Nondiffusive decay of gradient-driven fluctuations in a free-diffusion process (original) (raw)
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Effect of gravity on the dynamics of nonequilibrium fluctuations in a free-diffusion experiment
Annals of the New York Academy of Sciences, 2006
Diffusion is commonly believed to be a homogeneous process at the mesoscopic scale, being driven only by the random walk of fluid molecules. On the contrary, very large amplitude, long wavelength fluctuations always accompany diffusive processes. In the presence of gravity, fluctuations in a fluid containing a stabilizing gradient are affected by two different processes: diffusion, which relaxes them, and the buoyancy force, which quenches them. These phenomena affect both the overall amplitude of fluctuations and their time dependence. For the case of free diffusion, the time-correlation function of the concentration fluctuations is predicted to exhibit an exponential decay with correlation time depending on the wave vector q. For large wave vector fluctuations, diffusion dominates, and the correlation time is predicted to be 1 / (Dq2). For small wave vector fluctuations, gravitational forces have time to play a significant role, and the correlation time is predicted to be proporti...
Universal behavior of nonequilibrium fluctuations in free diffusion processes
2000
We show that giant nonequilibrium fluctuations are present during the free diffusive remixing occurring in ordinary liquid mixtures and in macromolecular solutions. The static structure factor of the fluctuations is measured by using a quantitative shadowgraph technique. We show that structure factors at different times and from different samples can be rescaled onto a single master curve without any adjustable parameter, thus giving strong evidence that nonequilibrium fluctuations are a universal feature of free diffusion processes.
Giant fluctuations in diffusion processes
Journal of Physics: Condensed Matter, 2000
Concentration fluctuations in a homogeneous mixture are in general small and confined to the molecular lengthscale. It has been recently predicted that stressed fluids should exhibit anomalously large fluctuations. We will show here that anomalously large fluctuations also occur when macroscopic concentration gradients relax to the uniform state via diffusion. The measurements have been taken by low angle static light scattering and shadowgraphs. We find that at larger wavevectors the amplitude of the fluctuations diverges as q −4 . It is also found that gravity stabilizes at a constant value the fluctuations below a critical wavevector. We will present data on a mixture close to a consolution critical point. Recent results will also be presented on ordinary liquid mixtures, a polymer and a protein solution. These new results have been obtained by means of a quantitative shadowgraph technique. They confirm that giant fluctuations are always associated with mass flow due to diffusion across a macroscopic gradient.
Dynamics of intermediate to late stage concentration fluctuations during free diffusion experiments
Two experiments were performed using shadowgraph visualization technique in order to study the dynamics of concentration fluctuations. Our experimental setup includes an objective attached to the CCD camera to increase the field of view. Using two colloidal suspensions, one with gold and the other with silica colloid, we extracted both the structure factors and the correlation time during the intermediate to late stages of concentration fluctuations during a free diffusion experiment. The temporal evolution of fluctuations is qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. Our experiment reveals significant differences between concentration fluctuations induced in silica and gold colloids. The difference is explained not only in terms of the particle size but due to the possible plasmonic interaction between gold nanoparticle and the incident light.
Slowing-down of non-equilibrium concentration fluctuations in confinement
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.
The European physical journal. E, Soft matter, 2014
In a binary fluid mixture subject to gravity and a stabilizing concentration gradient, concentration non-equilibrium fluctuations are long-ranged. While the gradient leads to an enhancement of the respective equilibrium fluctuations, the effect of gravity is a damping of fluctuations larger than a "characteristic" size. This damping is visible both in the fluctuation power spectrum probed by static and the temporal correlation function probed by dynamic light scattering. One aspect of the "characteristic" size can be appreciated by the dynamic analysis; in fact at the corresponding "characteristic" wave vector q* one can observe a maximum of the fluctuation time constant indicating the more persistent fluctuation of the system. Also in the static analysis a "characteristic" size can be extracted from the crossover wave vector. According to common theoretical concepts, the result should be the same in both cases. In the present work we provide ...
The Journal of Chemical Physics, 2012
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.
Scientific Reports, 2015
Diffusion processes are accompanied by the appearance of non-equilibrium fluctuations, whose size distribution on Earth is strongly affected by the gravity force. In microgravity and at steady state, these fluctuations exhibit generic scale invariance and their size is only limited by the finite dimension of the system. In this work, we investigate experimentally and computationally the development of non-equilibrium fluctuations during a thermophoretic process in microgravity. Both experiments and simulations show that during the onset of fluctuations the scale invariance is present at large wave vectors. In a broader range of wave vectors simulations predict a spinodal-like growth of fluctuations, where the amplitude and length scale of the dominant mode are determined by the thickness of the diffuse layer.
Fluctuations in Diffusion Processes in Microgravity
Annals of the New York Academy of Sciences, 2006
It has been shown recently that diffusion processes exhibit giant nonequilibrium fluctuations (NEFs). That is, the diffusing fronts display corrugations whose length scale ranges from the molecular to the macroscopic one. The amplitude of the NEF diverges following a power law behavior ∝ q −4 (where q is the wave vector). However, fluctuations of wave number smaller than a critical "rolloff" wave vector are quenched by the presence of gravity. It is therefore expected that in microgravity conditions, the amplitude of the NEF should be boosted by the absence of the buoyancy-driven restoring force. This may affect any diffusion process performed in microgravity, such as the crystallization of a protein solution induced by the diffusion of a salt buffer. The aim of GRADFLEX (GRAdient-Driven FLuctuation EXperiment), a joint project of ESA and NASA, is to investigate the presence of NEFs arising in a diffusion process under microgravity conditions. The project consists of two experiments.