Apparatus for measuring pressure-driven transport through channels at high Knudsen numbers

The pressure-driven gas flow through micro- and nano-porous structures is particularly interesting for innovative technologies such as microelectromechanical and nano-mechanical-electrical systems. The classical continuum assumption breaks down for rarefied flow through channels with a characteristic dimension comparable to the mean free path of the gas. Theories based on molecular interactions have been formulated to predict the flow at high Knudsen numbers. Measuring rarefied gas flow experimentally is a challenge since only a few studies have been able to determine flowrates in the molecular flow regime. Here we present the design of an experimental apparatus, which can be used to measure the flow of gases through nano- and microscale channels in the flow regimes where molecular effects are critical. The equations used to design the apparatus are given, focusing on the slip and transition flow regimes (together sometimes called "Intermediate flow regime"). A channel with a diameter of 325 μm ± 5μm and a length of 2 mm was tested experimentally with the apparatus for a wide range of Knudsen numbers (10^-2 < Kn < 1 × 10⁵) demonstrating its suitability through the slip and transition regime (2.23 × 10^-2 < Kn < 2.26).