Large-scale Lattice-Boltzmann simulations of the collective behaviour of swimming microorganisms
Many microorganisms live in aqueous environments, like the ocean or inside larger organisms. As such, most have evolved the ability to propel themselves, or swim, through the fluid they inhabit – they are motile. Their motion is often directed, for example in order to escape threats, or seek out food. In many cases, a swimmer's movement is a combination of random reorientations, and a response to chemical stimulus – the latter called chemotaxis.
While the swimming patterns of individual organisms are relatively well understood for several species, additional phenomena that are yet poorly investigated arise when several swimmers are considered together, at high concentrations. Each microorganism creates a fluid flow around itself as it moves, which affects other particles in the solution, including other swimmers. The shape of the flow field, and thereby its effects, depends on the species in question. Different organisms can have significantly different propulsion mechanisms, potentially leading to dissimilar collective behaviour.
My work focuses on investigating the effects of these hydrodynamic interactions. Notably, in the case of rear-actuated microswimmers (so-called pushers) such as the commonly studied bacterium E. coli, the generated flow field leads to chaotic, collective motion over comparatively large distances. This is referred to as bacterial turbulence, due to the similarities between the arising swimming patterns and regular turbulence - the flows seen for example around airplane wings.
It has also been observed that the presence of swimmers can lead to dramatically increased mixing of passive (non-swimming) tracer particles in a fluid. Due to the flows, the particles diffuse much quicker than what can be predicted as stemming from pure thermal agitation. Curiously, in cases where the tracers have elongated, ellipsoidal shapes, they tend to diffuse faster along their minor axes than along their major axis. This is exactly the opposite of what is seen in regular diffusion, and an occurrence we still do not have an adequate explanation for.
As a member of Joakim Stenhammar’s research group, I aim to understand these phenomena primarily through the use of large-scale computer simulations. We utilize the lattice Boltzmann method, which involves solving the Navier-Stokes equation on a lattice. Together with principles from statistical physics, our approach is tailored for explaining these and other aspects of the collective behaviour of swimming microorganisms.