Periodic Reporting for period 4 - CollectSwim (Individual and Collective Swimming of Active Microparticles)
Okres sprawozdawczy: 2022-03-01 do 2023-08-31
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.
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.
Chemically-active colloids must establish local chemical gradients in order to self-propel. The CollectSwim project significantly improved the physical understanding of the fundamental principles underlying this individual propulsion and the role of the particle design and environment. In contrast with phoretic particles, active droplets’ motion critically rely on the tight coupling of chemical transport with the self-generated interfacial flows. Despite the intrinsic difficulties arising from this nonlinearity, the project succeeded in overcoming several important technical and fundamental hurdles to unravel the role of hydro-chemical coupling in the droplets’ interactions inside a suspension and with a confining boundary, explaining for the first time how droplets can indeed propel so close to rigid walls, an observation that so far defied our intuition that viscous dissipation would preclude them to do so. A first-of-a-kind analysis of their interactions and resulting dynamics was also obtained using advanced mathematical techniques and simulations.
Building upon this individual modeling, the project developed several important numerical and analytical frameworks and methodologies for computing the interaction dynamics of a few active colloids all the way up to the self-organization and dynamics of a suspension. These rely either on particle-based approaches, where each individual colloid is described, and continuum modeling, thus focusing on the description of the statistical evolution of the particles’ organization. The latter allowed the team to analyze for the first time the rheological response of a suspension of chemically-active particles, demonstrating a reduction of the effective viscosity for some well-chosen set of physico-chemical properties.
The results of CollectSwim pave the way for new experimental approaches and applications such as the control of the suspension through careful design or physico-chemical forcing of the colloids.
Building upon this individual modeling, the project developed several important numerical and analytical frameworks and methodologies for computing the interaction dynamics of a few active colloids all the way up to the self-organization and dynamics of a suspension. These rely either on particle-based approaches, where each individual colloid is described, and continuum modeling, thus focusing on the description of the statistical evolution of the particles’ organization. The latter allowed the team to analyze for the first time the rheological response of a suspension of chemically-active particles, demonstrating a reduction of the effective viscosity for some well-chosen set of physico-chemical properties.
The results of CollectSwim pave the way for new experimental approaches and applications such as the control of the suspension through careful design or physico-chemical forcing of the colloids.
CollectSwim obtained several important results but two of them are particularly remarkable and go well beyond the state of the art of the discipline at the start of the project.
The first one corresponds to one of the main objectives of the project, namely the characterization of the rheological response of chemically-active suspensions. For the first time, using a kinetic approach to describe a dilute suspensions of phoretic particles, our team demonstrated the possibility to effectively reduce the viscosity of the active fluid by a careful tuning of the properties of the particles so as to trigger specific self-organisation in a shear flow.
The second major and unexpected result of the project brings a new paradigm in the understanding of chemically-active droplet propulsion. These fascinating yet particularly simple active colloids (e.g. oil droplets in water with large amount of tension-active molecules) rely on an instability to break spatial symmetry and swim in a specific direction. In experiments, they are generally denser or lighter than the suspending fluid and are therefore found very close to a horizontal wall. This tight proximity would be expected to oppose the droplet motion due to the increased dissipation: yet, they are commonly observed to swim in such settings! For the first time, CollectSwim provided both a fundamental physical understanding and a mathematical demonstration of the origin of this phenomenon: confinement does increase the friction on the wall but it also most importantly prevents chemical diffusion, thus enhancing locally the chemical gradients that drive the flow and droplet motion. As such, confinement not only does not hinder propulsion but actually promotes it.
The first one corresponds to one of the main objectives of the project, namely the characterization of the rheological response of chemically-active suspensions. For the first time, using a kinetic approach to describe a dilute suspensions of phoretic particles, our team demonstrated the possibility to effectively reduce the viscosity of the active fluid by a careful tuning of the properties of the particles so as to trigger specific self-organisation in a shear flow.
The second major and unexpected result of the project brings a new paradigm in the understanding of chemically-active droplet propulsion. These fascinating yet particularly simple active colloids (e.g. oil droplets in water with large amount of tension-active molecules) rely on an instability to break spatial symmetry and swim in a specific direction. In experiments, they are generally denser or lighter than the suspending fluid and are therefore found very close to a horizontal wall. This tight proximity would be expected to oppose the droplet motion due to the increased dissipation: yet, they are commonly observed to swim in such settings! For the first time, CollectSwim provided both a fundamental physical understanding and a mathematical demonstration of the origin of this phenomenon: confinement does increase the friction on the wall but it also most importantly prevents chemical diffusion, thus enhancing locally the chemical gradients that drive the flow and droplet motion. As such, confinement not only does not hinder propulsion but actually promotes it.