Self-focusing and jet instability of a microswimmer suspension

Three-dimensional (3D) numerical simulations are performed on suspensions composed of puller-like microswimmers that are sensitive to light (phototaxis) mimicking microalgae in a Poiseuille flow. Simulations are based on the numerical resolution of the flow equations at low Reynolds numbers discretized on a 3D grid (finite differences). The model reproduces the formation of a central jet of swimmers by self-focusing [Phys. Rev. Lett. 110, 138106 (2013)] but also predicts an instability of the jet, which leads to its fractionation in clusters. We show that this instability is due to hydrodynamic interactions between microswimmers, which attract each other along the flow direction. This effect was not observed in the experiments conducted on dilute suspensions (i.e., where hydrodynamic interactions are weak). This phenomenon is peculiar for pullers for which collective motions are usually not observed on such a time scale. With this modeling, we hope to pave the way toward a better understanding of concentration techniques of algae (a bottleneck challenge in industrial applications).