Hydrodynamic correlations and instabilities in stress-balanced pusher-puller mixtures

Multispecies swarms are found for microorganisms living in microfluidic environments where they can take advantage of collective motions during transport and spreading. Nevertheless, there is a general lack of physical understandings of the origins of the multiscale unstable dynamics. Here we build a computation model to study the binary suspensions of rear- and front-actuated microswimmers, or respectively the so-called "pusher" and "puller" particles, that have different populations and swimming speeds. We perform direct particle simulations to reveal that even in the scenarios of "stress-balanced" mixtures which produce approximately zero net extra stresses, the longtime dynamics can exhibit non-trivial density fluctuations and spatially-correlated motions. We then construct a continuum kinetic model and perform linear stability analysis to reveal the underlying mechanisms of hydrodynamic instabilities. Our theoretical predictions qualitatively agree with numerical results and explain the onsets of the observed collective motions.