Tilt effects on experimental measurement of squeeze film damping in microsystems

Previous experimental measurements of squeeze film damping forces in the MEMS field indicated a fundamental discrepancy from classical lubrication theory. This paper presents an explanation for the discrepancy employing a combined experimental and theoretical approach. In the past, analysis of experimental data assumed parallel surfaces, although a non-uniform fluid film thickness due to plate tilting may occur in practice. Small inclinations and misalignments are usually overlooked in theoretical treatment; however, this study finds that they could have a dramatic impact on the observed forces. To investigate this effect, a compact linear solution for hydrodynamic squeeze film damping forces was developed via a perturbation method assuming incompressible fluid bounded by tilted plates undergoing small normal vibrations. The results show that the inclination causes asymmetric pressure distribution and decreased fluid force. In fact, almost unmeasurably small angular tilt can cause significant change in the fluid force. The theoretical predictions employing the tilt model are found to agree well with the experimental measurements. These findings are of critical importance to the accurate determination of hydrodynamic lubrication forces and design of dynamic MEMS subjected to squeeze film damping.