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[Master 2] Brownian dynamics simulations of autophoretic active particles

par Marie Jardat, Pierre Illien, Vincent Dahirel - 23 octobre 2019

Période de stage : 5 mois min. entre janvier et juillet/septembre 2020

Encadrement : Pierre Illien, Vincent Dahirel et Marie Jardat (équipe MEM)

Description of the project

The ability to design active colloids and to accurately control their motion in a fluid environment is a key challenge of physical chemistry, and has motivated a significant amount of work over the past few years. In this context, diffusiophoretic transport has emerged as a prominent mechanism for non-equilibrium activity and self-propulsion. It has been known for many decades that the interactions between a particle immersed in a fluid and an inhomogeneous concentration of solute can lead to force-free and torque-free propulsion [1]. ’Active’ colloids typically bear an anisotropic coating of a catalyst that enhances some chemical transformation of the solute at a given side of the colloid. The colloid herefore self-generates gradients of concentration and responds to them by a net displacement in the fluid.

Recently, Michelin et al. showed that such anisotropy is not necessary for locomotion and that the nonlinear interplay between surface osmotic flows and solute advection can produce spontaneous and self-sustained motion of isotropic particles [2]. Solving the classical phoretic framework for isotropic particles emitting a solute isotropically, they proved that there exists a critical particle size above which spontaneous symmetry-breaking and autophoretic motion occur (Fig. 1a). Subsequently, Izri et al. proposed an experimental system of isotropic water droplets in a medium made of oil and surfactants that spontaneously self-propel, and autophoresis was put forward as the mechanism responsible for self-propulsion.

In spite of these observations, the understanding of this mechanism is still poorly understood. Indeed, the analytical theory rely on the analysis of mean-field advection-diffusion equations, that treats the solvent as a continuous media and neglect the interaction between the solute molecules. In particular, accounting for the interactions between the solute molecules will lead to jamming and depletion effects in front of and behind the tracer, that can modify significantly the effects predicted by the mean-field theory (Fig. 1b). Therefore, there is a need for a microscopic theory for autophoresis around active colloids.

Specific techniques or methods

In this context, we propose to perform Brownian dynamics simulations of the whole system (active colloid and solute molecules), accounting for the interactions between all the particles. The intern will first reproduce numerically the observations expected from the mean-field theory (with non-interacting solute molecules), and will study the effect of crowding interactions afterwards. The transport properties of the active colloids will be quantified, and the influence of different parameters (relative size of the colloid and the solute molecules, strength and shape of the interaction potential...) will be explored


[1] P. Illien, R. Golestanian, A. Sen, Chem. Soc. Rev.46, 5508 (2017).
[2] S. Michelin, E. Lauga, D. Bartolo, Phys. Fluids25, 061701 (2013).
[3] Z. Izri, M. van der Linden, S. Michelin, O. Dauchot, Phys. Rev. Lett.113, 248302 (2014)