We use dynamic near field scattering to measure the dynamics of concentration non equilibrium fluctuations at the steady-state of Soret separation. The analysis reveals that above a threshold wave vector q c , the dynamics is governed by diffusion while at smaller wave vectors, gravity dominates. From the measurements, we extract both the mass diffusion and the Soret coefficients. Comparing our results with literature data, we find good agreement confirming that the proposed experimental technique can be considered a sound approach for the study of thermodiffusion processes.

Fluctuations in a fluid are strongly affected by the presence of a macroscopic gradient making them long-ranged and enhancing their amplitude. While small-scale fluctuations exhibit diffusive lifetimes, larger-scale fluctuations live shorter because of gravity, as theoretically and experimentally well-known. We explore here fluctuations of even larger size, comparable to the extent of the system in the direction of the gradient, and find experimental evidence of a dramatic slowing-down in their dynamics. We recover diffusive behaviour for these strongly-confined fluctuations, but with a diffusion coefficient that depends on the solutal Rayleigh number. Results from dynamic shadowgraph experiments are complemented by theoretical calculations and numerical simulations based on fluctuating hydrodynamics, and excellent agreement is found. The study of the dynamics of non-equilibrium fluctuations allows to probe and measure the competition of physical processes such as diffusion, buoyancy and confinement.PACS numbers: 05.70.Ln, 42.30.Va It is well established that fluctuations are long-ranged in systems out-of-equilibrium [1-3], even far from critical points where the long-range behaviour is observed also in equilibrium conditions [4]. In a binary fluid mixture subject to a stabilizing (vertical) temperature or concentration gradient, the coupling between the spontaneous velocity fluctuations and the macroscopic gradient results in giant concentration fluctuations in the quiescent state [3, 5]. Gravity quenches the intensity of fluctuations with length scales larger than a characteristic (horizontal) size 2π/q s related to the dimensionless solutal Rayleigh number Ra s of the system [5, 6]:where β s = ρ −1 (∂ρ/∂c) is the solutal expansion coefficient, ρ the fluid density, g the gravity acceleration, c the concentration (mass fraction) of the denser component of the fluid, ∇c the modulus of the concentration gradient, D the mass diffusion coefficient, ν the kinematic viscosity, and q s a characteristic solutal wave vector. Vertical boundaries suppress fluctuations larger than the confinement length L in the direction of the gradient [3, 7]. Gravity also accelerates the dynamics of the fluctuations for wavenumbers smaller than q s via buoyancy effects, leading to non-diffusive decay of large-scale fluctuations [8].The dynamics of concentration non-equilibrium fluctuations (c-NEFs) in the presence of a vertical concentration gradient in a binary liquid mixture can be characterized in terms of the Intermediate Scattering Function (ISF or, equivalently, normalized time correlation function) f (q, t), with f (q, 0) = 1. At first approximation the ISF can be modeled by a single exponential with decay time τ (q) depending on the analysed wave vector q.Available theories accounting for the simultaneous presence of diffusion (d) and gravity (g) [9, 10], but not for confinement, predict for a stable configuration (Ra s < 0):where the wave vector is expressed in its dimensionless formq = qL and τ s = L 2 /D is the typical so...

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Compositional grading within a mixture has a strong impact on the evaluation of the pre-exploitation distribution of hydrocarbons in underground layers and sediments. Thermodiffusion, which leads to a partial diffusive separation of species in a mixture due to the geothermal gradient, is thought to play an important role in determining the distribution of species in a reservoir. However, despite recent progress, thermodiffusion is still difficult to measure and model in multicomponent mixtures. In this work, we report on experimental investigations of the thermodiffusion of multicomponent n-alkane mixtures at pressure above 30 MPa. The experiments have been conducted in space onboard the Shi Jian 10 spacecraft so as to isolate the studied phenomena from convection. For the two exploitable cells, containing a ternary liquid mixture and a condensate gas, measurements have shown that the lightest and heaviest species had a tendency to migrate, relatively to the rest of the species, to the hot and cold region, respectively. These trends have been confirmed by molecular dynamics simulations. The measured condensate gas data have been used to quantify the influence of thermodiffusion on the initial fluid distribution of an idealised one dimension reservoir. The results obtained indicate that thermodiffusion tends to noticeably counteract the influence of gravitational segregation on the vertical distribution of species, which could result in an unstable fluid column. This confirms that, in oil and gas reservoirs, the availability of thermodiffusion data for multicomponent mixtures is crucial for a correct evaluation of the initial state fluid distribution.

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In a fluid system driven out of equilibrium by the presence of a gradient, fluctuations become long-ranged and their intensity diverges at large spatial scales. This divergence is prevented vertical confinement and, in a stable configuration, by gravity. Gravity and confinement also affect the dynamics of non-equilibrium fluctuations (NEFs). In fact, small wavelength fluctuations decay diffusively, while the decay of long wavelength ones is either dominated by buoyancy or by confinement. In normal gravity, from the analysis of the dynamics one can extract the diffusion coefficients as well as other transport properties. For example, in a thermodiffusion experiment one can measure the Soret coefficient. Under microgravity, the relaxation of fluctuations occurs by diffusion only and this prevents the determination of the Soret coefficient of a binary mixture from the study of the dynamics. In this work we propose an innovative self-referencing optical method for the determination of the thermal diffusion 2 ratio of a binary mixture that does not require previous knowledge of the temperature difference applied to the sample. The method relies on the determination of the ratio between the mean squared amplitude of concentration and temperature fluctuations. We investigate data from the GRADFLEX experiment, an experiment flown onboard the Russian satellite FOTON M3 in 2007. The investigated sample is a suspension of polystyrene polymer chains (MW=9,100g/mol, concentration 1.8wt%) in toluene, stressed by different temperature gradients. The use of a quantitative shadowgraph technique allows to perform measurements in the absence of delicate alignment and calibration procedures. The statics of the concentration and temperature NEFs are obtained and their ratio is computed. At large wave vectors the ratio becomes constant and is shown to be proportional to the thermal diffusion ratio of the sample.

In this work we propose a methodology, based on molecular dynamics simulations, to quantify the influence of segregation and thermodiffusion on the initial state distribution of the fluid species in hydrocarbon reservoirs. This convection-free approach has been applied to a synthetic oil composed of three normal alkanes and to a real acid gas. It has been found that the thermodiffusion effect induced by the geothermal gradient is similar (but opposite in sign) to that due to segregation for both mixtures. In addition, because of the combined effect of thermal expansion Guillaume Galliero guillaume.galliero@univ-pau.fr and thermodiffusion, it has been observed that the density gradient can be reversed, in the presence of a geothermal gradient. These numerical results emphasize the need of improving our quantification of thermodiffusion in multicomponent mixtures. The SCCO-SJ10 experiments will be a crucial step towards this goal.

Abstract. Diffusion and thermal diffusion processes in a liquid mixture are accompanied by long-range non-equilibrium fluctuations, whose amplitude is orders of magnitude larger than that of equilibrium fluctuations. The mean square amplitude of the non-equilibrium fluctuations presents a scale-free power law behavior q −4 as a function of the wave vector q, but the divergence of the amplitude of the fluctuations at small wave vectors is prevented by the presence of gravity. In microgravity conditions the non-equilibrium fluctuations are fully developed and span all the available length scales up to the macroscopic size of the systems in the direction parallel to the applied gradient. Available theoretical models are based on linearized hydrodynamics and provide an adequate description of the statics and dynamics of the fluctuations in the presence of small temperature/concentration gradients and under stationary or quasi-stationary conditions. We describe a project aimed at the investigation of Non-EquilibriUm Fluctuations during DIffusion in compleX liquids (NEUF-DIX). The focus of the project is on the investigation of the non-equilibrium fluctuations in complex liquids, trying to tackle several challenging problems that emerged during the latest years, such as the theoretical predictions of Casimir-like forces induced by non-equilibrium fluctuations; the understanding of the non-equilibrium fluctuations in multi-component mixtures including a polymer, both in relation to the transport coefficients and to their behavior close to a glass transition; the understanding of the non-equilibrium fluctuations in concentrated colloidal suspensions, a problem closely related with the detection of Casimir forces; and the investigation of the onset of fluctuations during transient diffusion. We envision to parallel these experiments with state of the art multi-scale simulations.