A crowd at the entrance to a narrow corridor behaves differently than in a spacious hall: some people get pressed against the walls, others are pushed forward. Roughly the same happens when an elongated particle finds itself in a fluid teeming with self-propelled dumbbells. Until now, it was unclear why such a particle moves faster along its long axis than across it, and how the crowd density affects its speed.

In a computer model, scientists placed an ellipse in a two-dimensional suspension of active dumbbells that repel each other upon contact. Each dumbbell consists of two beads connected by a spring and swims forward under its own force. When dumbbells are scarce, they only occasionally bump into the ellipse, which barely moves. But as the crowd thickens, the ellipse starts to accelerate: its speed and rotation increase more than tenfold.

However, acceleration is not infinite. Above a threshold density, dumbbells form a dense ring around the ellipse, acting like a cage that restricts motion. The more active the dumbbells, the earlier this threshold is reached. So there is an optimal density: below it, the crowd helps; above it, it hinders.

Earlier experiments with bacteria gave contradictory results: in some cases, particles diffused faster across, in others along. Now it is clear that it all depends on how active swimmers accumulate at the surface. The computer model isolated only collisions, without fluid flows, and showed that this mechanism alone is sufficient for directed transport.

Let's return to the corridor: the crowd at the entrance creates uneven pressure, and an elongated object squeezed in it will move along its long axis. So too with an ellipse in an active suspension: its path is dictated not only by its shape but also by how the crowd self-organizes at its surface. This opens the way to controlling microparticle motion in living systems, where such crowds are everywhere.