Elsevier

Acta Astronautica

Volume 176, November 2020, Pages 204-215
Acta Astronautica

Data quality assessment of Diffusion Coefficient Measurements in ternary mIXtures 4 (DCMIX4) experiment

https://doi.org/10.1016/j.actaastro.2020.06.020Get rights and content

Highlights

  • 4th campaign of the DCMIX project on board the ISS.

  • New interferogram contrast definition detecting lasers mode hopping.

  • Phase-stepping quality parameter introduction for phase-shifting interferometry.

  • Assessment of the temperature regulation during experiments.

  • Level of environmental disturbances control of the station during the campaign.

Abstract

Thermodiffusion, which leads to a component separation in a mixture due to the thermal gradient, still does not have an unambiguous microscopic picture. Therefore, experimental studies, especially in convection free environment, are important. As part of the 4th campaign on the DCMIX project, thermodiffusion experiments on three compositions of the toluene-methanol-cyclohexane ternary mixture, on a mixture of fullerene-tetralin-toluene and on a mixture of polystyrene-toluene-cyclohexane have been performed in microgravity conditions on board the International Space Station. A binary mixture of polystyrene-toluene has been filled into the companion cell for the campaign. The Selectable Optical Diagnostics Instrument (SODI), wich is a two-wavelength Mach-Zehnder interferometer for the ternary mixtures, plus a monochromatic Mach-Zehnder interferometer for the binary mixture, has been used in order to obtain the temperature and the concentration fields in the cells. Precisely, it is a 5-steps phase-shifting interferometry technique which is implemented with SODI, producing by means of laser illumination a set of 5 phase-shifted images of π/2 between them as function of the time. A total of 58 runs of various durations and at different mean temperatures have been conducted. Here, we evaluate the contrast of the interferograms, the quality in the phase stepping, the stability of the thermal regulation of the experiments and the level of environmental disturbances on board the space station during the campaign.

Introduction

The aim of the Diffusion Coefficient Measurements in ternary mIXtures (DCMIX) project is to perform, in gravity-free environment thermodiffusion experiments, in order to provide quantitative measurements of mass diffusion and Soret coefficients on ternary mixtures of different origins. These series of experiments are performed on board the International Space Station (ISS) making use of the Selectable Optical Diagnostics Instrument (SODI) installed within the Microgravity Science Glovebox (MSG) inside the Destiny U.S. Laboratory of the ISS. The instrument comprises a two-wavelength (670 nm and 935 nm) Mach-Zehnder interferometer (MZI) with the possibility to change alignment and magnification. It provides the possibility to measure diffusion and Soret coefficients in binary and ternary mixtures. It is recognized that it is impossible to perform very extensive and systematic thermodiffusion studies in a microgravity environment due to the naturally limited availability of experiment time. With present technology, the coverage of the full ternary composition diagram of a single ternary system on a dense grid would require measurement times on the order of years. This is why the improvement of the reliability and the understanding of ground based measurements has always been considered an important objective of the DCMIX project. For flight experiments, 5 ternary and 1 binary mixtures can be studied at the same time, delivered to the ISS in the form of a cell array pre-filled with the liquid samples. Within the framework of the DCMIX project, four measurement campaigns were carried out on the ISS.

The first campaign, DCMIX1, was completed in January 2012. The samples were ternary mixtures of three hydrocarbons (tetralin, isobutylbenzene and dodecane). These compounds serve as model systems for the oil industry and the associated binaries are known as the so-called Fontainebleau benchmark systems. An experimental benchmark study was organised comparing ground and microgravity results at the point with mass fractions 0.80/0.10/0.10 for the ternary mixture THN-IBB-C12 [1]. A large part of the DCMIX team was involved in processing of the selected runs [2].

The second campaign, DCMIX2, was completed in January 2014. The ternary mixtures were composed of toluene, methanol and cyclohexane. This system is of particular interest due to the existence of a miscibility gap and a consolute critical point. Its investigation in ground based experiments is significantly more complex than in case of the DCMIX1 mixtures due to double diffusive convective instabilities [3]. It was shown that Soret separation increases at least by one order of magnitude towards the demixing zone. An unexpected experimental result was obtained in the binary cell filled with a toluene-cyclohexane mixture: the thermodiffusion coefficient DT is temperature independent for this mixture [4]. For the ternary mixture a linear dependence of the Soret coefficients on temperature was established. Processing the same set of images by different teams showed that the results depend on the methodology used to obtain the optical phase [5]. Furthermore, a small discrepancy in the optical phase (10%–15%) can lead to a large discrepancy between the results due to the propagation of errors at the state points with poor optical contrasts and/or ill-conditioned contrast factor matrices.

DCMIX3 was originally scheduled to be brought to the ISS in October 2014. Unfortunately, due to the catastrophic failure of the CRS Orb-3 mission during lift off, the first cell array was lost. A second cell array was built and successfully delivered to the ISS aboard SpaceX CRS-9 in July 2016. Experiments were completed on 17 November 2016. The DCMIX3 samples were ternary mixtures of water, ethanol, and triethylene glycol, for which a sign change of the Soret coefficient along the binary water-ethanol boundary was already known from the literature. Motivated by the microgravity experiments, a second sign change was discovered along the ethanol-triethylene glycol boundary in accompanying laboratory experiments. Sign changes of the Soret coefficient render these mixtures particularly sensitive to unwanted convective instabilities under gravity conditions. For the mixture of symmetric composition with equal mass fractions it has been possible to obtain consistent results between the microgravity measurements and the laboratory ground experiments performed with optical beam deflection and with a thermogravitational column [6]. The evaluation of the other cells has not yet been finished. 5-steps phase-shifting interferometry technique relies on acquiring a set of 5 images with π/2 phase shift between them, which is called a stack. Some of the DCMIX3 measurements were affected by laser instabilities that render the standard phase stepping evaluation procedure impossible. A new evaluation scheme, which is based on single image analysis, has been developed, which allows evaluating also the malformed image stacks without information loss. This alternative technique proves the robustness of the SODI instrument, which can deliver valid data even in situations where some components of the instrument fail to work as designed [7].

For the 4th campaign of the project, which is the subject of this work, the scientific team wanted to maximize the scientific return of the mission by maximizing on the diversity of the chemical systems sent into space. Taking into account novelty and importance of the obtained results for the ternary mixture, three cells of DCMIX4 (1–3) have been filled with toluene-methanol-cyclohexane mixtures at concentrations different from those of DCMIX2 and closer to phase separation, in order to confirm the slowing-down of the mass diffusion and the divergence of the Soret coefficients. In Fig. 1 the ternary composition map is reported. All three mixtures have the same content of methanol (25% by mass). The composition of the first cell is the one closer to the demixing zone (shown as the shaded area) at small toluene concentration. The analysis of the cells 1–3 has been coordinated by the team headed by V. Shevtsova (VS), from Université libre de Bruxelles (ULB), Belgium. Another sample of DCMIX4 includes a mixture of fullerene (C60)-tetralin-toluene, as the first complex mixture including nanoparticles [8]. The results obtained by measuring this sample are expected to provide added value and a high impact in this very active research area with so many applications: advanced manufacturing, health, nanotechnologies and biotechnology [9]. The study of the cell 4 has been coordinated by the team headed by M.M. Bou-Ali (MMBA), from Mondragon Goi Eskola Politeknikoa (MGEP), Spain. The ternary mixture of polystyrene-toluene-cyclohexane has been chosen mainly because the two eigenvalues of the mass diffusion coefficients matrix are expected to be well separated, by a factor of about 10, as this mixture includes a polymer as one of the components, namely the polystyrene, having a much larger molecular weight than the other two [[10], [11], [12]]. This implies that the time evolution of the concentration profiles measured by SODI two-wavelength diagnostics will be easier to analyse and interpret, as the different contributions can be also separated on the basis of their kinetics. A binary mixture of polystyrene-toluene completes the cell array in order to establish reference values for the polymer in a molecular solvent. The analysis of cells 5–6 has been carried out by the team headed by H. Bataller (HB) and F. Croccolo (FC) from Université de Pau et des Pays de l’Adour (UPPA), France. The sample content of all the cells is detailed in Table 1. For component numbering, a hydrodynamic approach has been adopted, corresponding to a decreasing order of density; i.e. component 1 is the denser one and so on.

The liquids are placed into quartz Soret cells of 0.5 ml (5 mm (H) x 10 mm (L) x 10 mm (W)). The smallest dimension is in the direction of the thermal gradient. These cells are combined in a cell array that contains five such cells, called primary, with ternary mixtures and one, called companion cell, with the binary mixture. The cell array was delivered to the ISS inside the unmanned Cygnus cargo ship launched by the Antares rocket on 17th November 2018. It was installed inside SODI on Wednesday 12th December 2018 by the NASA astronaut Serena Auñon-Chancellor. The first scientific experiment started on 13th December 2018 and the tests continued until 4th March 2019. All the experiments were handled by the Spanish User Support and Operations Center (EUSOC) in Madrid. During the campaign, a report was written on the good progress of the experiment and on the basis of the downloaded telemetry data [13]. To date, all of the stacks have been collected and made available to the scientific team. Under the coordination of ESA, the first task for the scientific team was to assess the quality of the data in order to identify failing runs and make recommendations for future data processing. In this article we report this work of data quality assessment.

Section snippets

Selectable Optical Diagnostics Instrument

The DCMIX experiments rely on optical probing of refractive index changes in order to infer time resolved spatial composition changes. Since ternary mixtures are described by two independent composition variables, the use of two different readout wavelegths is needed. SODI consist of a two-color MZI equipped with two lasers operating at 670 nm and 935 nm (designated MR and MN for Moving Red and Moving Near-infrared) that probes successively the primary cells. For the binary mixtures there is

Evaluation of optical data

Before extraction of required phase information from interference images, their quality has to be individually and independently estimated.

Vibrational characterization

Because of a limited amount of data available for post-flight analysis and the absence of possibility to repeat the experiment, it is important to clarify all unknown sources of errors. Perturbations of some microgravity experiments is usually attributed to the effect of the residual accelerations environment. The recent study [19] showed the daily onboard environment of the ISS does not interfere with diffusion controlled experiments. However, the transient (pulse-like) acceleration of

Summary and conclusions

As part of the 4th campaign on the DCMIX project, thermodiffusion experiments on 3 compositions of the toluene-methanol-cyclohexane ternary mixture, on a mixture of fullerene-tetralin-toluene and on a mixture of polystyrene-toluene-cyclohexane have been performed in microgravity conditions on board the International Space Station. A binary mixture of polystyrene-toluene has been filled into the companion cell for the campaign. In order to obtain the temperature and the concentration fields in

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work has been developed in the framework of the cooperative project DCMIX (No. AO-2009-0858/1056) of the European Space Agency and the Russian Space Agency (Roscosmos)/TsNIIMash. AM, QG, YG, VS, SVV and VY acknowledge support from the PRODEX program of Belgian Federal Science Policy Office (BELSPO). The Mondragon group acknowledges the support of the ATNEMFLU program (ESP2017-83544-C3-1-P) of the MINECO project from Spain and MMMfavIN (KK-2020/00099) of the Basque Government. FC, LGF and

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