Editorial for the Special Issue on Gas Flows in Microsystems

The last two decades have witnessed a rapid development of microelectromechanical systems (MEMS) involving gas microflows in various technical fields [...].

static and dynamic characteristics of ultra-thin film gas lubrication in journal microbearings were investigated by Wu et al. [4] under various operative conditions and structure parameters. On the basis of the fractal geometry theory and the Boltzmann slip correction factor, the authors demonstrated that high values of the eccentricity ratio and bearing number tend to significantly increase the principal stiffness coefficients, and the fractal roughness surface considerably affects the ultra-thin film damping characteristics compared to smooth surface bearing. Controlling gas damping at microscale is also of high interest for the development of new compliant resonant microsystems. Mirzazadeh and Mariani [5] developed simple analytical solutions to estimate the dissipation in the ideal case of air flow between infinite plates, at atmospheric pressure, for application to comb-drive actuators. The results of numerical simulations were also reported to assess the effect of the finite size of actual geometries on damping.
These fundamental papers underline the importance of experimental data for validating simplified or more complex models. Unfortunately, the amount of experimental data on gas microflows is very limited, compared to the high number of numerical studies. The main difficulty, as explained in the review by Brandner [6], is due to the fact that conventional measurement techniques (for temperature, pressure, etc.) cannot be adapted to gas microflows, due to their intrusiveness and/or low signal delivery, especially when timely and spatially correlated measurements are required. In that review, the potential of nuclear magnetic resonance and magnetic resonance imaging for analyzing gas microflows is discussed. Some issues linked to the intrusiveness of sensors, even highly miniaturized, are also treated in the paper by Mironov et al. [7], in which the interaction between a Pitot microtube and a supersonic microjet is investigated.
The last series of papers published in this Special Issue are devoted to specific microsystems designed for the control or the analysis of gas microflows. One specific phenomenon experienced in rarefied gas flows is thermal transpiration, which allows the design of thermally driven pumps without any moving mechanical part. These so-called Knudsen pumps are very appealing for a number of applications requiring the control of a pressure, a flow rate, or the intake of a gas sample. Lopez Quesada et al. [8] provided some guidelines for the design of Knudsen micropumps based on architectures adapted to target applications which can require a high vacuum, a high flowrate, or a compromise between these two parameters. Their work is based on kinetic modeling and simulations, but takes into account some manufacturing constraints. Zhang et al. [9] focused their numerical analysis on the behavior of N 2 -O 2 gas mixtures in a more classic design of the Knudsen pump. The thermal transpiration efficiency is related to the molecular mass of the gas and, even with a molecular mass close to that of O 2 , N 2 was submitted to a stronger thermal transpiration effect. In addition, the lighter gas, N 2 , could effectively promote the motion of the heavier gas, O 2 . If separation of gas species from a mixture is of practical interest at a microfluidic level, it is also the case of mixing. Meskos et al. [10] numerically investigated the mixing process of two pressure-driven rarefied gas flows between parallel plates and evaluated the mixing length using a DSMC approach. They proposed a simple approach to control the output mixture composition, by only adding a splitter in an appropriate location of the microsystem's mixing zone. This mixer was working in a steady state, differently from the option analyzed by Noël et al. [11] who proposed a new multi-stage design of pulsed micromixer. For example, they demonstrated that, for a 1 s pulse of pure gas (formaldehyde) followed by a 9 s pulse of pure carrier gas (air), an effective mixing up to 94-96% was obtained at the exit of the micromixer. There is currently a high demand for compact, accurate, and rapid gas detectors. Several papers in this Special Issue are focused on this subject. Khan et al. [12] developed a toluene detector based on deep ultraviolet (UV) absorption spectrophotometry. They implemented two types of hollow-core waveguides, namely, a glass capillary tube with aluminum-coated inner walls and an aluminum capillary tube, and obtained limits of detection of 8.1 ppm and 12.4 ppm, respectively. Rezende et al. [13] proposed a micro milled microfluidic photoionization detector of volatile organic compounds. The device does not require any glue, which facilitates the easy replacement of components, and the estimated detection limit is 0.6 ppm for toluene without any amplification unit. Finally, Lara-Ibeas et al. [14] developed a compact prototype of gas chromatograph equipped with a preconcentration unit, able to detect sub-ppb levels of benzene, toluene, ethylbenzene, and xylenes (BTEX) in gaseous mixtures. Detection limits of 0.20, 0.26, 0.49, 0.80, and 1.70 ppb were determined for benzene, toluene, ethylbenzene, m/p-xylenes, and o-xylene, respectively.
We wish to thank all authors who submitted their papers to this Special Issue. We would also like to acknowledge all the reviewers for dedicating their time to provide careful and timely reviews to ensure the quality of this Special Issue.

Conflicts of Interest:
The authors declare no conflict of interest.