Hydrodynamic interactions dominate the structure of active swimmers' pair distribution functions

Microswimmers often exhibit surprising patterns due to the nonequilibrium nature of their dynamics. Collectively, suspensions of microswimmers appear as a liquid whose properties set it apart from its passive counterpart. To understand the impact of hydrodynamic interactions on the basic statistical features of a microswimmer's liquid, we investigate its structure by means of the pair distribution function. We perform particle-based simulations of microswimmers that include steric effects, shape anisotropy, and hydrodynamic interactions. We find that hydrodynamic interactions considerably alter the orientation-dependent pair distribution function compared to purely excluded-volume models like active Brownian particles and generally decrease the structure of the liquid. Depletion regions are dominant at lower filling fractions, while at larger filling fraction, the microswimmer liquid develops a stronger first shell of neighbors in specific directions, while losing structure at larger distances. Our work is a first step toward a statistico-mechanical treatment of the structure of microswimmer suspensions.