arXiv subjects

Florence Raynal

Publications and source records attributed to Florence Raynal.

10 recordsLinked to original sources

arXiv2025

Experimental study of diffusiophoresis in a cellular flow

In this article, we study experimentally the dispersion of colloids in a two-dimensional, time independent, Rayleigh-B\'enard flow in the presence of salt gradients. Due to the additional scalar, the colloids do not follow exactly the Eulerian flow-field, but have a (small) extra-velocity $\mathbf{v}_{\mathrm{dp}} = D_{\mathrm{dp}} \nabla \log C_s$, where $D_\mathrm{dp}$ is the phoretic constant and $C_s$ the salt concentration. Such a configuration is motivated by the theoretical work by Volk \textit{et al.} (JFM, 2022), which predicted enhanced transport or blockage in a stationary cellular flow depending on the value of a blockage coefficient. By means of High Dynamical Range Light Induced Fluorescence, we study the evolution of the colloids concentration field at large P\'eclet number. We find a good agreement with the theoretical work although a number of hypotheses are not satisfied, as the experiment is non homogeneous in space and intrinsically transient. In particular we observe enhanced transport when salt and colloids are injected at both ends of the Rayleigh-B\'enard chamber, and blockage when colloids and salt are injected together and phoretic effects are strong enough.

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arXiv2022

Phoresis in cellular flows: from enhanced dispersion to blockage

In this article, we study numerically the dispersion of colloids in a two-dimensional cellular flow in the presence of an imposed mean salt gradient. Owing to the additional scalar, the colloids do not follow exactly the Eulerian flow field, but have a (small) extra-velocity proportional to the salt gradient, $\mathbf{v}_\mathrm{dp}=\alpha\nabla S$, where $\alpha$ is the phoretic constant and $S$ the salt concentration. We study the demixing of an homogenous distribution of colloids and how their long-term mean velocity $\mathbf{V_m}$ and effective diffusivity $D_\mathrm{eff}$ are influenced by the phoretic drift. We observe two regimes of colloids dynamics depending on a blockage criterion $R=\alpha G L/\sqrt{4 D_cD_s}$, where $G$ is the mean salt gradient amplitude, $L$ the length scale of the flow and $D_c$ and $D_s$ the molecular diffusivities of colloids and salt. When $R<1$, the mean velocity is strongly enhanced with $V_m \propto \alpha G \sqrt{Pe_s}$, $Pe_s$ being the salt P\'eclet number. When $R> 1$, the compressibility effect due to the phoretic drift is so strong that a depletion of colloids occurs along the separatrices inhibiting cell-to-cell transport.

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arXiv2022

Residence time distributions for in-line chaotic mixers

We investigate the distributions of residence time for in-line chaotic mixers; in particular, we consider the Kenics, the F-mixer and the Multi-level laminating mixer, and also a synthetic model that mimics their behavior and allows exact mathematical calculations. We show that whatever the number of elements of mixer involved, the distribution possesses a $t^{-3}$ tail, so that its shape is always far from Gaussian. This $t^{-3}$ tail also invalidates the use of second-order moment and variance. As a measure for the width of the distribution, we consider the mean absolute deviation and show that, unlike the standard deviation, it converges in the limit of large sample size. Finally, we analyze the performances of the different in-line mixers from the residence-time point of view when varying the number of elements and the shape of the cross-section.

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arXiv2021

Mixing and unmixing induced by active camphor particles

In this experimental study, we report on the mixing properties of interfacial colloidal floaters (glass bubbles) by chemical and hydrodynamical currents generated by self-propelled camphor disks swimming at the air-water interface. Despite reaching a statistically stationary state for the glass bubbles distribution, those floaters always remain only partially mixed. This intermediate state results from a competition between (i) the mixing induced by the disordered motion of many camphor swimmers and (ii) the unmixing promoted by the chemical cloud attached to each individual self-propelled disk. Mixing/unmixing is characterized globally using the standard deviation of concentration and spectra, but also more locally by averaging the concentration field around a swimmer. Besides the demixing process, the system develops a "turbulent-like" concentration spectra, with a large-scale region, an inertial regime and a Batchelor region. We show that unmixing is due to the Marangoni flow around the camphor swimmers, and is associated to compressible effects.

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arXiv2019

Kolmogorovian active turbulence of a sparse assembly of interacting swimmers

Active matter, composed of self-propelled entities, forms a wide class of out-of-equilibrium systems that display striking collective behaviors among which the so-called active turbulence where spatially and time disordered flow patterns spontaneously arise in a variety of {active systems}. De facto, the active turbulence naming suggests a connection with a second seminal class of out-of-equilibrium systems, fluid turbulence, and yet of very different nature with energy injected at global system scale rather than at the elementary scale of single constituents. Indeed the existence of a possible strong-tie between active and canonical turbulence remains an open question and a field of profuse research. Using an assembly of self-propelled interfacial particles, we show experimentally that this active system shares remarkable quantitative similarities with canonical fluid turbulence, as described by the celebrated 1941 phenomenology of Kolmogorov. Making active matter entering into the universality class of fluid turbulence not only benefits to its future development but may also provide new insights for the longstanding description of turbulent flows, arguably one of the biggest remaining mysteries in classical physics.

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arXiv2019

Diffusiophoresis, Batchelor scale and effective P\'eclet numbers

We study the joint mixing of colloids and salt released together in a stagnation point or in a globally chaotic flow. In the presence of salt inhomogeneities, the mixing time is strongly modified depending on the sign of the diffusiophoretic coefficient $D_\mathrm{dp}$. Mixing is delayed when $D_\mathrm{dp}>0$ (salt-attracting configuration), or faster when $D_\mathrm{dp}<0$ (salt-repelling configuration). In both configurations, as for molecular diffusion alone, large scales are barely affected in the dilating direction while the Batchelor scale for the colloids, $\ell_{c,\mathrm{diff}}$, is strongly modified by diffusiophoresis. We propose here to measure a global effect of diffusiophoresis in the mixing process through an effective P\'eclet number built on this modified Batchelor scale. Whilst this small scale is obtained analytically for the stagnation point, in the case of chaotic advection, we derive it using the equation of gradients of concentration, following Raynal \& Gence (\textit{Intl J. Heat Mass Transfer}, vol. 40 (14), 1997, pp. 3267--3273). Comparing to numerical simulations, we show that the mixing time can be predicted by using the same function as in absence of salt, but as a function of the effective P\'eclet numbers computed for each configuration. The approach is shown to be valid when the ratio $D_\mathrm{dp}^2/D_s D_c \gg 1$, where $D_c$ and $D_s$ are the diffusivities of the colloids and salt.

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arXiv2018

Advection and diffusion in a chemically induced compressible flow

We study analytically the joint dispersion of Gaussian patches of salt and colloids in linear flows, and how salt gradients accelerate or delay colloid spreading by diffusiophoretic effects. Because these flows have constant gradients in space, the problem can be solved almost entirely for any set of parameters, leading to predictions of how the mixing time and the Batchelor scale are modified by diffusiophoresis. We observe that the evolution of global concentrations, defined as the inverse of the patches areas, are very similar to those obtained experimentally in chaotic advection. They are quantitatively explained by examining the area dilatation factor, in which diffusive and diffusiophoretic effects are shown to be additive and appear as the divergence of a diffusive contribution or of a drift velocity. An analysis based on compressibility is developed in the salt-attracting case, for which colloids are first compressed before dispersion, to predict the maximal colloid concentration as a function of the parameters. This maximum is found not to depend on the flow stretching rate nor on its topology (strain or shear flow), but only on the characteristics of salt and colloids (diffusion coefficients and diffusiophoretic constant) and the initial size of the patches.

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arXiv2015

Diffusiophoresis at the macroscale

Diffusiophoresis, a ubiquitous phenomenon that induces particle transport whenever solute concentration gradients are present, was recently observed in the context of microsystems and shown to strongly impact colloidal transport (patterning and mixing) at such scales. In the present work, we show experimentally that this nanoscale mechanism can induce changes in the macroscale mixing of colloids by chaotic advection. Rather than the decay of the standard deviation of concentration, which is a global parameter commonly employed in studies of mixing, we instead use multiscale tools adapted from studies of chaotic flows or intermittent turbulent mixing: concentration spectra and second and fourth moments of the probability density functions of scalar gradients. Not only can these tools be used in open flows, but they also allow for scale-by-scale analysis. Strikingly, diffusiophoresis is shown to affect all scales, although more particularly the small ones, resulting in a change of scalar intermittency and in an unusual scale bridging spanning more than seven orders of magnitude. By quantifying the averaged impact of diffusiophoresis on the macroscale mixing, we explain why the effects observed are consistent with the introduction of an effective P\'eclet number.

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arXiv2014

The distribution of "time of flight" in three dimensional stationary chaotic advection

The distributions of "time of flight" (time spent by a single fluid particle between two crossings of the Poincar\'e section) are investigated for five different 3D stationary chaotic mixers. Above all, we study the large tails of those distributions, and show that mainly two types of behaviors are encountered. In the case of slipping walls, as expected, we obtain an exponential decay, which, however, does not scale with the Lyapunov exponent. Using a simple model, we suggest that this decay is related to the negative eigenvalues of the fixed points of the flow. When no-slip walls are considered, as predicted by the model, the behavior is radically dfferent, with a very large tail following a power law with an exponent close to -3.

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arXiv2014

Chaotic mixing in effective compressible flows

We study numerically joint mixing of salt and colloids by a chaotic velocity field $\mathbf{V}$, and how salt inhomogeneities accelerate or delay colloid mixing by inducing a velocity drift $\mathbf{V}_{\rm dp}$ between colloids and fluid particles as proposed in recent experiments \cite{Deseigne2013}. We demonstrate that because the drift velocity is no longer divergence free, small variations to the total velocity field drastically affect the evolution of colloid variance $\sigma^2=\langle C^2 \rangle - \langle C \rangle^2$. A consequence is that mixing strongly depends on the mutual coherence between colloid and salt concentration fields, the short time evolution of scalar variance being governed by a new variance production term $P=- \langle C^2 \nabla \cdot \mathbf{V}_{\rm dp} \rangle/2$ when scalar gradients are not developed yet so that dissipation is weak. Depending on initial conditions, mixing is then delayed or enhanced, and it is possible to find examples for which the two regimes (fast mixing followed by slow mixing) are observed consecutively when the variance source term reverses its sign. This is indeed the case for localized patches modeled as gaussian concentration profiles.

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