Spatiotemporal organization in an active viscoelastic suspension: a dissipative particle dynamics study.

Microorganisms such as bacteria and spermatozoa often inhabit confined viscoelastic environments. These organisms exhibit self-organization/collective dynamics in such complex surroundings. Here, we report a simulation study of active particle suspensions in viscoelastic fluids under confinement-representing an experimental scenario where motile organisms suspended in aqueous viscoelastic fluid are surrounded by an oil medium. We employed dissipative particle dynamics, a particle-based mesoscopic approach, to model the system with minimalistic ingredients and qualitatively reproduced some of the experimental observations by Liu et al. Nature, 2021, 590 (7844), 80-84. The collective dynamics within the suspended drop, mediated by the viscoelastic nature of the medium, manifest into two steady state configurations, namely a unidirectional vortex or an oscillatory vortex. We present a phase diagram for the drop's steady state configuration as a function of system parameters, such as strength and packing fraction of active agents, polymer concentration etc.