Hydrodynamic interactions characterise active rheology of soft microswimmer suspensions

Swimming microorganisms dynamically respond to their external environment, resulting in self-organised collective behaviour. To accurately predict the flow field at the continuum scale, it is essential to understand the rheological properties of active suspensions. Microorganisms in nature exhibit a wide range of deformabilities, and former studies have reported that they play an important role in hydrodynamic interactions. However, the effect of cell deformability on suspension rheology has not yet been clarified. In this study, we developed a soft microswimmer model to represent general swimming microorganisms to investigate rheological properties such as apparent viscosity and normal stress differences of their suspensions. The model consisted of a capsule with a hyperelastic membrane and propelled itself by a torque distribution modelling the ciliary activity of a ciliate. Our results demonstrated that interparticle hydrodynamic interaction is a critical factor inducing symmetry breaking in the time-averaged orientational distribution in the low shear flow. For the ellipsoidal swimmers, these interactions promote orientational distribution biased towards the elongational direction, resulting in inherent viscosity variation in microbial suspensions. In addition, this orientational distribution is distinctly different from that observed in solitary swimmers, causing the significant reduction (increase) in the apparent viscosity for the pusher (puller). Analysis varying membrane deformability showed that the softer membrane is less likely to exhibit the effects of the active stress on specific viscosity. These findings demonstrate the important role of elastohydrodynamic interactions between swimmers in suspension rheology and support the development of continuum models for complex microbial systems.