micromachines Article Experimental Validation of a Novel Generator of Gas Mixtures Based on Axial Gas Pulses Coupled to a Micromixer Florian Noël 1,2,3 , Claire Trocquet 2 , Christophe A. Serra 3 and Stéphane Le Calvé 1,2, *   Citation: Noël, F.; Trocquet, C.; Serra, C.A.; Le Calvé, S. Experimental Validation of a Novel Generator of Gas Mixtures Based on Axial Gas Pulses Coupled to a Micromixer. Micromachines 2021, 12, 715. https:// doi.org/10.3390/mi12060715 Academic Editor: Stefano Zampolli Received: 8 March 2021 Accepted: 8 June 2021 Published: 18 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 ICPEES UMR 7515, Université de Strasbourg/CNRS, F-67000 Strasbourg, France; noel.florian.gim@gmail.com 2 In’Air Solutions, 25 Rue Becquerel, F-67087 Strasbourg, France; claire.trocquet@gmail.com 3 Institut Charles Sadron (ICS) UPR 22, Université de Strasbourg/CNRS, F-67000 Strasbourg, France; ca.serra@unistra.fr * Correspondence: slecalve@unistra.fr Abstract: In this work, a novel generator of gas mixtures previously numerically investigated and based on axial gas pulses coupled to a micromixer has been conceived, manufactured, and validated. Standard gaseous pollutant mixtures and pure nitrogen or pure air were introduced in a microdevice designed to generate alternating axial gas pulses which were downstream homogenized by means of a multi-stage modular micromixer. The dilution, and therefore the final pollutant concentration, was controlled by two parameters: the ratio between the times of each of the two gas pulses and the partial pressure of the pollutant(s) mixture added to the device. The gas mixture generator was coupled to an analyzer to monitor the concentration of aromatic pollutants. The response time was optimized to be lower than 2 min in accordance with the analytical instrument. The quantity of pollutants measured at the micromixer’s outlet increased linearly with the expected gas concentration of 3.7–100 ppb generated by this novel microfluidic generator and fitted perfectly with those obtained by a reference gas dilution bench. At 5 ppb, the precision on the concentration generated is close to that obtained with the conventional gas mixing bench, i.e., around 10%. Keywords: gas generator; gas mixing; pulsed flow; micromixer; multi-stages; Benzene; aromatic com- pounds 1. Introduction The mixing of different compounds with a known and precise ratio is often of great in- terest in many areas. On industrial level, this is the case for mixtures involving solids, such as in agriculture [1–3], pharmaceutical and cosmetics industries [4–7], food industry [8,9] or the manufacture of certain materials such as ceramics [10,11]. Other applications involve mixing liquids, such as for the food industry [8,12], refining fuels when choosing octane level [13,14], or managing chlorine levels in swimming pools [15]. Finally, gas mixtures are also used in many fields, for example in the beer brewing process [16], for the generation of anesthetic gases [17,18] and the direct mixing of shielding gases for coated electrode arc welding (SAEE or SMAW), but also for the generation of standard gases. These latter cases are useful in many applications, since they can be stored in cylinders and reused for other processes [19,20] or can be directly consumed online in the case of the calibration of an analytical device [21–23]. In addition, analytical instruments with low detection limits have been in much demand for many decades. Thus, they require accurate and reliable calibrations. For example, liquid standards are used for the calibration of spectrophotometers [24,25] and High Performance Liquid Chromatography (HPLC) devices. Standard gases are mainly used for the calibration of sensors [26–28], gas chromatographs [29,30] and other fluorescence detectors [31]. This calibration, on which the reliability of the measurements made by the analytical apparatus strongly depends, is crucial and must be accurate, repeatable, and reproducible in order to guarantee the quality of measurements over time. Micromachines 2021, 12, 715. https://doi.org/10.3390/mi12060715 https://www.mdpi.com/journal/micromachines