Dense assemblies of self-propelling rods (SPRs) may exhibit fascinating collective behaviors and anomalous physical properties that are far away from equilibrium. Using large-scale Brownian dynamics simulations, we investigate the dynamics...
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.
I. INTRODUCTIONActive suspensions of swimming microorganisms, such as bacteria or algae, can exhibit fascinating collective behaviors that feature large-scale coherent structures, enhanced mixing, ordering transition, and anomalous diffusion [1,2]. To uncover the multiscale origins of the collective dynamics, people have constructed micro-mechanical models that can capture the distinctive swimming mechanisms for various microswimmers. Figure 1(a) sketches the so-called "pusher" and "puller" particles to characterize a class of microswimmers that generate thrust at the rear (e.g., E. Coli) and at the front (e.g., Chlamydomonas) of the body [3], which respectively exert extensile and contractile dipolar extra stresses on the surrounding fluid. So far, our physical
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