A microswimmer in open fluid moves in a straight line (plus rotational diffusion). Add obstacles and the motion changes qualitatively. Simulations of squirmers navigating random obstacle arrays reveal that the interaction between self-propulsion, hydrodynamic flows, and confinement creates effective long-range diffusion from short-range mechanisms (arXiv:2603.21285).
Different swimmer types suffer different fates. Pushers (which generate thrust from behind, like bacteria) and pullers (which pull from the front, like algae) exhibit asymmetric trapping. Pushers get trapped at obstacle corners through a geometric mechanism — their flow field aligns with the corner geometry and locks them in place. Pullers get trapped dynamically — they orbit near obstacles in stable loops created by their own flow fields. The trapping mechanisms are structurally different despite producing the same macroscopic outcome (immobilization).
At high enough obstacle density, a “hopping-and-trapping” regime emerges. Swimmers escape one trap only to fall into the next, creating a random walk between trapping sites. The effective diffusion coefficient depends on the ratio of trapping time to hopping time — a quantity set by near-field hydrodynamic interactions that are sensitive to the swimmer's detailed geometry.
The structural insight: the disordered environment doesn't just slow down the swimmer. It selects for swimmer types. Pushers survive in sparse environments but jam in dense ones. Pullers develop dynamic traps that are stable only at certain speeds. The obstacle field acts as a filter, not a barrier — what gets through depends on the coupling between self-propulsion and the local flow topology. The same maze admits different swimmers with different probabilities, and the selection criterion is hydrodynamic, not geometric.