GAS WALL INTERACTIONS OF RAREFIED GASES IN MEMS: A NEW EXPERIMENTAL DEVICE WITH INTEGRATED SENSORS

A newly designed device for the experimental characterization of rarefied pressure driven gas flows is presented. The device is intended for both the thermal and the hydrodynamic analysis of gas flowing inside a single micro channel. The innovative feature of the present design is the integration of temperature and pressure sensors inside the micro channel itself. The sensors are fabricated as an array on longitudinal thin film membranes, installed on a polymeric supporting frame. A peculiar multi layer configuration of the device allows, on the one hand, the mounting of the sensor layer as a separate channel upper wall and, on the other hand, the interchangeability of the test sections. The sensors directly face the gas stream, registering the temperature and the pressure profiles along the channel. This gives additional information about the gas behavior compared to the simplified assumption of a linear profile developing between the measured inlet and outlet values. Examples of integrated sensors in micro channels have been already realized; however the majority of them are in silicon channels, as only with silicon fabrication technologies the sensor sizes can be reduced down to the micron range. This of course limits the field of application of the obtained results. In the present case, although silicon technology is still employed for the sensor manufacturing, this refers to the sensor wall only, while the remaining three-wall-channel is machined into an exchangeable foil. The foil, in principle, can be made of any material and can be easily replaced. Several channel dimensions and materials, as well as different surface roughness levels can be tested with the same device, making it very flexible and suitable for a broad characterization of gas wall interactions. A future experimental campaign will investigate the influence of roughness and material on the flow and the heat transfer characteristics. This is a very critical point in rarefied gas applications in MEMS, as the similarities with conventional flows as well as new features need to be identified and analyzed. With the described configuration it is possible to have insight into the flow parameters and to understand the actual behavior of gases under specific flow conditions. This is important in order to obtain a proper modeling and to validate the results of simulations and calculations on rarefied gas flows in micro channels.