Collective behaviour in suspensions of pushers and pullers

Abstract: Microswimmer is a term that denotes small selfpropelling agents, such as bacteria, algea or artificial robots. Depending on the flow field they set up they can be categorized into two classes: pushers (e.g. E. coli ) and pullers (e.g C. reinhardtii ). The occurrence of jets, eddies and flocking have been observed in systems of dens pusher suspension. This phenomenon is called active turbulence. The transition to active turbulence has been attributed to hydrodynamic interactions reorienting and aligning pushers, whereas the same type of hydrodynamic interactions destroy collective motion in puller suspensions. This thesis sets out to investigate the properties of active turbulence in mixtures of pushers and pullers by means of particle-based Lattice Boltzmann simulations. Two body interactions between microswimmers and properteis regarding fluid statistics and swimmer statistics in 3-dimensional suspensions were investigated. Similar studies were also performed on a class of nonswimming particles, so called shakers. Our results shows that the presence of pullers in a pusher suspension can inhibit active turbulence. Furthermore we have observed that a suspension consisting equal parts pullers and pushers will display properties similar to a noninteracting system.