Description:

Small particles suspended in a viscous fluid interact via long-range hydrodynamic interactions. In this talk, we will explore different ways to use and control hydrodynamic interactions to achieve specific functions, such as self-assembly, enhanced fluid mixing and particle transport, with synthetic and biological microswimmers.

First, we will focus on torque-driven synthetic particles, called microrollers. Recent experiments and simulations have shown that microrollers can self-assemble into stable motile structures, called « critters ». Using a dynamical system framework, we will identify the mechanisms that lead to spontaneous self-assembly in hydrodynamic systems. We will show that pairs of microrollers can exhibit hydrodynamic bound states whose nature depends on a dimensionless number, denoted B, that compares the relative strength of gravitational forces and torque-driven flows. We will characterize these various states in phase space and analyze the bifurcations of the system as B varies. In particular, I will show that there is a critical value of B for the existence of stable motile orbiting, “leapfrog”, trajectories, reminiscent of the « critters » previously observed.

In the second part of the talk, we will focus on the mixing and transport properties of biological suspensions. In addition to enabling movement towards environments with favourable living conditions, swimming by microorganisms has also been linked to enhanced mixing and improved nutrient uptake by their populations. Experimental studies have shown that Brownian tracer particles exhibit enhanced diffusion due to the swimmers, while theoretical models have linked this increase in diffusion to the flows generated by the swimming microorganisms, as well as collisions with the swimmers. Using large-scale numerical simulations, we will examine tracer displacements and effective tracer diffusivity in micro-algae suspensions. By isolating effects such as hydrodynamic or steric interactions, we will provide physical insight into experimental measurements of the tracer displacement distribution. In addition, we will extend results to the semi-dilute regime where the swimmer–swimmer interactions affect tracer transport and the effective tracer diffusivity no longer scales linearly with the swimmer volume fraction.

Professor Blaise Delmotte, LadHyX, Ecole Polytechnique

Host: Michelle Driscoll

Keywords: Physics, Astronomy, Complex Systems