hydrodynamic bearing simulation methods are described in the Rotordynamic Analysis Guide. The information presented in this guide only concerns microstructures.
One method for assessing the effect of thin films for microstructures is to use thin film fluid elements based on the Reynolds number (which is known from lubrication technology and rarified gas physics) to calculate the stiffening and damping effects. These effects can then be added to the overall system model. Separate element types are used to assess squeeze and slide film effects.
If the following conditions are not satisfied, the effects of the thin film cannot be assessed using thin film elements:
The governing Reynolds equation limits the application of thin film analyses to structures with lateral dimensions much greater than the gap separation.
The pressure change across the gap must be much smaller than the ambient (surrounding) pressure.
Any viscous heating effects must be ignored.