DCMIX2

DCMIX2 : Diffusion coefficients in mixtures

Subject

Space experiment, diffusion and Soret coefficients in ternary mixtures, demixing zone, consulate point, interferometry.

Start/end dates

January 2011 – December 2015

Description

DCMIX project is focused on the investigation of the mass transport in ternary mixtures including experiment a series of microgravity experiments. Different formalisms and theories exist in literature to describe the thermodiffusion effect in multicomponent mixtures but there is a lack of experimental data for their validations. There exist only a few experimental techniques which allow studying the binary systems with negative Soret coefficients. The need in trustworthy measurements is obvious. The long-term vision is to understand kinetic of thermal diffusion in multicomponent systems.

Application of modern optical methods requires (N-1) independent light sources for the mixture with N components. Two light sources, λ=670 nm and λ=925 nm are available inside SODI onboard the ISS to analyze ternary mixture. DCMIX2 is the second experiment in series of five; the first one DCMIX-1 analyzed the hydrocarbon mixture (TNH-IBB-nC12) which displays large region of positive Soret coefficients.

The subject of DCMIX2 is the mixture Toluene (T) / Methanol (M) / Cyclohexane (CH). The preliminary ground experiment reveals strong instability which indicates at negative Soret coefficients in a large region of compositions. In this respect, an orbital laboratory provides an ideal environment for the measurements due to the absence of buoyancy driven convection. In addition, this mixture (T-M-CH) exhibits a miscibility gap and for a long time attracts the attention of scientists from different areas of research. For available lights on the ISS five compositions of (T-M-CH) mixture have been selected as the results of the preliminary experimental study of condition numbers of matrix (∂nλ,j/∂ci), i,j=1,2. They are shown inside triangle on the logo above.

Breaking News:
On 29 November 2013 DCMIX2 cell array arrived at the ISS and was carefully integrated inside SODI by the astronaut Mike Hopkins. The check-out of the optical settings was successfully completed in two instead of seven days as planned. Scientific experiments started on 1 December and finished on 24 January 2014. Before the Christmas break the experiments were running 24 hours a day, 7 days a week. It allowed performing three experimental runs in each cell with ternary mixtures and 9 experiments in the binary cell.

These experimental runs consisted of two steps: thermodiffusion and diffusion. Fractions of images for each run were downloaded via telemetry and a quick cross check showed that all the images were scientifically good!!! There are no issues with the images quality!

On the scientific side, one of the highlights of the DCMIX2 experiment is that in two cells (4 and 5) close to the demixing zone we expect giant Soret effect. Our preliminary analysis of refractive index variation has demonstrated that in these cells the signals are 10-15 times stronger compared with the other ones.

The life time of the cells was limited by 16 December but the first vapour bubble was found after the Christmas break. While the primary science objectives had been met, it was proposed to perform a series of shorter runs to assess the change in the samples compositions occurring with time since filling. These shorter runs do include the thermodiffusion study only (Soret phase), but not the relaxation phase afterwards. Actually, diffusion coefficients can also be retrieved from transient profiles. On 15 January the images check-out showed the presence of bubbles in most of the samples. A higher mean temperature of 31°C was imposed to possibly get rid of the vapour bubbles present in the cells. All these script alterations allowed to perform additional 14 runs in the ternary cells and 3 runs in the binary cell after the Christmas and New Year break.

A complementary science, short thermo/solute capillary (Marangoni) convection runs were also conducted. Each of these runs lasted about 40 min and in some of them bubble drifts were observed. In total, 16 experiments on Marangoni convection were executed.

Flight opportunities

International Space Station (ISS): the tentative launch date is 2013.

Results

• A new approach is proposed to cross-check conflicting measured values of the thermodiffusion coefficients in ternary mixtures.
• Taylor dispersion technique for independent measuring of mass diffusion coefficients has been developed.
• A simple mathematical approach has been developed to verify applicability of chosen two wave lengths for measurements of mass transport coefficients in ternary mixtures.
• The contrast factors have been measured for whole concentration field of DCMIX mixtures and compared with theoretical predictions according to different models. The mixture with best possible contrast factors are identified for microgravity experiments.

The details on these particular latest findings one may find in recent selected publications:

• Shevtsova V., Santos C., Sechenyh V., Legros J.C., Mialdun A. Diffusion and Soret in ternary mixtures. Preparation of the DCMIX2 experiment on the ISS. Microgravity Sci. Technology, 2013, DOI: 10.1007/s12217-013-9349-6
• Mialdun A., Shevtsova V., Communication: New approach for analysis of thermo-diffusion coefficients in ternary mixtures. J. Chem. Phys. (2013) 138, 161102
• Sechenyh V., Legros J.C., Shevtsova V., Measurements of optical properties in ternary mixture: cyclohexane--toluene – methanol. J. Chem. Engineering Data, (2012) 57, 1036-1043.
• Mialdun A., Sechenyh V., Legros J.C. , Ortiz de Zárate J., Shevtsova V, Investigation of Fickian diffusion in the ternary mixture of 1,2,3,4-tetrahydronaphthalene, isobutylbenzene and dodecane, J. Chem. Physics, (2013) 139, 104903.