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Postdoc on diffusiophoresis in a Rayleigh-Bénard cell, ENS de Lyon / LMFA, France

ENS de Lyon and LMFA
Contact :
Romain.volk@ens-lyon.fr
Florence.raynal@ec-lyon.fr


We propose to study how mixing is affected by diffusiophoresis in a Rayleigh-Bénard cell. The config- uration chosen is a long cell, where the velocity is almost 2D just above the instability threshold, and where multiple convection cells are present. The geometry of the cell is very similar to the one used by Solomon & Gollub [4].

 

Post-doc position: diffusiophoresis in a Rayleigh-Bénard cell

The post-doc position (1 year), will start in september 2020 and is mostly experimental.

Diffusiophoresis [1-3] is the spontaneous movement of large particles (colloids) under the effect of a salt gradient; it results in a velocity of the type

vdp =DdplogS, (1)

where Ddp is the diffusiophoretic coefficient and is the salt concentration. Besides the simplicity of the coupling, where the particles are coupled to salt whereas salt evolves freely, the resulting dynamics are rather complex. It is possible to tune the quality of mixing, depending on whether the particles are introduced with the salt (“salt-in”, delayed mixing) or whether the solvant is initially salted (“salt-out”, enhanced mixing).

We propose to study how mixing is affected by diffusiophoresis in a Rayleigh-Bénard cell. The config- uration chosen is a long cell, where the velocity is almost 2D just above the instability threshold, and where multiple convection cells are present. The geometry of the cell is very similar to the one used by Solomon & Gollub [4].

The goal is to study how anomalous diffusion is affected by diffusiophoresis. Many configurations will be studied

• Colloids and salt are introduced together at one side
• Colloids are introduced at one side and salt on the other side • Well mixed case with salt introduced at one side: demixing

Scheme of the experimental apparatus: because of the dimensions of the cell, the flow is mostly 2D near onset. Colloids or salt are initially introduced at one side or the other, depending on the configuration chosen.

All the experiments will be compared to 2D numerical experiments.

Because a Rayleigh-B ́enard apparatus implies temperature gradients, the case of thermophoresis (spon- taneous movement of particles under the effect of a temperature gradient) will also be tested.

References:
[1] Ab ́ecassis, B., Cottin-Bizonne, C., Ybert, C., Ajdari, A. & Bocquet, L., (2009) Osmotic manipulation

of particles for microfluidic applications. New J. of Physics 11 (7), 075022.
[2] Raynal, F., Bourgoin, M., Cottin-Bizonne, C., Ybert, C. & Volk, R., (2018) Advection and diffusion

in a chemically induced compressible flow. J. Fluid Mechanics 847, pp228243. HAL
[3] Raynal, F. & Volk, R., (2019) Diffusiophoresis, Batchelor scale and effective P ́eclet numbers. 
J. Fluid

Mechanics 876, pp 818-829. HAL
[4] Solomon T.H. & Gollub J.P.(1988) Passive transport in steady RayleighB ́enard convection. 
Phys.

Fluids 31, 1372. Contacts:

́
Florence Raynal, LMFA, Ecole centrale de Lyon. E-mail: florence.raynal@ec-lyon.fr

Romain Volk, LP ENS de Lyon. E-mail: romain.volk@ens-lyon.fr