Variety of flow patterns in a liquid bridge subjected to a gas stream.

We present an experimental and two-phase computational study of convection in a liquid bridge ([Formula: see text]) that develops under the action of a parallel gas flow. The study focuses on tracking the evolution of hydrothermal waves by increasing the applied temperature difference [Formula: see text] and the temperature of gas moving at the velocity [Formula: see text]. Our experiments revealed certain regularity in the change of oscillatory states with an increase in the control parameters. Above the instability threshold, the nonlinear dynamics passes through three oscillatory regimes, which are repeated in a somewhat similar way at higher values of the control parameters. They are periodic, quasi-periodic with two or three frequencies and multi-frequency state when the Fourier spectrum is filled with clusters of duplex, triplex or higher numbers of frequencies. Three-dimensional numerical simulation, complemented by a deep spectral analysis, sheds light on the evolution of the flow pattern observed in experiments. The developed methodology identified conditions for the existence of a multi-frequency regime such as the presence of a weak low-frequency mode that can modulate strong high-frequency modes, the existence of strong azimuthal modes with different wavenumbers and the [Formula: see text] mode, and the structured combination of peaks in the Fourier spectrum. This article is part of the theme issue 'New trends in pattern formation and nonlinear dynamics of extended systems'.

Diffusion in liquid mixtures.

The understanding of transport and mixing in fluids in the presence and in the absence of external fields and reactions represents a challenging topic of strategic relevance for space exploration. Indeed, mixing and transport of components in a fluid are especially important during long-term space missions where fuels, food and other materials, needed for the sustainability of long space travels, must be processed under microgravity conditions. So far, the processes of transport and mixing have been investigated mainly at the macroscopic and microscopic scale. Their investigation at the mesoscopic scale is becoming increasingly important for the understanding of mass transfer in confined systems, such as porous media, biological systems and microfluidic systems. Microgravity conditions will provide the opportunity to analyze the effect of external fields and reactions on optimizing mixing and transport in the absence of the convective flows induced by buoyancy on Earth. This would be of great practical applicative relevance to handle complex fluids under microgravity conditions for the processing of materials in space.

Transport properties of n-ethylene glycol aqueous solutions with focus on triethylene glycol-water.

Soret effect and diffusion in triethylene glycol (TEG)-water mixtures were investigated as a function of concentration at 25 °C by means of optical digital interferometry, with the use of a classical Soret cell. Diffusion D, thermal diffusion DT, and Soret ST coefficients are described for the full concentration range and an analysis is made individually for TEG-water mixture and within a series of n-ethylene glycol (n-EG) aqueous systems. All coefficients decrease with increasing the concentration of TEG and n-EG. ST shows a change of sign with concentration, and this change is directly related to the ability of the n-EG molecule to establish hydrogen bonding with water. Diffusion and thermal diffusion coefficients present a plateau behavior with increasing concentration, showing the occurrence of changes in the preferential interactions in aqueous solution with concentration and meaning that, at high TEG composition, ether oxygens can be involved in the molecular interactions.

The Soret coefficients of the ternary system water/ethanol/triethylene glycol and its corresponding binary mixtures.

Thermodiffusion in ternary mixtures is considered prototypic for the Soret effect of truly multicomponent systems. We discuss ground-based measurements of the Soret coefficient along the binary borders of the Gibbs triangle of the highly polar and hydrogen bonding ternary DCMIX3-system water/ethanol/triethylene glycol. All three Soret coefficients decay with increasing concentration, irrespective of the choice of the independent component, and show a characteristic sign change as a function of temperature and/or composition. With the exception of triethylene glycol/ethanol at high temperatures, the minority component always migrates toward the cold side. All three binaries exhibit temperature-independent fixed points of the Soret coefficient. The decay of the Soret coefficient with concentration can be related to negative excess volumes of mixing. The sign changes of the Soret coefficients of the binaries allow to draw far-reaching conclusions about the signs of the Soret coefficients of the corresponding ternary mixtures. In particular, we show that at least one ternary composition must exist, where all three Soret coefficients vanish simultaneously and no steady-state separation is observable.

European Space Agency experiments on thermodiffusion of fluid mixtures in space.

This paper describes the European Space Agency (ESA) experiments devoted to study thermodiffusion of fluid mixtures in microgravity environment, where sedimentation and convection do not affect the mass flow induced by the Soret effect. First, the experiments performed on binary mixtures in the IVIDIL and GRADFLEX experiments are described. Then, further experiments on ternary mixtures and complex fluids performed in DCMIX and planned to be performed in the context of the NEUF-DIX project are presented. Finally, multi-component mixtures studied in the SCCO project are detailed.

Preliminary analysis of Diffusion Coefficient Measurements in ternary mIXtures 4 (DCMIX4) experiment on board the International Space Station.

In the frame of the Diffusion Coefficient Measurements in ternary mIXtures 4 (DCMIX4) project the thermodiffusion experiments were conducted on the International Space Station (ISS) in the Selectable Optical Diagnostics Instrument (SODI) which is on orbit since 2009. We describe the results of the preliminary analysis of images downloaded during the execution of DCMIX4 in order to check the quality of the running experiments and, if needed, adjust the experiment parameters for the following runs. The quick analysis of raw data showed that they are meaningful and will allow to obtain the transport coefficients of examined ternary and binary mixtures.

The Soret effect in ternary mixtures of water+ethanol+triethylene glycol of equal mass fractions: Ground and microgravity experiments.

Measurements of the Soret and thermodiffusion coefficients of a symmetric ternary mixture with equal mass fractions of water, ethanol, and triethylene glycol have been performed by two-color optical beam deflection (2-OBD) and the thermogravitational column technique (TGC) in the laboratory and under microgravity conditions in the Selectable Optical Diagnostics Instrument (SODI) aboard the International Space Station. The results from all three experimental techniques agree within the experimental error bars, which result mainly from the inversion of the contrast factor matrices. TGC shows by far the lowest, 2-OBD the highest error amplification. The microgravity measurements are in between. The agreement with the microgravity results shows that thermosolutal convection could be well controlled in the 2-OBD experiments by a proper orientation of the temperature gradient. Despite the different condition numbers, the results are invariant under the choice of the independent compositions. Based on the orientation of the confidence ellipsoid in the ternary composition diagram, not all coefficients are equally affected by experimental errors. Although there are appreciable uncertainties for water and ethanol, the Soret and the thermodiffusion coefficients of triethylene glycol could be obtained with a good accuracy due to the favorable orientation of the confidence ellipsoid. We have found that water behaves thermophobic, corresponding to a positive Soret coefficient, whereas both ethanol and triethylene glycol are thermophilic with negative Soret coefficients. This resembles the behaviour of the binary system ethanol/water above the ethanol concentration of the sign change.

Benchmark values for the Soret, thermodiffusion and molecular diffusion coefficients of the ternary mixture tetralin+isobutylbenzene+n-dodecane with 0.8-0.1-0.1 mass fraction.

With the aim of providing reliable benchmark values, we have measured the Soret, thermodiffusion and molecular diffusion coefficients for the ternary mixture formed by 1,2,3,4-tetrahydronaphthalene, isobutylbenzene and n-dodecane for a mass fraction of 0.8-0.1-0.1 and at a temperature of 25°C. The experimental techniques used by the six participating laboratories are Optical Digital Interferometry, Taylor Dispersion technique, Open Ended Capillary, Optical Beam Deflection, Thermogravitational technique and Sliding Symmetric Tubes technique in ground conditions and Selectable Optical Diagnostic Instrument (SODI) in microgravity conditions. The measurements obtained in the SODI installation have been analyzed independently by four laboratories. Benchmark values are proposed for the thermodiffusion and Soret coefficients and for the eigenvalues of the diffusion matrix in ground conditions, and for Soret coefficients in microgravity conditions.

Measurement of thermodiffusion coefficient of hydrocarbon binary mixtures under pressure with the thermogravitational technique.

It was designed and constructed a new thermogravitational column able to operate at high pressures (up to 50 MPa). This new thermogravitational column is of the cylindrical type with closed ends. It is made of stainless steel. The length of the column is 0.5 m and the gap between its two walls is variable. First, the column was validated at atmospheric pressure by means of measurements of the thermodiffusion coefficient of well-known binary mixtures. Then, this new thermogravitational column was used to measure the thermodiffusion coefficient of the binary mixtures 1,2,3,4-tetrahydronaphtalene/isobutylbenzene, 1,2,3,4-tetrahydronaphtalene/n-dodecane, and isobutylbenzene/n-dodecane at high pressures and within the pressure range between 0.1 and 20 MPa at a mean temperature of 25 °C. We have found a linear dependence between the thermodiffusion coefficient and the pressure.

A comprehensive study of diffusion, thermodiffusion, and Soret coefficients of water-isopropanol mixtures.

We report on the measurement of diffusion (D), thermodiffusion (D(T)), and Soret (S(T)) coefficients in water-isopropanol mixtures by three different instrumental techniques: thermogravitational column in combination with sliding symmetric tubes, optical beam deflection, and optical digital interferometry. All the coefficients have been measured over the full concentration range. Results from different instruments are in excellent agreement over a broad overlapping composition (water mass fraction) range 0.2 < c < 0.7, providing new reliable benchmark data. Comparison with microgravity measurements (SODI/IVIDIL (Selected Optical Diagnostic Instrument/Influence of VIbration on DIffusion in Liquids)) onboard the International Space Station and with literature data (where available) generally gives a good agreement. Contrary to theoretical predictions and previous experimental expectations we have not observed a second sign change of S(T) at low water concentrations.

Inclined layer Soret instabilities.

Linear stability of a binary mixture buoyant return flow in a differentially heated inclined infinite layer is investigated by asymptotic long-wave analysis and pseudospectral Chebyshev numerical solutions. The Soret coefficient is negative so that thermodiffusion separates the species with the heavier component migrating to the hot wall, thus, promoting unstable stratification except in the classical Rayleigh-Benard arrangement. It is shown that longitudinal instabilities with small wave numbers are triggered at any finite temperature difference at all angles of inclination except very close to the horizontal heated from the above or below arrangements. Numerical results are given for a specific water-ethanol mixture and are in excellent agreement with the asymptotic results. As is well known the heated from below horizontal layer is overstable while that heated from above is doubly-diffusive unstable. Transition from the longitudinal stationary instabilities in inclined layers to these instabilities in horizontal layers is also given for this mixture.

Soret coefficients in some water-methanol, water-ethanol, and water-isopropanol systems.

In this article, we re-examine the published experimental values of the Soret coefficients of a few water-alcohol systems at a mean temperature of 37.5 degrees C, because we feel that these published values (including microgravity values) are doubtful. The main technique applied is the use of thermogravitational columns to determine the thermodiffusion coefficient. The obtained values did not agree with the published values; worse, sometimes the sign was different.