Particle-Size Effects on the Enhanced Diffusion of Tracer Particles in Microswimmer Suspensions

Abstract: Active suspensions of microswimmers such as bacteria or microalgae are found in oceans or lakes, and within living organisms, such as the human body. These suspensions can exhibit complex flow patterns and enhanced diffusion of passive tracer particles due to the advection from the long-ranged dipolar flow fields of the swimmers. The diffusion of tracers at varying radii is poorly understood but one recent experimental study points to a non-monotonic behaviour with a certain radius that maximizes diffusion. In this thesis, we study the effect of nonlinearities in the flow field on the effective diffusion of spherical tracer particles. To do so, we model the swimmers as force dipoles which create a known fluid field around them. In a single-swimmer-single-tracer simulation, this field is used directly to study the advection while a lattice Boltzmann simulation allows for many-particle simulations. For non-interacting swimmers, corresponding to very dilute suspensions, we find that the diffusion coefficient as a function of tracer radius is non-monotonic, although it is convex in the probed range. However, the simple one-swimmer-one-tracer simulation indicates that the many-particle simulation is only valid below a certain tracer radius (R = 2.5 × the swimmers’ length) where the function is slightly decreasing. In this range, for interacting swimmers, the effect of interaction is increased for both of the studied swimmer types, pushers and pullers.