Knudsen Force Actuation and Sensing in Microsystems

Graduate Student: Drew Strongrich

The generation of forces and moments on structures immersed in rarefied non-isothermal gas flows has received limited practical implementation since first being discovered over a century ago. The formation of significant thermal convective stresses requires both large thermal gradients and characteristic dimensions which are comparable to the gas molecular mean free path. For macroscopic geometries this necessitates a combination of impractically large temperatures and very low pressures. At the microscale, however, these conditions are easily achieved, allowing the effects to be exploited, namely, for gas-property sensing and microstructure actuation. We introduce and experimentally evaluate performance of a Microelectromechanical In-plane Knudsen Radiometric Actuator (MIKRA), a self-contained device having Knudsen thermal force generation, sensing, and tuning mechanisms integrated onto the same platform. Sensitivity to ambient pressure, temperature gradient, as well as gas composition is demonstrated. Results are presented in terms of a non-dimensional force coefficient, allowing measurements to be directly compared to previous experimental and computational data on out-of-plane cantilevered configurations.

Figure 1: Unpackaged MIKRA device shown on U.S. penny for scale

Figure 2: Scanning electron microscope image of packaged device with labels detailing location of primary components

Figure 3: Temperature distribution under application of 100 mW/heater at 10 Pa. Measurements confirm the existence of thermal gradients on the order of 10⁶ K/m.

Figure 4: Shuttle displacement and Knudsen force magnitude in air and helium under various applied powers as a function of ambient pressure.