Despite their tiny size, bacteria can profoundly alter the properties of their environment: through their spontaneous organization and the mechanical energy they inject locally by their swimming motion, they generate significant mechanical stresses that can reduce (or even cancel) the effective viscosity of the fluid they swim in. The importance and opportunities offered by this example go well beyond biology, and physicists and engineers alike are fascinated by the possibility to synthesize, analyze and exploit similar synthetic “active fluids”. Chemically-active particles and droplets offer indeed a minimal design: with no moving they simply tap into the physico-chemical energy of their immediate microscopic environment in order to self-propel. Despite the tremendous attention received by these now canonical systems, a detailed and quantitative physics-based modelling of their dynamics at the individual and collective levels has so far remained elusive.
Bridging this gap of understanding and modeling is the overarching objective of the CollectSwim project, led by Prof Sébastien Michelin at the Hydrodynamics Laboratory (LadHyX) of Ecole Polytechnique. To this end, it builds upon the theoretical and numerical expertise of the team in order (i) to obtain efficient models of such chemically-active suspensions’ behaviour from a detailed understanding at the particle level of the coupling between hydrodynamic flows and physico-chemical processes, and (ii) to exploit these models to quantify the macroscopic properties of these active fluids as well as to explore their controllability. Indeed, a better understanding of their macroscopic response to particular forcings or stimuli could eventually provide a route for designing ``tunable’’ fluids whose physical properties can be dynamically acted upon via the design or actuation of the colloids.
