Figure 2. Microparticle behaviour within the active suspension.

(a) Typical microparticle trajectory (∼210 s) in the Hele–Shaw experiment at N_c=4.84±0.13 × 10⁶ cells ml⁻¹. Colour represents instantaneous speed (colour bar unit: μm s⁻¹). The trajectory shows three types of dynamics: Brownian motion and loop-like perturbations (yellow-green blobs) followed by rare and large jumps (red lines). Inset: representative trajectory of a purely Brownian particle in the same setup, lasting ∼210 s. (b) A representative entrainment event: as the cell swims from the left to the right of the panel, it drives the colloid along the dashed line. Scale bar, 20 μm. The event lasts 1.16 s.