Self-organisation of spermatozoa in bulk suspensions via unsteady elastohydrodynamic interactions

We developed a mechanical model of spermatozoal swimming in bulk suspensions. We traced the spatiotemporal elastohydrodynamic interactions and found that spermatozoa engaged in self-organisation: flagellar undulatory motion generated a stable polar order. Conventional microswimmer theories use time-averaged stress fields to predict that the cellular structure is unstable. However, our present results reveal a novel self-organisation mechanism attributable to unsteady hydrodynamic interactions. We thus contribute to the theory of active soft matter dynamics.