Microgravity Sci. Technol. (2014) 25:275–283DOI 10.1007/s12217-013-9349-6

Diffusion and Soret in Ternary Mixtures. Preparationof the DCMIX2 Experiment on the ISS

Valentina Shevtsova · Cecilia Santos · Vitaliy Sechenyh ·Jean Claude Legros · Aliaksandr Mialdun

Received: 30 April 2013 / Accepted: 25 July 2013 / Published online: 17 August 2013© Springer Science+Business Media Dordrecht 2013

Abstract We report on ground-based studies in course ofpreparation of the experiment DCMIX2 (Diffusion coef-ficient in mixtures) to be performed on the InternationalSpace Station (ISS). In microgravity experiment the diffu-sion and thermodiffusion coefficients will be measured atsix points with different compositions of the ternary mix-ture Toluene-Methanol-Cyclohexane. This mixture attractsattention of the researchers, as it has a miscibility gap andpresumably large region with negative Soret coefficients.By using Optical Digital Interferometry we have measuredthe variations of refractive index �n in the full param-eter space of concentrations. The regions of stable andunstable behavior of system were determined from the time-dependent behavior of �n. The system is hydrodynamicallyunstable in a wide region of compositions where Soret mea-surement should be done in microgravity. We present theresults of the study of the compatibility of this mixture withoften used sealing materials: Viton (R), Chemraz (R) anddifferent types of Kalrez. To facilitate the expected theoreti-cal studies we have measured density and thermal expansionin the points of the interest.

Keywords Diffusion · Thermodiffusion · Soret ·Vibrations · Interferometry

V. Shevtsova (�) · C. Santos · V. Sechenyh ·J. C. Legros · A. MialdunMRC -Microgravity Research Centre,Universite Libre de Bruxelles (ULB) EP - CP165/62,Avenue F.D. Roosevelt 50, 1050 Brussels, Belgiume-mail: vshev@ulb.ac.be

Introduction

Many efforts are currently aimed to study thermodiffusion(Soret effect) in multicomponent mixtures. Recent advancesin finding some intrinsic rules governing behaviour of mix-tures on the basis of properties of single components,e.g. thermophobicity suggested by Hartmann et al. (2012),excite even more interest in such studies with the ambitiousgoal of expanding this regularity from binary equimo-lar compositions to the whole concentration coverage forcertain classes of ternary mixtures.

Experiments in mixtures with more than two compo-nents impose severe practical difficulties with respect tobinaries. The first steps on the way from binary to ternarysystems have already been done, see Blanco et al. (2010)and Koniger et al. (2010). Nevertheless, the results obtainedby various methods are quite different and a method fortheir comparison was recently suggested by Mialdun andShevtsova (2013).

The most often used ternary mixture used by oil indus-try for their numerical modeling is composed by 1,2,3,4-tetrahydronaphthalene (THN), isobutylbenzene (IBB) anddodecane (nC12) as it includes molecules of different fami-lies (polycyclic, alkane, aromatic). The different pairs of thecomponents of the mixture were used by scientific commu-nity as binary benchmark liquids (see Platten et al. 2003;Mialdun and Shevtsova 2011a; Croccolo et al. 2012;Gebhardt et al. 2013) and it seems that this mixture also actsas a benchmark mixture among the ternary ones.

Nowadays along with revisited old techniques for mea-suring the thermodiffusion new ones appear. The modernoptical techniques provide very good accuracy and reli-ability for measurement transport coefficients in binarymixtures since no perturbation is introduced into the diffu-sive process. The situation with ternary mixtures is more

276 Microgravity Sci. Technol. (2014) 25:275–283

complicated as the sign of the Soret coefficients of the var-ious components could be different and it destabilizes thesystem as discussed by Ryzhkov and Shevtsova (2009),Shevtsova et al. (2006). In this respect, orbital laboratoriesprovide an ideal environment for the measurements due tothe absence of buoyancy driven convection.

In the framework of a cooperative international projectsupported by ESA, European scientists expect to obtain reli-able benchmark results on the ISS for different types ofmixtures and validate their ground-based techniques. Instru-ment SODI (Selectable Optical Diagnostic) is equippedwith two wavelength diagnostic which enables to mea-sure Soret and diffusion coefficients in ternary mixtures.Accordingly, in the first experiment DCMIX1 on the ISS thebenchmark mixture THN–IBB–nC12 was investigated.

This article is organized as follows. First, the experi-mental design and procedure are briefly outlined. Then, theanalysis of benefits and disadvantages of the test mixturefor the second experiment DCMIX2, which is going to beimplemented in 2013, is presented. Fundamental theoreticaland experimental results of ground activities on the subjectare summarized in the next paragraphs. The conclusions aredrawn at the end of article.

SODI Instrument on the ISS

The SODI instrument (Selectable Optical DIagnostic),launched to ISS in 2009 and operated inside the Micro-gravity Science Glovebox, is a multifunctional instrument.The basic feature of the instrument is the use of differentexchangeable methods for optical probing of objects underinvestigation. The SODI instrument enables the possibil-ity to perform thermal diffusion experiments. It includesan optical interferometer, and its real time results (images)can be sent to researchers via telemetry. The first experi-ment inside SODI facility, IVIDIL (Influence of VIbrationson DIffusion in Liquids) has examined diffusion controlledphenomena in binary mixtures with and without imposedvibrations, see Mazzoni et al. (2010), Shevtsova et al.(2011b).

The following project, DCMIX (Diffusion Coefficientsin MIXtures), is aimed at measuring the diffusion andSoret coefficients in ternary mixtures. Presently the projectincludes three experiments with mixtures of different ori-gins. The experiment DCMIX1 has been recently per-formed with nC12-THN-IBB mixture, DCMIX2 will oper-ate with Toluene-Methanol-Cyclohexane (further Tol-Meth-Ch) mixture and DCMIX3 with Water-Ethanol-TEG mix-ture. The authors of this paper are working on the prepara-tion of DCMIX2 experiment.

Both experiments, IVIDIL and DCMIX analyze diffusivephenomena in liquids using the same two-step procedure:

separation by thermodiffusion and back isothermal diffu-sion. However, they have different optical design. Bothuse the concept of Mach-Zehnder interferometer but withdifferent ways to obtain the optical phase data from rawinterferometric images. The IVIDIL approach is based on2D Fourier Transform, e.g. see Mialdun and Shevtsova(2008, 2011b), while DCMIX uses phase shift technique,e.g. Hariharan et al. (1987), Mialdun et al. (2013). In thegeneral case of N-component mixture, (N −1) independentdiagnostics are required, for example (N − 1) laser beamsof different wavelengths.

Experimental Design and Procedure

The five transparent rectangular cells 10 mm × 10 mm ×5 mm are filled with different compositions of the sameternary system. Since two independent concentrationsshould be measured in a ternary mixture, two laser diodesemitting light with different wavelengths that ensure differ-ent optical behavior of the components are required for acomplete composition analysis. The SODI/DCMIX is basedon a two-wavelength (670 nm and 935 nm) Mach-Zehnderinterferometer with the possibility of changing alignmentand magnification (see Fig. 1). It is complex, but verysensitive and precise. The instrument includes two opticalbridges, a moving and a fixed one.

Movable optical parts, consisting of illumination blockand imaging block (CCD camera plus optics), successivelyinspects all five cells to follow the evolution of separation,as it is shown in Fig. 2. The interferometric patterns fortwo wavelengths are successively recorded with the smalldelay in time. The fixed optical part monitors only the cellwith binary mixture using laser λ = 670 nm. The temper-ature difference �T is applied across the cell to maintainthermodiffusion process.

In multicomponent fluid mixtures, subjected to a temper-ature gradient at a constant pressure, the diffusive transportis caused not only by gradients of concentrations but alsoby gradients of temperature; when the latter tend to sepa-rate species, the former always tend to homogenize themback. All the coefficients characterizing both effects entersimultaneously into the mathematical model of the pro-cess. If we denote the mass fraction of component i by ci ,∑N

i=1 ci = 1, then the diffusive flux of this component canbe written as

Ji = −ρ

N−1∑

k=1

Dik∇ck − ρD′T i∇T i = 1, 2, ..., N (1)

where Dik are the mass diffusion coefficients, and D′T i are

the thermal diffusion coefficients. Since the diffusive flux

Microgravity Sci. Technol. (2014) 25:275–283 277

Fig. 1 Flight implementation oftwo-color Mach-Zehnderinterferometer

Imaging blockIllumination block

CCDLasersources670 nm

935 nm

Soretcell

Referencebeam

Objectbeam

Fiber splitters

must vanish in the two dilute limits ci = 0 and ci = 1,the thermal diffusion coefficients are usually representedas D′

T i = ci0(1 − ci0)DT i where ci0 is the mean massfraction of the i-component. In a binary mixture there aretwo independent coefficients (DT and D), while in case ofternary it grows to six (two thermal- and four mass diffusioncoefficients). The appearance of cross-diagonal diffusion(Dij , i �= j) significantly complicates the measurement ofthe thermodiffusion coefficients in ternary in comparison tobinary mixtures. The Soret coefficients ST i in ternary mix-tures depends on four mass diffusion coefficients Dij andon thermal diffusion coefficients D′

T i , see Shevtsova et al.(2011a)

S′T 1 = D′

T 1D22 − D′T 2D12

D11D22 − D12D21,

S′T 2 = D′

T 2D11 − D′T 1D21

D11D22 − D12D21. (2)

Imaging blocks

Illumination blocks

Cell array

P5P4P3P2P1 C6

Fixedbridge

Movingbridge

Fig. 2 Principal scheme of the experimental setup. Five cells are filledwith ternary and one with binary mixtures

All experimental runs will last a long time with thepurpose to reach steady state. Experiment in each sam-ple includes 15h for thermodiffusion step, and then 15hfor isothermal diffusion. The Soret coefficients S′

T i can beextracted from observations in the transient regime or atsteady state, when diffusion fluxes vanished, Ji = 0. Themass diffusion coefficients Dij can be determined from thetransient behavior in both regimes. After finishing the exper-iment in one cell, the optical system is displaced to anothercell. To avoid non complete homogeneity at the end of thediffusion step, an experiment in the first cell will be con-ducted again only after the scanning of all five cells. Thecell with binary mixture can be monitored continuously, butagain, to avoid residual concentration, the experiments inbinary mixture will be conducted each second run. As result,it is assumed to conduct 20 experimental runs with ternarymixtures (applying different �T or mean temperature) and10 experimental runs with binary mixture. Experimentalruns are conducted in automatic regime according to thescript established before flight.

Choice of the DCMIX2 Test Mixture

The ternary liquid mixture Toluene-Methanol-Cyclohexanewas selected for experiments in ISS due to some of itsspecific features.

First, it exhibits a miscibility gap and for a long timeattracts the attention of scientists from different areas ofresearch. The system with demixing contains critical point,so-called consolute point. On the basis of the statisticalphysics it was derived that the diffusion coefficient D

should drastically diminish approaching to the critical point,while thermodiffusion coefficients DT remains almost con-stant. In binary mixture the Soret coefficient ST = DT /D

grows as ∼ 1/D. Experimentally an increase of Soret coef-ficient by four orders of magnitude has been observed inpolymer solutions by Voit et al. (2007). A similar behavior isexpected in the ternary systems. The map of the mixture inmass fraction is shown in Fig. 3 where green points outlinethe demixing zone at 298.15 K.

278 Microgravity Sci. Technol. (2014) 25:275–283

Second, the mass diffusion coefficients in the wide rangeof compositions have recently been measured at 298.15 Kby Grossmann and Winkelmann (2009a, b) using the Taylordispersion technique. The points, at which diffusion coeffi-cients were measured are shown by blue dots in Fig. 3. Inde-pendent data on diffusion are very important to cross-checkthe future results, especially, at the step of fitting proce-dure. The values of the diffusion coefficients are influentialbecause the sign of the Soret coefficients in ternary mixturedepends on the mass diffusion coefficients, see Eq. 2.

Third, this mixture possess a region of compositionswhere the Soret effect is negative, i.e. denser componentssegregate to the hot regions due to thermal diffusion. Thenegative Soret sign has a destabilizing effect on the sys-tem in the gravity field and these ternary mixtures cannotbe treated quantitatively in terrestrial conditions, at least, bynow. The reported data by Bou-Ali et al. (2000) in the con-stituent binary mixture Meth-Tol showed a sign change atmass fraction of methanol c = 0.23. To cross-check, weconducted experiments with three compositions of this mix-ture and the results of all measurements are summarized inFig. 4. The comparison shows an excellent agreement. Forbinary Meth-Ch the measurements were conducted in thesmall range of mass fractions 0.13 < cCh < 0.28 byStory and Turner (1969) and the authors reported about thepositive Soret coefficient in this region with respect to lessdenser component (Ch). A negative sign was reported in asingle point with concentration of cyclohexane close to 1with respect to Ch.

Fig. 3 Map of Toluene-Methanol-Cyclohexane in mass fractions.Green dots show the region with demixing at T = 298.15 K. Bluedots indicate compositions with mass diffusion coefficients measuredby Grossmann and Winkelmann (2009a, b). Red dots show selectedDCMIX2 points

Fig. 4 Measured Soret coefficients of Tol-Meth as a function of massfraction of Toluene. Rhombus corresponds to the results by Bou-Aliet al. (2000) and triangles show our measurements using ODI

Fourth, the mixture satisfied to the safety condition on theISS and its use was approved by toxical level assessment.

Along with the attractive features, this system has thenegative one: non compatible with a variety of sealing mate-rials, namely due to high solvent permeation and sorptionin the sealing material. This point will be addressed insection “Analysis of Sealing Materials”.

Results and Discussion

Choice of the Compositions of the Mixture

There are two equivalent ways of choosing optimal exper-imental conditions: by choosing a concentration set or bychoosing a proper wavelengths. In the case of interest forsome specific liquids one should search for proper lightsources because for the prescribed wavelengths not allcompositions are suitable. To conclude, the available laserdiodes inside SODI (670 nm and 935 nm) are not suitablefor arbitrary liquids and concentrations. Below one may findshort explanation.

Optical measurements on the ISS (or at the ground lab-oratory) provide variation of the refractive index in spaceand in time. Let us denote this �n1 and �n2 the changeof refractive index due to variation of c and T for the mea-surements at wavelength λ1 and λ2. To obtain concentrationprofiles one needs to solve the system of equations

(∂n1

∂c1

)

T0,c20

�c1 +(

∂n1

∂c2

)

T0,c10

�c2 = �n1 (3)

(∂n2

∂c1

)

T0,c20

�c1 +(

∂n2

∂c2

)

T0,c10

�c2 = �n2. (4)

Microgravity Sci. Technol. (2014) 25:275–283 279

Equations 3–4 can be written in a vector form where thematrix A should be determined from preparatory laboratoryexperiments,

A�c = �n where �n =(

�n1

�n2

)

A =(

∂n1∂c1

∂n1∂c2

∂n2∂c1

∂n2∂c2

)

. (5)

By inversion of the matrix A, we find the variations of theconcentration fields,

�c = A−1�n,

and then the Soret coefficients can be defined.The method described above is simple and straightfor-

ward, but there is a serious obstacle to its practical imple-mentation because the matrix A can be ill-conditioned. Foreach matrix one can calculate the control parameter, so-called condition number. Obviously, before carrying out theSoret measurements it is necessary to estimate the condi-tion number of the matrix A. To understand the essence ofthe problem the appendices of the paper by Shevtsova et al.(2011a) are recommended.

The matrix A for Tol-Meth-Ch mixture was determinedby the measurements of refractive indices for the full con-centration space in two wavelengths. The refractive indiceswere determined at 66 points for both wavelengths λ =670 nm and λ = 925 nm. Continuous 3D surfaces inthe form n1f = nf (c1, c2), n2f = nf (c2, c3), n3f =nf (c1, c3) were constructed on the basis of the experimen-tal points with the aim to fit the set of data points as closelyas possible for a specified function nf . The smoothness ofthe surface is important as the final target is the calculationof the derivatives with a good accuracy. The polynomial fitfor n1f (c1, c2) was chosen as

n1f (c1, c2) =i,j=4∑

i,j=0

αij ci1 c

j

2 (6)

where the coefficients αij depend on the concentrationsset under consideration and they were sought by the non-linear regression method, see Sechenyh et al. (2012). Thepolynomials for other functions were constructed in a sim-ilar way. The concentration derivatives ∂nλj

/∂ci, i, j =1, 2, vital for the determination of ST and Dij werecalculated according to Eq. 6. The values on trianglesides, i.e. for binary mixtures, were compared with lit-erature data. The correctness of the fitting functionswas also controlled by calculating the sum of the three

derivatives obtained for each wavelength, which must beequal to zero

� =(

∂n

∂c1

)

c2

+(

∂n

∂c2

)

c3

+(

∂n

∂c3

)

c1

= 0.

Although it is only internal consistency check, taking intoaccount that we present experimental results, it provides usconfidence to the obtained derivatives.

Then, the condition numbers were calculated for matricesA. The smaller is condition number, the better is precision ofthe results. The variation of the condition number over com-position space is shown in Fig. 5 for mixture Tol-Meth-Ch.The tabulated values of refractive indices, fitting coeffi-cients as well as contrast factors are given in recent paperby Sechenyh et al. (2012).

The compositions of ternary mixtures are selected alongthe two lines in the direction of consolute point, as shownin Fig. 6. In the vicinity of the critical point exists a tur-bidity region where mixture is non-transparent. To avoidit, the experimental points are selected at some distancefrom demixing zone. The numbered experimental cells withselected composition are listed in Table 1.

Physical Properties of the Mixture

Along with refractive index we have measured density andthermal expansion in the points of the interest to facili-tate the expected theoretical studies. For these measure-ments each mixture was prepared in a narrow-distributedconcentration around the values of interest. An electronicbalance of Sartorius (model 1712, ser. no. 35070147) withan accuracy of 10−2 mg/30 g was used. The weights of allcomponents were measured with an accuracy ±5 · 10−5 g.The density was measured at different temperatures usingAnton Paar densimeter DMA 5000M. The densities of allthe mixtures are linear function of temperature and for twomixtures they are shown in Fig. 7. The values of densityand thermal expansion coefficients βT are given in the lasttwo columns of Table 1. The difference in density betweenthe mixtures as well as in thermal expansion coefficients isabout 5 %.

Analysis of Sealing Materials

The process of permeation of chemicals through polymersis a combination of two processes, absorption of the chem-ical in the polymer and diffusion through the polymer.There is a limit to the amount of the chemicals that can beabsorbed under particular set of conditions. The often usedseal materials do not provide high resistance to permeationof Tol-Meth-Ch mixture. Experiment with Tol-Meth-Ch

280 Microgravity Sci. Technol. (2014) 25:275–283

Fig. 5 Isolines of conditionnumbers of matrix A whenλ1 = 670 nm and λ2 = 925 nm

and compositions for DCMIX2experiment (Sechenyh et al.2012). Shaded region outlinesdemixing zone. The selectedpoints for DCMIX2 experimentare shown by circles withnumbers

system was supposed to be uploaded to the ISS in 2011together with DCMIX1 mixture. However, two weeks afterthe cell filling bubbles appeared in the cells with welldegassed mixtures. The conclusion drawn after the detailedanalysis was that seals made of VITON were not compatiblewith the mixture.

We have performed comprehensive study on the perme-ation of Tol-Meth-Ch through different sealing materialswhich are made from polymers (elastomers). A piece ofelastomer under the test was immersed into studied liquid

Fig. 6 The selection of the experimental points for DCMIX2 experi-ment in the regions, where condition number is relatively small. Thechosen points are numbered

mixture of a particular concentration. Each sample had beenimmersed into approximately 10 ml liquid volume in tightlyclosed glass flask. Samples were stored at atmosphericpressure and room temperature. Time to time samples hadbeen picked up from the flasks, dried externally by filterpaper and weighted by the same balance as described insection “Physical Properties of the Mixture”. The study wascontinued during 25 days, the samples were weighted afew times per day in the first week and then once per day.Figure 8 shows the relative increase of elastomer mass withtime due to permeation of the mixture Tol/Meth/Ch withthe concentration in mass fractions 0.25/0.15/0.60 (cell #1)by three different materials: Viton, Chemraz and Kalrez. Asecond attempt to fill the cells for the experiment on theISS with the mixture was in 2012, using sealing materialChemraz 505 (75 shore) as the first line of O-rings andVITON as second line of O-rings. The compatibility testswere performed in company Qinetiq (http://www.QinetiQ.be), which designed and developed SODI instrument. The

Table 1 Compositions of the cells with DCMIX2 mixture

Cell Tol Meth CH ρ βT × 103

number kg/m3 1/K

1 25 15 60 790.27 1.23

2 45 15 40 807.65 1.20

3 65 15 20 827.86 1.16

4 20 40 40 788.94 1.23

5 30 30 40 796.33 1.18

6 40 – 60 802.62 1.17

Microgravity Sci. Technol. (2014) 25:275–283 281

Fig. 7 Variation of density with temperature for the mixtures in thecell#1 and cell#5

bubbles appear in cell #1 and cell #3 two months after fillingthe cells with mixtures of Tol-Meth-Ch. The used sealingmaterials are listed in Table 2.

Among the studied 12 seals here we discuss only those,which have already been used in SODI (Viton 75, Chemraz505) and the best one, Kalrez 6375. Usually, the compat-ibility of seal is assumed as acceptable when saturation isless than 1 %. Because the bubbles appear on the shortesttime scale in the cell #1, we present results in Fig. 8 for thisparticular composition. Among the studied 12 seals the lessresistant is the Viton which weight was increased by 5 % inless than 15 days and it is still far from saturation. Chem-raz seal attains saturation in about 2 weeks and the relativemass increase is about 1.4 %. Our study demonstrated thatfor all the six mixtures in Table 1 the best seal is that madeof Kalrez 6375, for which the change of mass is less than1 % after 25 days. After the completing of weighing, thesamples of elastomers remains in the mixtures for more than7 months and no modification of the elastomer was visible,chemical compatibility was excellent.

Despite this results the following attempt to uploadexperiment on the ISS in 2013 will be done with theseals made of Chemraz expecting the late load access.Because the change of the seal at this step would further

Table 2 Materials used for sealing

Purpose Material Supplier

Plug seals VITON (51414) Eriks

Membrane CHEMRAZ 505 Humitab

FFPM SM03-03-010-505

Glass seals VITON (Gsk RX FPM) Eriks

Fig. 8 Permeation of Tol-Meth-Ch mixture with the concentrationin mass fraction 0.25/0.15/0.60 (cell #1) through different sealingmaterials

delay experiment, all measurements will be conducted onthe ISS as fast as possible and, preferably, during first twomonths after the filling.

Hydrodynamic Instability

It is common knowledge that light fluids rise while heavyfluids sink in the gravity field. The most obvious case is theisothermal Rayleigh-Taylor instability when a more densefluid is placed a on top of the less dense one. Unstabledensity stratification might be established in a binary mix-ture with a negative Soret effect in the case of heatingfrom above: the denser liquid is accumulated on the lessdenser one. Situation is more complicated in ternary mix-tures where the final scenario depends on the sum of Soretcoefficients.

Using Optical Digital Interferometry (ODI) with onewave length λ = 670 nm we have performed Soret exper-iments in ternary mixture Tol-Meth-Ch in wide regionof compositions. The experimental technique and pic-ture processing are described in details by Mialdun andShevtsova (2008, 2009, 2011a). One wave length experi-ments allow to determine the total variation of the opticalphase in whole field of view or, by other words, net vari-ation of the refractive index, �n = �n1(x, z, t) definedin Eq. 3, although it doesn’t allow to determine both Soretcoefficients.

All experiments were conducted in the rectangular cellof height L = 6.1 mm (in the direction of tempera-ture gradient) imposing �T = 6 K, resulting in gradient≈ 1 K/mm. The duration of the experiments was about12h. These experiments allow us to determine the time

282 Microgravity Sci. Technol. (2014) 25:275–283

dependent behavior of the refractive index difference nearthe hot and cold walls, �n = �n(z = L) − �n(z = 0).To illustrate stability of the flow in the gravity field, theevolution of �n(t) with time is shown Fig. 9 for the threerepresentative cases: stable, semi-stable and unstable. Thesethree cases correspond to different initial compositions.Figure 9 clearly shows that even in the case of unstableregime �n attains stationary value after 7h. ODI techniqueenables to observe the concentration field in the entire cross-section of the cell and this has allowed us to identify, thatthis stationary behavior of �n simply means that there is nochange near the walls, but convection continues to evolve inthe interior region.

Conducting such experiments at different compositions,we have determined the areas where the concentration fieldeither is non perturbed or convective flow occurs in themixture. Figure 10 presents stability map in the full param-eter space of concentrations. The concentrations, at whichexperiments were carried out, are shown by small dots. Themajor part of the parameter space of concentrations displayunstable behavior and Soret measurement should be done inmicrogravity. The only region, rich by methanol and poorin cyclohexane is gravitationally stable and can be mea-sured in laboratory experiments. Notice, that the demixingzone is entirely inside unstable region. In the experimentson the ISS usually the temperature gradient is �T/L =10 K/5 mm = 2 K/mm.

Fig. 9 Evolution of the difference in the refractive index near hotand cold walls at different stability regime in Tol-Meth-Ch mixturewhen �T/L ≈ 1 K/mm: semi-stable (0.597/0.258/0.145), stable(0.364/0.428/0.208) and unstable (0.20/0.40/0.40)

Fig. 10 Stability map of the flow obtained during the Soret experi-ments when �T/L ≈ 1 K/mm. The boundary between stable (yellow)and unstable (blue) regions is shown by dashed line. The demixingzone is shaded with squares

The stability of a stratified system very much depends onthe temperature difference and on the cell height. The stabil-ity map shown in Fig. 10 correspond to �T/L ≈ 1 K/mmand it will be shifted applying another �T (�= 6 K) orchoosing different thickness L (�= 6.1 mm) of the cell.

Conclusions

In the frame of DCMIX project the measurements of dif-fusion and thermodiffusion coefficients will be conductedin the mixtures of different origins using SODI instru-ment on the ISS. We have reported motivations for theselection of Tol-Meth-Ch mixture for DCMIX2 experimentand ground-based activities in preparation of microgravityexperiment.

The refractive indices (RI) have been measured in thismixture in the whole concentration space at two wavelengths, available in the SODI instrument. Knowledge of RIallowed calculation of the contrast factors and conditionsnumbers of the corresponding matrices. The mixture con-centrations have been selected for microgravity experimentin the region of low condition numbers at different distancesfrom demixing zone.

Tol-Meth-Ch mixture is chemically active and not com-patible with often used seals (elastomers). We presentedthe results of the studying the compatibility of this mixturewith Viton, Chemraz and Kalrez. The density and thermalexpansion have been measured in the points of interest.

Microgravity Sci. Technol. (2014) 25:275–283 283

The previous measurements of Soret coefficients inbinary mixture Tol-Meth are confirmed. We have performedcomprehensive study of the hydrodynamic stability of themixture in the gravity field. The ternary system is hydro-dynamically stable only in a small region of concentrationswhere the mixture is rich by methanol and poor in cyclohex-ane. The most interesting region around the demixing zoneis unstable and, presently, the themodiffusion coefficientscan only be measured in microgravity.

Acknowledgments This work is supported by the PRODEX pro-gramme of the Belgian Federal Science Policy Office, ESA.

References

Blanco, P., Bou-Ali, M.M., Platten, J.K., de Mezquia, D.A.,Madariaga, J.A., Santamaria, C.M.: Thermodiffusion coefficientsof binary and ternary hydrocarbon mixtures. J. Chem. Phys. 132,114506 (2010)

Bou-Ali, M.M., Ecenarro, O., Madariaga, J.A., Santamaria, C.M.,Valencia, J.J.: Measurement of negative Soret coefficients in a ver-tical fluid layer with an adverse density gradient. Phys. Rev. 62,1420 (2000)

Croccolo, F., Bataller, H., Scheffold, F.: A light scattering study of nonequilibrium fluctuations in liquid mixtures to measure the Soretand mass diffusion coefficient. J. Chem. Phys. 137, 234202 (2012)

Gebhardt, M., Kohler, W., Mialdun, A., Yasnou, V., Shevtsova, V.: Dif-fusion, thermal diffusion, Soret coefficients, and optical contrastfactors of the binary mixtures of dodecane, isobutylbenzene and1,2,3,4-tetrahydronaphthalene. J. Chem. Phys. 138, 114503 (2013)

Grossmann, T., Winkelmann, J.: Part 1. Toluene-rich area. J.Chem.Eng. Data 54, 405 (2009a)

Grossmann, T., Winkelmann, J.: Part 2. Toluene-rich area near theBinodal curve. J.Chem. Eng. Data 54, 485 (2009b)

Hariharan, P., Oreb, B.F., Eiju, T.: Digital phase-shifting interferom-etry: a simple error-compensating phase calculation algorithm.Appl. Opt. 26, 2504 (1987)

Hartmann, S., Wittko, G., Kohler, W., Morozov, K.I., Albers, K.,Sadowski, G.: Thermophobicity of liquids: heats of transport inmixtures as pure component properties. Phys. Rev. Lett. 109,065901 (2012)

Koniger, A., Wunderlich, H., Kohler, W.: Measurement of diffusionand thermal diffusion in ternary fluid mixtures using a two-coloroptical beam deflection technique. J. Chem. Phys. 132, 174506(2010)

Mazzoni, S., Shevtsova, V., Mialdun, A., Melnikov, D., Gaponenko, Y.,Lyubimova, T., Saghir, Z.: Vibrating liquids in space. Europhys.News 41, 14 (2010)

Mialdun, A., Shevtsova, V.: Development of optical digital interferom-etry technique for measurement of thermodiffusion coefficients.Int. J. Heat Mass Transfer 51, 3164 (2008)

Mialdun, A., Shevtsova, V.: Measurement of Soret coefficients: openquestions. Microgravity Sci. Technol. 21, 31 (2009)

Mialdun, A., Shevtsova, V.: Measurement of the Soret and diffusioncoefficients for benchmark binary mixtures by means of digitalinterferometry. J. Chem. Phys. 134, 044524 (2011a)

Mialdun, A., Shevtsova, V.: Digital interferometry as a powerful toolto study the thermodiffusion effect. C. R. Mecanique 339, 362(2011b)

Mialdun, A., Shevtsova, V.: Communication: new approach for anal-ysis of thermodiffusion coefficients in ternary mixtures. J. Chem.Phys. 138, 161102 (2013)

Mialdun, A., Minetti, C., Gaponenko, Y., Shevtsova, V., Dunois,F.: Towards SODIDCMIX experiments on the ISS: analysisof the thermal performance. Microgravity Sci. Technol. 25, 83(2013)

Platten, J.K., Bou-Ali, M.M., Costeseque, P., Dutrieux, J.F., Kohler,W., Leppla, C., Wiegand, S., Wittko, G.: Benchmark values for theSoret, thermal diffusion and diffusion coefficients of three binaryorganic liquid mixtures. Phil. Mag. 83, 1965 (2003)

Ryzhkov, I.I., Shevtsova, V.M.: Long-wave instability of a multicom-ponent fluid layer with the Soret effect. Phys. Fluids 21, 014102(2009)

Shevtsova, V., Melnikov, D.E., Legros, J.C.: Onset of convection inSoret-driven instability. Phys. Rev. E 73, 047302 (2006)

Shevtsova, V., Sechenyh, V., Nepomnyashchy, A., Legros, J.C.: Anal-ysis of the application of optical two-wavelength techniques tomeasurement of the Soret coefficients in ternary mixtures. Philos.Mag. 91, 3498 (2011a)

Shevtsova, V., Lyubimova, T., Saghir, Z., Melnikov, D., Gaponenko,Y., Sechenyh, V., Legros, J.C., Mialdun, A.: IVIDIL: on-board g-jitters and diffusion controlled phenomena. J. Phys. Conf. Ser. 327,012031 (2011b)

Sechenyh, V., Legros, J.C., Shevtsova, V.: Measurements of opti-cal properties in binary and ternary mixtures containing cyclo-hexane, toluene, and methanol. J. Chem. Eng. Data 57, 1036(2012)

Story, M.J., Turner, J.C.R.: Flow cell studies of thermal diffusionin liquids. Part 5.Binary mixtures of CH3OH with CCl4, ben-zene and cyclohexane at 25 ◦C. Trans. Faraday Soc. 65, 1523(1969)

Voit, A., Krekov, A., Kohler, W.: Laser-induced structures in a poly-mer blend in the vicinity of the phase boundary. Phys. Rev. E 76,011808 (2007)