The role of activity in the nonlinear rheology and flow processes of active fiber (Turbatrix aceti) suspensions

Nematodes, with their active motility and fiber-like structure, serve as excellent models for studying active fluid behavior. This study explores the nonlinear rheology of active and inactive Turbatrix aceti suspensions using large amplitude oscillatory shear experiments and applying the sequence of physical processes method to assess Cole–Cole plots and Trefoil profiles in addition to storage and loss moduli (G′ and G″) and loss factor [tan(δ)] to explore the interplay between activity-induced and externally applied flow effects. At low frequencies, active suspensions display a stable viscoelastic state [(G′≈G″)] without crossover, transitioning to a viscous regime at higher strain amplitudes. Inactive/passive suspensions, in contrast, exhibit viscoelastic gel-like behavior (G′>G″) with clear crossover points. The transition points shift to lower strains at higher frequencies for both active and passive suspensions. At f = 4 Hz, which matches with the nematode's natural undulation frequency (4–6 Hz), active and passive suspensions display similar viscous behavior, indicating that the influence of nematode activity is effectively suppressed by the external flow field. Our study shows that higher frequencies reduce the applied shear strain threshold required to overcome nematode-driven activity, with external power dominating at f = 4 Hz. These findings advances our understanding of activity–flow interactions in complex fluids and lays the groundwork for practical applications, such as the design of nematode-inspired microrobots, microfluidic mixers, and design of on-demand flow generating systems for medical and industrial applications.