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-Bénard 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 Péclet 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-Bénard chamber, and blockage when colloids and salt are injected together and phoretic effects are strong enough.

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