The molecular tagging velocimetry (MTV) is a well-suited technique for velocity field measurement in gas flows. Typically, a line is tagged by a laser beam within the gas flow seeded with light emitting acetone molecules. Positions of the luminescent molecules are then observed at successive times and the velocity field is deduced from the analysis of the tagged line displacement and deformation. However, the displacement evolution is expected to be affected by molecular diffusion, when the gas is rarefied. Therefore, there is no direct and simple relationship between the velocity field and the measured displacement of the initial tagged line. This paper addresses the study of tracer molecules diffusion through a background gas flowing in a channel delimited by planar walls. Tracer and background species are supposed to be governed by a system of coupled Boltzmann equations, numerically solved by the direct simulation Monte Carlo (DSMC) method. Simulations confirm that the diffusion of tracer species becomes significant as the degree of rarefaction of the gas flow increases. It is shown that a simple advection–diffusion equation provides an accurate description of tracer molecules behavior, in spite of the non-equilibrium state of the background gas. A simple reconstruction algorithm based on the advection–diffusion equation has been developed to obtain the velocity profile from the displacement field. This reconstruction algorithm has been numerically tested on DSMC generated data. Results help estimating an upper bound on the flow rarefaction degree, above which MTV measurements might become problematic

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Rarefied gas flows have a central role in microfluidic devices for many applications in various scientific fields. Local thermodynamic non-equilibrium at the wall-gas interface produces macroscopic effects, one of which is a velocity slip between the gas flow and the solid surface. Local experimental data able to shed light on this physical phenomenon are very limited in the literature. The molecular tagging velocimetry (MTV) could be a suitable technique for measuring velocity fields in gas micro flows. However, the implementation of this technique in the case of confined and rarefied gas flows is a difficult task: the reduced number of molecules in the system, which induces high diffusion, and the low concentration of the molecular tracer both drastically reduce the intensity and the duration of the exploitable signal for carrying out the velocity measures. This work demonstrates that the application of the 1D-MTV by direct phosphorescence to gas flows in the slip flow regime and in a rectangular long channel is, actually, possible. New experimental data on phosphorescence emission of acetone and diacetyl vapors at low pressures are presented. An analysis of the optimal excitation wavelength is carried out to maximize the intensity and the lifetime of the tracer emission. The experimental results demonstrate that a little concentration of about 5-10 % of acetone vapor excited at 310 nm or of diacetyl vapor excited at 410 nm in a helium mixture at pressures on the order of 1 kPa provides an intense and durable luminescent signal. In a 1-mm deep channel, a gas flow characterized by these thermodynamic conditions is in the slip flow regime. Moreover, numerical experiments based on DSMC simulations are carried out to demonstrate that an accurate measurement of the velocity profile in a laminar pressure-driven flow is possible for the rarefied conditions of interest.

Molecular tagging velocimetry is a little-intrusive technique based on the properties of specific molecules able to emit luminescence once properly excited. Several variants of this technique have been successfully developed for analyzing external gas flows or internal gas flows in large systems. There is, however, very few experimental data on molecular tagging velocimetry for gas flows in mini or microsystems, and these data are strongly affected by the molecular diffusion of the tracer molecules. In the present paper, it is demonstrated that the velocity field in gas microflows cannot be directly deduced from the measured displacement field without taking into account Taylor dispersion effects. For that purpose, the benchmark case of a Poiseuille gas flow through a rectangular channel 960 µm in depth is experimentally investigated by micro molecular tagging velocimetry using acetone vapor as a molecular tracer. An appropriate reconstruction method based on the advection diffusion equation is used to process the data and to correctly extract the velocity profiles. The comparison of measured velocities with flowrate data and with theoretical velocity profiles shows a good agreement, whereas when the reconstruction method is not implemented, the extracted velocity field exhibits qualitative and quantitative anomalies, such as a non-physical slip at the walls. The robustness of the reconstruction method is demonstrated on flows with light and heavy molecular species, namely helium and argon, at atmospheric as well as at 2 lower pressure conditions, for which diffusion effects are stronger. The obtained results represent an encouraging step for future analysis of rarefied gas microflows.

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We consider the implementation of a friction contact angle model in a Navier-Stokes VoF-CSF solver for the simulation of moving contact lines at the nano-scale. A liquid-liquid interface confined in a Couette flow generated by two solid walls moving at the same velocity in opposite directions is considered to discuss the relevance of the friction model. The simulations are compared with a reference case obtained using MD simulations by Qian et al. [46]. We show that the Navier Stokes simulations are able to reproduce the MD simulations for both the interface shape and the velocity field. The appropriate contact line friction is found to be grid convergent and of the same order as the friction measured in MD simulations. A detailed investigation of the interface shape has revealed an auto-similar linear profile in the center of the channel. Close to the wall the interface shape follows the classical Log evolution given by the Cox relation despite the wall confinement.