Squirmers — model microswimmers that generate surface flows to propel themselves — dissipate energy differently depending on whether they push or pull. In moderately dense suspensions under shear, pushers burn more energy than pullers at low shear rates, a gap that widens with increasing density. This contradicts expectations from bacterial suspension studies, where the distinction between pusher and puller rheology follows a different pattern. The disagreement traces to a structural difference: apolar shakers interact through a different hydrodynamic multipole than self-propelled bacteria.
At high shear rates, the distinction vanishes. External flow overwhelms internal activity, and both types behave like passive spheres. But in the low-shear regime — where the swimmer's own activity dominates the local flow — the microstructure diverges. Pushers develop stronger nematic alignment and directional pair correlations, both of which increase effective viscosity. The rheological signature is not a direct consequence of swimming mode but of how activity and confinement organize the local geometry.
Activity in a suspension doesn't just add energy — it restructures the medium through which that energy must dissipate. The relationship between internal motion and macroscopic resistance is mediated by the microstructure that activity itself creates. Self-organization is not separate from dissipation; it is the mechanism by which dissipation takes its particular form.
(arXiv:2603.00392)