An analysis of the different significant length scales allows us to show the major part played by rarefaction in gas microflows, and the different flow regimes encountered in microchannels. The main theoretical and experimental results from the literature about steady pressure-driven gas microflows are summarized. Because it is very frequent in microchannels, the slip flow regime is more detailed and the question of appropriate choice of boundary conditions is discussed. It is shown that using second-order boundary conditions allows us to extend the applicability of the slip flow regime to higher Knudsen numbers that are usually relevant to transition regime. The case of pulsed flows is also presented, for this kind of flow is frequently encountered in micropumps. The influence of slip on the frequency behavior (pressure gain and phase) of microchannels is illustrated. When subjected to sinusoidal pressure fluctuations, microdiffusers reveal a diode effect which depends on the frequency. This diode effect may be reversed, when the depth is shrunk from a few hundred to a few micrometers. Thermally driven flows in microchannels are also described. They are particularly interesting for vacuum generation, using microsystems without moving parts.

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