Bifurcations in suspensions of confined biased microswimmers

We study the emergence of collective dynamics in a suspension of confined Stokesian swimmers whose orientation is set by external cues. With an initially symmetric system, we show that increasing the strength of the alignment as well as the suspension density leads to a successive bifurcations, first to unsteady flows and eventually to symmetry-breaking at the level of the entire suspension. We study the system numerically, elucidate analytically the physical mechanism responsible for the instability, and relate our results to recent experimental findings.