Tuning mixing of suspension by particles (re-)active to their local environment. – TunaMix

Based on promising preliminary results by the applicant consortium, this project aims at exploring how novel local-scale mechanisms, providing «environment sensing» capabilities, can be harvested to control macro-scale mixing of particles and macromolecules. While usual strategies to improve mixing generally consist in stirring the fluid to promote advection and compensate the low efficiency of molecular diffusion, we propose here a new strategy, which consists in promoting the small-scale mobility of the mixed entities by exploiting phoretic «environment sensing» phenomena. Practically these phoretic mechanisms can be as simple as colloids responding to a weakly inhomogeneous chemical or thermal background, and as generic as swimming microorganisms in oceans or bio-reactors. Such mechanisms jeopardize the classical description of particles transport, showing that the smallness of intrinsic mobility is not a fate, but that «environment sensing» can unveil novel levers to control the interplay of particles with a mixing flow. For instance the group at ILM has shown that diffusiophoresis (enhanced mobility in presence of chemical gradients) can enhance by order of magnitudes (compared to simple Brownian diffusion) the effective mobility of colloids in presence of controlled salinity gradients. In this project, we will explore how «environment sensing» combined to fluid mechanics can be used to control (enhance, reduce, or even reverse) mixing of particles in flows ranging from micro-channels to macro-scale chaotic, and eventually turbulent, systems. The relevance of this approach has already been proven in preliminary studies by the partner ILM, who coupled for the first time chaotic advection and diffusiophoresis to tune the mixing of colloids in micromixers. This promising result has motivated further exploration of «environment sensing» processes in macroscopic systems, leading to preliminary collaborations inside the present consortium combining partners with a broad range of expertise in «soft condensed matter» and in turbulent and chaotic mixing. This consortium has demonstrated for the first time the persistence of chemical phoretic effects across scales, up to truly macro-scales in a 5 cm wide Hele-Shaw chaotic mixing cell. In addition, numerical simulations by the consortium, pointed out that these phenomena are associated with the appearance of effective compressibility effects related to the non divergence-free of the colloids diffusiophoretic velocity field. The present project naturally builds on these preliminary successes of the consortium, showing the relevance of our bottom-up approach, to either tune nano/micro-scale phoretic phenomena to trigger upscale mixing effects, or understand how such small-scale phoretic (eventually self-phoretic) phenomena can impact large scale transport processes. Specifically, we mention here the following new fascinating perspectives: the striking possibility to un-mix particles (demixing); the possibility that small-scale «environment sensing» plays a role in macroscopic turbulent mixing of particles; the possibility to bridge the theoretical description of «environment sensing» in the presence of advection to the extensive studies of inertial particles in turbulence; the possibility to extend the concept of «environment sensing» to active particles (capable of self-propulsion) in complex flows. This rich context shows that combining the expertise of the group at ILM, whose pioneering experimental results have opened a new outstanding domain of investigation of particles under «environment sensing», and of the groups at the ENS Lyon and LMFA, with a unique expertise in the study of particles in complex and turbulent flows (including collective effects) offers extremely promising perspectives to make headway in the understanding of new fascinating effects on particles mixing.